Metric system

The metric system is a system of units for measurement that are all interrelated in a mathematically convenient way. For example, a force of one newton applied to an area of one square metre is exactly one pascal of pressure, and one newton applied over a distance of one meter is one joule of energy. As another example, one liter is one cubic decimeter.1 dm3 = 0.1 m × 0.1 m × 0.1 m = 0.001 m3 One liter of water has a mass of approximately one kilogram and requires 4184 joules (this is the definition of one kilocalorie) to raise the temperature by one kelvin, assuming no phase transitions.One degree Celsius also works in this case, because the size of one kelvin is equal to the size of one degree Celsius. The modern version of the metric system, SI, has been designed with the intent ultimately to deprecate legacy units around the world, by incorporating the most recent developments in metrology into the definitions of its base units, and by learning from the shortcomings of previous attempts at devising such a system of units (e.g. and 
Such systems are called and are used in a decimal fashion for smaller and larger measurements, using a system of prefixes to indicate orders of magnitude. This makes the system incredibly easy to work in. It has been almost universally adopted for use around the world, and is commonly used in scientific work. Nowadays, it is standard for undergraduate students in science and engineering to be taught using only SI units, although different systems of units are used at the graduate level and in the literature. For example, books and research papers in electromagnetism and quantum mechanics tend to use Gaussian units while those in particle physics Lorentz-Heaviside units.Jackson, John David. Classical Electrodynamics. 3rd ed., Wiley, 1999. Preface.Griffiths, David J. Introduction to Electrodynamics. 4th ed,. Pearson, 2013. Advertisement.Readers who have studied elementary chemistry have no doubt encountered yet another non-SI unit, the atomic mass unit, used in the periodic table of the elements.
==The SI==
The metric system had been discussed as early as the 16th and 17th centuries. Its first implementation came in 1799 during the French Revolution. The modern version of the metric system, first announced in 1960, is the Système International (International System), abbreviated "SI", which defines the specific units used in science. The SI follows strict mathematical principles such as coherency: each physical dimension (such as length, mass, or energy) has a single unit associated to it, and these units are interrelated by dimensional analysis, always using a factor of one. This means that the mathematical relationship between any two physical dimensions is identical to the relationship between the units that measure them; in modern physics and engineering, equations are almost always written in SI units for this reason.
For example, since Newton's Second Law defines force as the product of mass and acceleration, it follows that the SI unit of force, the newton, must be equivalent to the product of the SI units of mass (the kilogram) and acceleration (the metre per second squared). Thus 1 N = 1 kg·m·s-2, in terms of the SI base units.
Some common metric units are not SI units, but relate directly to them. For instance, the SI considers the litre (l or L) to be a special name for the cubic decimetre (dm3); the SI unit of volume, following dimensional analysis (i.e., volume is the cube of length), is the cubic metre.
One of the great strengths of the SI in terms of promoting science literacy is that it emphasises the connections between different kinds of physical dimension: the watt (power unit) is one joule (energy unit) per second (time unit). Pre-metric units of power and energy, such as horsepower and British Thermal Units (BTU), lack this transparent relationship, and thus their relationships to other units are obscure and extremely difficult to understand. In practice, when using the older units, it is common to "cheat" and convert the inputs to SI units, calculate a result in SI units, and then apply conversion factors to get a result in the desired unit.
==Customary units==
Was the catch-all term for the older systems then in use; a confusing, usually a needlessly complicated collection of arbitrary items with differing ratios devoid of any continuity. Most countries had their own sets, which would sometimes differ within countries; the Spanish 'yard' was between 77 and 155 centimetres and the 'pound' was between 350 and 575 grams, for example. Clearly a situation where every town had their own unique measures was not conductive to trade, construction, education, or production. The fact that the metric system started in France – a land which apparently had half a million different units in use – is no surprise.
The obvious benefits led to a gradual adoption worldwide; most of Europe by 1914, most of the world by 1939. Almost all new nations adopted metric on independence or soon after. The declaration of it as the 'International System' in 1960 wasn't a statement of intention or ambition, but of fact.
Except the Anglosphere, that is.
=== 'English' measures ===
Like the other customary systems, it's both archaic and arcane; 12 inches to the foot, 3 feet to the yard, 1,760 yards or 5,280 feet to the mile. Apparently, this comes from human measurements; an inch is a width of a thumb, a foot is an adult forearm (yes, I know), and a yard one full stride. Then there's ounces, pounds, and tons for mass and fluid ounces, pints, quarts, and gallons for volume. All with their own ratios. Confused yet?
But a few measurements are not equal – there is a nautical/air mile (1,852 meters), while a land mile is around 13% shorter at about 1,609 meters, for example. There's the occasional use of almost-extinct units; such as fathoms (6 feet or 6 cubic feet, used in seafaring and mining), furlongs (220 yards, horse-racing), or the engineer's chain (100 feet, land surveying), which is not the same as the other two types of chain... And pound is not a pound when measuring gunpowder or precious metals - instead of the avoirdupois pound (454 g) divided in 16 ounces, some things are weighed in troy pounds (373 g) of 12 ounces. Thus one pound of feathers weighs more than one pound of gold, but one ounce of gunpowder weighs more than one ounce of lead.
The worst aspect is that what is believed by some to be one system is in fact two; they diverged in 1824 when the British created the 'Imperial System' for their EmpireAlso a formal abolishment of Scottish units., but the United States continued with the older one (now known as 'US Customary'). Now, an Imperial ton is 2,240 pounds, while an American ton is 2,000. The 'international foot' and the American 'survey foot' is off by 32 millimetre per mile. The American fluid ounce is 4% larger than the Imperial, but there's 32 less ounces in a US gallon, leaving it 17% smaller than its Imperial variant. And just to make things easier, they use the same symbols - to the chagrin of say, homebrewers looking at a recipe and wondering if the 'gallon' of beer is 3.78l or 4.45l.
The difference in the length of inch in the sixth decimal doesn't sound much, but in the fine mechanics, it is. Take the Merlin aircraft engine, used during WW2. Originally made by the British Rolls-Royce Company, the design was shared with Packard in 1940 by the British 'Tizard Mission'Where the British effectively did a scientific/engineering striptease; showing the Americans their workings on jet propulsion, electronics, radar, high explosives and most importantly, what would become the atom bomb.. Problem was, American-made parts caused British-made engines to leak oil while American-made engines would seize up with British-made parts. Which became even more confusing as at times you ended up with 'American-model' planes using British Merlins and 'British-model' planes using American Merlins (causing frankly unnecessary complication in supply-lines).
Yet, despite all this confusion and difficulty, metrication in the Anglosphere has been partial at best.
===In the United States=== 
While the general American public couldn't use metric units to save their lives, it is used widely in the United States among entrepreneurs, scientists, engineers, the military, and hospitals, the last of which tend to have scales that measure in kilograms for the hospital's information on the patient and can switch to pounds for laypeople who aren't completely familiar with the system (and they assume every patient is not). In addition, hospitals often use 24-hour time like mainland Europe.
The general American public, has, however, fiercely resisted the adoption of the metric system, with most simply being uncomfortable with change, and many even claiming that the metric system is "confusing" (when in reality they simply don't try to learn it or find it difficult at first because they are not used to it). In spite of this, everyone in the U.S. is quite comfortable with two-liter bottles of carbonated sweetened beverages, 9 mm bullets, and nutritional information and medicine measured in grams and milligrams (example: a fat-free product saying it has 0 g of fat).Some businesses manage to sell beverages in 2-quart (1.89 L) bottles/cartons which make them slightly more profitable. (N.B.: An Imperial quart is 1.16 US quarts, and is in fact larger than a litre at about 1140 mL. And every alcoholic knows perfectly well that 750 mL is close enough to a "fifth" (1/5 US gal) that it doesn't really matter.
Of course, there have been some crackpot right-wing responses to the metric system that will go as far as to claim that adoption of the metric system would destroy America and/or the American identity, in one way or another (i.e., a moral panic). For example, Dean Krakel, Director of the National Cowboy Hall of Fame: 
Never mind that that was before communism was really a thing, and it was done by the kilometre.
and Dave Barry (who — it should be noted — is a humorist): 
In 2019, Tucker Carlson ran a segment in which he referred to the metric system as "creepy", "inelegant", and "the yoke of tyranny" and stated that it was part of the New World Order, and he and his guest used various stupid arguments that are commonly used by people unwilling to change to the metric system which will be refuted later in this article. He ended the segment by stating "I'll accept the kilometer when we accept the Euro, never!"
Interstate 19, located in Arizona between Tucson and the Mexican border, is notably the only highway in the United States to use the metric system, and it is a relic of Gerald Ford's attempt to metrify. An attempt to change it to imperial units in 2009 failed as a result of backlash from the local residents and business owners who were too used to the metric units.Untold Arizona: How Arizona's Metric Highway Nearly Lost Its Unique Status, Christina Estes, KJZZ 5 April 2019
===In the United Kingdom===
The British offer an interesting counterpart to the American experience, because while they decided roughly the same time to 'go metric' (1965 instead of 1975) the Brits decided to a) make it legally mandatory to convert and b) actually stick with it (well, sorta). The majority of the legwork was done between 1965 and 1985, though some sectors had stupidly long 'dual use' periods (loose produce for consumers until 2000), others suffered from lack of enforcement (British schools didn't hugely bother until the 1990s) and in a few areas the Government itself was the backslider (which is why Brits buy petrol in litres but measure road distances in miles)Inching Towards the Meter: Britain, Europe and the Politics of Economic Integration University of Manchester, 2 May 2017.. At first, resistance was led by a combination of the obtuse, wilfully ignorant and nostalgia fetishists but the movement started to make real headway after the upswing of Euroscepticism after the signing of the of 1992.Seen by opponents as being the plan to make a 'United States of Europe'. Now, 'Brussels' was held to be the cause of 'the loss of British units', and a stubborn clinging onto them was held to be a sign of 'patriotism'. This belief gained traction into the late 1990s as the final transition periods expired… which led to the - which a then little-known agitator named Nigel Farage and a crew from this tiny political party called UKIP goaded alongWe owe the Metric Martyrs a formal pardon Nigel Farage.com, 31 May 2022., supported by the right-wing press such as the Daily Express and Daily Mail. 
Like the American experience, 'support for Imperial units' became an increasingly partisan affair (it became part of the 'Brexit coalition'), however it was always on the back foot because unlike the Americans, the metric system had generally become the status quo and as such, they were advocating the return of a system which younger cohorts (as well as millions of settled foreigners) had only a hazy to nil working knowledge of and thus, our 're-Imperialisers' ran into the same obtuse laziness which had hindered metrification in the 1970sResistance increasing the moment British Millennials and Gen Z realise you can't easily use a calculator on pounds and inches.. What's more, it had become increasingly tainted by crank magnetism - that it became increasingly clear to the wider British public that the sorts of people who got all het up over 'defending yards and stones' were also the sorts who seemed to view 'Europe' as the source of all evil, had a tendency of xenophobic mutterings, bore a barely-concealed Islamophobic streak 30.48cm wide and was likely to be a general social reactionary and technological luddite.
Things got to a head during the final flailing months of the Johnson regime, when the Edwardian cosplayer Jacob Rees-Mogg (at this point 'Minister of Brexit Opportunities') showed he thought it was grand to re-adopt a system that had been ditched before he'd been potty-trained, ordering a survey forthwith.Govt pays for 'ludicrous' study on benefits of reintroducing imperial units after Brexit LBC, 23 February 2022. Despite attempting to tune the study to fit their preconceptions and with other Tory blockheads championing the idea,Tory MP has said the UK will adopt imperial measures to bring back 'British culture and heritage.' Euronews, 30 May 2023. this study didn't go well; businesses and educators alike were at a loss of where these 'benefits' were, engineers were strongly againstEngineers raise concerns over proposed switch to imperial measurements New Civil Engineer, 13 October 2023. and the public aroused enough to the point 99% of respondents were solidly against. Not much of a shock that his successor's successor Rishi Sunak (after dumping the pilot) quietly took the plan behind the shed and shot it - though hardly because he was firmly for the metric system, more that it simply a culture war edge which wasn't delivering electorallyImperial Measurements: the Spurious Brexit Dividend that Failed to Divide Byline Times, 18 January 2024..
Thus, the status quo resumed; with the Imperial system continuing their slow slide into the mists of time (with the exception of a few holdouts such as road distances) and with a generally dwindling band of people who think this is somehow all wrong and should be reversed. Ultimately, it may be globalisation which kills off Imperial - that in a world where everything is seemingly made in China it is perhaps wise to know the accurate measures in metric because even if the seller gives dimensions in Imperial it very well be an approximation from metric.
=== In Ireland ===
The Imperial System was part of the British 'inheritance' which Eire got on leaving the United Kingdom in 1922, which the Irish maintained mainly for the basis they were still economically stuck within the British orbit (it was only in the 1980s where the UK share of Irish external trade went under 50%Ireland’s economy since independence: what lessons from the past 100 years? Economics Observatory, 30 April 2021.) and it was important to have 'compatibility' for your historically best export - that is being peopleExhibition tells stories of Irish immigration to Great Britain since 1970s The Guardian, 31 October 2023.. So when the British announced they were 'going metric', the Irish promptly followed suit (though they'd been considering it ever since Independence). There was significantly less hostility to this change, partly due to the fact that Imperial was associated with the British and by extension, of being 'a colony'Technically after 1800 they were not - having become a part of the UK due to an Act of Union - but the colonial socio-economic factors long remained.. Generally speaking, the Irish mirrored the British, partly due to the fact they had also joined the EEC in 1973 and thus, also felt the metric hand of Brussels - and like London, Dublin carved out 'opt outs'Future Of Inch Yard And Pint Secure - 2007 RTE., which means you're still capable of getting a pint of Murphy's when thirsty. Main difference is that the Irish decided to embrace the kilometre in the 1980s which meant they formally ditched the road-mile in 2005Ireland goes metric - fast The Independent, 8 February 2005.. However, a lot of Imperial still continues to be used unofficially by older folks like on the other side of the Sea; inches and feet for height, acres for land, stones for weight etc.
=== Elsewhere ===
Outside of the United States (and its associated states: the Federated States of Micronesia, Marshall Islands and Palau), the nations of Liberia, Myanmar, and Samoa also maintain the usage of the imperial system. Liberia and Myanmar, however, have made efforts towards metrication, and now at least in part use metric units. 
===Arguments against introducing metrication===
;It's too hard (for old people) to learn a new system!
:Appeal to laziness. While on the surface this makes a bit of sense, it promptly falls apart when you realise you will always have 'old people' and therefore, will be forbidden to change anything ever because it would upset them. Similar could be said regarding the phaseout of particular technologies such as analogue television, leaded petrol and DDT. What's more, previous metrification efforts (such as with the UK) show how it can be done with a relative lack of fuss - that you start with educating the youth to 'think in metric', allow soft metrification of popular/customary sizes and give long periods where both systems are used. Lastly, it is an inherently ageist comment because it makes the assumption that older people are incapable of learning new things. It's just that we've already learned how to measure stuff, don't want to have to do it again and use the excuse of 'think of the olds and their fossilised brains' as an excuse.
;Divisions into tenths are awkward to work with on a practical basis!
:Appeal to tradition. If using mental or paper-based arithmetic, chances are - assuming equal knowledge of both systems - that you'd be able to work out measures and calculations at roughly the same speed. Except the people making this argument don't have equal familiarity. Therefore, for them Imperial/customary system is 'faster'/'more accurate' than using a metric system they're rather hazy at. Younger Britons would provide a decent inversion of this; for them it's the Imperial system which is 'slower'/'less accurate' than the metric system due to unfamiliarity. 
:However, the metric system has one clear edge which has become ever-more glaring since the 1970s - that electronic calculators have become ubiquitous, and they work very well on a base-ten system, meaning that if armed with a calculator (which almost everyone with access to a working cellphone does) you'd be able to compute the metric results quicker. What's more, having decimals is a lot more convenient than working in eighths, or sixteenths of whatever to the point you see odd hybrid situations - such as talking of '3.2 inches' or '5.6 pounds'. 
;Fahrenheit degrees are smaller, so they offer greater precision than Celsius!
:Not even wrong, as this logic suggests that kilometres and centimetres are superior because 'they offer greater precision' with length measurements than miles and inches but there's a pretty high chance our Fahrenheit-lover wouldn't accept that. In practice, using a decimal point where appropriate solves all these apparent problems, so this is really a non-issue. Also, all modern medical thermometers are graduated in tenths of Celsius, which offers adequate precision. Lastly, there's not many 'everyday' situations where a difference between say, 90°F and 91°F is actually that important, and in most things where it is (like science and engineering) we find as a rule metric is already in general use.
;Fahrenheit is more practical for everyday use than Celsius! 100 °F gives a nice cutoff for really hot weather, and 0 °F for very cold weather!
:Appeal to stupidity/laziness. That you're either incapable or unwilling to learn and remember that (say) '10°C' is 'don't forget your coat' territory while '35°C' is the stage where you do need to seriously consider the risk of heatstroke. In this respect, Celsius is no more 'practical' than Fahrenheit but it's no less practical either - it's merely the way you've been taught to think.
:Furthermore, speaking of everyday life, one could easily argue that 0 °C is a better cutoff than 32 °F, since all temperatures below freezing are noted as negative values.
;Changing everything in the country would be too expensive!
:Sunk cost fallacy; there's costs in not adopting the metric system (which are eternal) while the cost of metrication is a one-off spend. What's more, a large percentage of items can undergo 'soft metrification'; that the customary measures/sizes can continue to be made but they are simply measured differently - for example, 'two pound' bags are marked as 909g and 'foot-longs' have the official length of 30.5cm. What's more, these days you'd be surprised how much 'stuff' is already metric. Lastly, if planned well and given sufficient transition-time, the change-over can be done relatively cleanly and without excessive cost. The only real defence here is if converting is allowed to remain entirely voluntary; because if it isn't enforced, sectors which don't see an obvious benefit from converting won't bother doing so (creating an uneven situation, rather like the one seen in the United States right now).
;Businesses will use the conversion to sneak in price rises!
:A credible risk. However, this can be made less likely by enforcing per-volume pricing in stores, having a decent transition period where people get to understand that '$5.99/kg' is '$2.72/lb' and publicly shaming companies who jack up prices.
;But we already DO use the metric system in science and engineering! Isn't that good enough?
:Which also means that millions of American children each year are required to learn two systems (and how to convert between them) for no obvious benefit. That's teaching-time and effort wasted. More importantly, more and more realms of 'ordinary' American life is 'in metric' - such as anyone who has served in the military, works in business with foreign suppliers or simply shops overseas (a much bigger chance in a world of Amazon and Temu) - which means a decent working knowledge of metric isn't the preserve for a few ivory tower PhDs anymore.
;Metric is better for scientific use, but English units are better calibrated for common use! For example, the gallon has the size it does because that is a good size for laypeople to measure volumes with, and a foot has roughly the size of a human foot!
:Appeal to familiarity; 'people are more aware of a gallon than a litre so can mentally visualise it better'. Clearly, this will work the other way when you're reared in a country where litres are used in normal activities and so you know '2l' is the same capacity as a large bottle of soda and '750ml' is a standard wine bottle. It's simple enough; if it's used, folks get to know it and judge accordingly. There's nothing stopping you using say, parts of your body for metric guestimate measures - that your thumb-little finger gap is about 15cm, your thumb width is 2cm, an 'arm-length' around 60cm etc Speaking of which, if you're so determined to retain the foot, why aren't you also using hands too?.
:The only real merit in this argument is the one which George Orwell pointed out way back in 1947 - that 'the metric system does not possess, or has not succeeded in establishing, a large number of units that can be visualised''As I Please' in Tribune, 14 March 1947 Telelib.. After all, there is nothing for liquids apart from millilitres and litres, while the older systems have gallons, pints, gills, tuns and so on. However, there is the relatively simple solution of metrification of existing units - such as the 'metric ton' (1,000kg), the German 'customs pound' (500g) and in the USA, the '(metric) Fifth' of alcohol (750ml). There wouldn't be much issue making a metric 'furlong' (200m) or 'cental' (50kg) - or even the coining of a couple of new ones to aid visualisation, such as the Hectare.
;What about kitchen measurements? Replacing cups and spoonfuls with, say, 75 milliliters? That doesn't have the right feel to it at all!
:Appeal to ignorance. A standard US 'teaspoon' is 4.9ml, 'tablespoon' is 14.8ml and 'cup' is 240ml. In places like Canada and Britain these has been metrified to 5ml, 15ml and 250ml respectively. Your cooking doesn't need that precise measures where an increase of ratios less than 4% is noticeable. Metric measuring spoons existIn fact you should own some measuring spoons period because most normal spoons aren't really 4.9ml/14.8ml in the first place. and 250ml is a quarter-litre which would show on any remotely decent jug. Plus, think of all those metric cookbooks you could now enjoy without having to sit down and laboriously convert all of the numbers back into 'English' measuresIncluding ironically, cookbooks from England..
;Drugs are measured in metric, especially cocaine and other "hard drugs"!
:Poisoning the well. Besides the fact that this claim is partly untrue (as ounces can/are used also, such as with marijuana in the UK) and we could also cite 5.56mm and 9mm firearms ammunition (which makes .50 cal rounds good because it's measured in inches). If most drugs are sold in metric these days, it's most likely because the drugs trade is global and all global businesses have gone metric.
;Metric Units are a communist atheist plot cooked up by some proto-communist during the French Revolution, just like the Guillotine and We prefer American units!
:What are these 'American' units you speak of? You use English units, given to you by your then colonial master which you were then unpleasant to later on. Pedanticness aside, while the metric system was invented in France during the Revolutionary Era, so was the 'Declaration of the Rights of Man and the Citizen', so this guilt by association is a bit stupid. Perhaps the only real evidence to 'Commies liking the metric system' was the fact the Soviet Union fully adopted the metric system in 1925, sweeping away all the traditional Russian units which were also as confusing and impractical as the English one.
==Metric time==
When the French revolutionaries brought in the new metric system, they also brought in the French Republican Calendar, which decimalized time with uniformly thirty-day months (five or six days being added at the end of each year to keep the seasons in step), ten-day weeks, ten-hour days,Working eight hours a day all of a sudden sounds a lot longer! 100-minute hours, and 100-second minutes. And catchy names like Brumaire and Thermidor, which we can only hope made some sense in French. This did not prove terribly popular and was abolished less than 15 years after it was brought in.
Nearly two centuries later, Saturday Night Live parodied it in their "Metric day" sketch.
Ironically, using a decimal system for the "arbitrary" increments of the day (as noted above), the "new second" would be within 20% of the "old second". So counting off would translate to "one nine hundred, two nine hundred, three nine hundred..." See how easy this would be? It's effectively just a way of stating as a decimal fraction the proportion of the day that has elapsed.
There are apps such as LogiClock which can display decimal time.
Strictly speaking, SI time is already a decimal standard: like any other SI unit, the second can be used with decimal prefixes. Whereas this is common with the reducing prefixes (milli-, micro-, nano-, etc.), the magnifying prefixes (kilo-, mega-, giga-, etc.) are not widely used in this way, because they don't align neatly with days, weeks, months, and years.A typical human lifespan, for example, is 2-3 Gs. Given that the way humans typically experience time is cyclical (unlike any other physical dimension, such as length, mass, or temperature), time is a rather unusual dimension. Moreover, any system of timekeeping that was decimalised on Earth would make no practical sense to humans living in deep-space habitats, or on colonies on other planets, which would have very different time cycles. So there is relatively little reason to adjust the SI – all units derived from the second, such as the joule, newton, and pascal – such that there are 100,000 seconds in a day.
China had decimal time before 1912, and they considered adopting the 12 hour clock as a retrograde step.
==Problems caused by having two systems==
It is often claimed that, while the metric system offers important advantages in science, there is no real harm to retaining the old units and using them in parallel with SI. However, there are many ways in which this "dual-units" approach can cause problems which are just as severe as those caused by the lack of metrication, leading some metric advocates to adopt a position of "don't duel with dual":http://www.metricationmatters.com/docs/WhatIsMetrication.pdf
*The incident of 1983 (not related to ) was the result of improper conversions that left a plane with less than half of its needed fuel for a trip. Incredible piloting prevented any deaths.
*In 1999, NASA's was lost due to unit mismatch in software: the output of the program that kept track of small thruster firings while the probe was en route reported these impulses in pound-force-seconds, but the program that used this data to update the spacecraft's known position expected the numbers to be in newton-seconds. A pound-force is more than 4 N, so the probe ended up being far closer to the Martian surface on its first orbit than it was supposed to, slicing into the upper Martian atmosphere extremely fast and burning up.
* There are frequent problems when doses of medicine are measured in non-metric units such as drams or teaspoons (the latter of which is readily confused with the tablespoon, a factor of three or four larger). Such confusions have repeatedly led to fatal overdoses, meaning that medicine has increasingly standardized on metric units, requiring all devices for measuring liquid medicines to display millilitres only, and asking parents to dispense medicines to their children using millilitres only.http://www.watfordobserver.co.uk/news/11845157.Looking_back__March_28__1969/https://www.pharmacist.com/health-care-providers-should-use-cups-measure-ml-0#main-contenthttp://www.thewesterlysun.com/opinion/columnguest/7015862-129/dr-maggie-kozel-dosing-advice-for-todays-parents.html
* Dose errors can also be caused by confusion regarding patient weight, since medicines are commonly prescribed in a quantity which is proportional to the body mass of the patient (e.g. milligrams of drug per kilogram of patient body mass); notably, no medication is commonly prescribed or described in terms of imperial units. In the US, confusion between pounds and kilograms could cause a dose to be wrong by a factor of 2.2, and in the UK, incorrect dosage due to imperial weighing in the past (e.g. a patient weight recorded as 13 stone 7 pounds and misread as 137 kg, which is about 60 % more) has forced standardization on metric measures for weighing patients, with imperial measurements being explicitly forbidden.http://metricviews.org.uk/2009/12/nhs-risking-patients-lives-with-imperial-scales/ This practice also allows ready calculation of the patient body mass index, which is the patient's body mass (kg) divided by the square of their height (m2).
* In the UK, due to the confusing situation of fuel being sold exclusively by the litre (which is a legal requirement) but driving distances typically still being measured in miles (due to inaction by the Department for Transport), there is no truly useful measurement of fuel economy. Neither miles per imperial gallon (since fuel has not been sold by gallons for decades) nor litres per 100 kilometres (since road signs and associated measurements are in miles and yards) is compatible with these conflicting units for fuel volume and road distance.
* Incompatibility between US Customary and metric units might have contributed to the failure of the US company Target's attempt to expand into Canada.http://www.zdnet.com/article/billion-dollar-failures-how-bad-decisions-and-poor-it-killed-target-canada/ It has also been suggested that similar incompatibilities have caused problems for Boeing when outsourcing work to other countries; their competitors (such as Airbus) have not faced such problems.http://agmetalminer.com/2011/08/16/not-so-fast-comac-c919-is-doa-but-boeing-and-airbus-duopoly-dead-anyway/ This point relates to the main reason for the push for metrication in the 1970s: the economic benefit to everyone of the world sharing a single unambiguous language of measurement.
* The education of children is hampered by their having to use two incompatible sets of units in their studies (as in the USA) or the conflict they observe between the units they use exclusively in school, and the older units that are still retained in the outside world (as in the UK). One of the main motivations for metrication was to prevent children from having to spend the enormous amounts of classroom time required to learn the extensive conversion factors necessary to use imperial and US customary units correctly, but until metrication is complete, this cannot truly happen, nor can children be free from the need to learn the clumsy conversion factors between imperial and metric units.
* Even the most trivial calculations are made unnecessarily difficult by imperial units, impeding people who use imperial from gaining an intuitive understanding of many measurements, e.g. how they scale up. The smooth and intuitive scaling of metric units does not pose such problems: if one knows what a drive down a 500-metre section of highway feels like, then one can easily reason that a 60-kilometre drive comprises 120 such sections; the same comparison between 500 yards and 60 miles is not so easily made. Likewise with innumerable scalings and comparisons of everyday measurements – if a steak is weighed in ounces, a bicycle in pounds, a man in stones, and a car in long tons and hundredweights, how does one get an intuitive feel of the ratios of their weights? To speak in an aphorism, one might say that the essence of stupidity is to make trivial things difficult, while the essence of intellect is to make difficult things trivial.
* Refusing to convert to the metric system makes travel more difficult for Americans. Americans unfamiliar with the metric system traveling outside of their country have to convert all distances, weights, measures, and temperatures to their system to get a feel for what the local figures mean. So it is a good idea for Americans to learn the metric system, even if they have no intention of using it at home.
* The current infant formula shortage in the domestic American infant formula market. The situation could have been eased or avoided completely simply by importing formula liquids and powders from any other nation; however, due to American ignorance and intransigence on the metric system and restrictive FPLA (Fair Packaging & Label Act of 1967) regulations requiring "dual units" on American packaging, an emergency session of congress was necessary to allow importation of metric-only labeled formulas to the United States. Furthermore, there was the added expense of supplying each formula with a conversion chart because Americans don't know a milliliter from a millipede, and they're quite proud of their insularity and illiteracy. 
==Specific impulse==
In SI, impulse (the integral of a force of a given magnitude applied over a given time, equivalent to the change in momentum due to the force) is measured in newton-seconds. An important measure of the power of a rocket fuel is its specific impulse — meaning the amount of impulse the fuel delivers per unit mass, or newton-seconds per kilogram.
In the imperial system, this measure becomes confused because the same word pound is used to describe both force and mass. So specific impulse, in that system, is "pound-seconds per pound" and is quoted as simply "seconds". This measure is so ingrained in rocketry that specific impulse is still quoted in seconds even though nobody in the rocketry field now uses imperial measurement.
The dimensions of specific impulse are, of course, not simply T but LT-1, same as velocity.
==How the metric system can be poorly implemented in real life==
Despite its simplicity, in the real world, the common implementation of the metric system can be problematic. Nature is of course indifferent to human conventions.
* Common use of non-decimal minutes, hours, and days for measuring time (but at least the hours are counted to 24, avoiding the AM/PM disaster).
** Related, the common use of km/h for measuring speed, which must be divided by 3.6 to get the SI speed in m/s.
** Electricity usage is usually billed in kilowatt-hours instead of joules (1 kWh=3.6 MJ). 
** Battery capacity (especially in mobile phones) is measured in milliampere-hours instead of the SI unit of electric charge coulombs (1 mAh=3.6 C).
* Fairly common use of "metric horsepower" or pferdestärke (instead of kilowatts) for measuring power of automotive engines. 
* While a proper metric system for shoe size exists, Mondopoint, it is really only used for ski boots and in NATO. The base unit for the most common shoe size system in Europe is the Paris point (two thirds of a centimetre), no less. Still way better than imperial shoe sizes, which are based on the barleycorn, with different offsets for male/female/child/UK/US shoes to make it more interesting.
* Clothes sizes cause problems to both metric and imperial, with most male sizes letter based and most female sizes two or three numbers smaller for the same actual size than a decade or two ago. This is why — naturally — online clothes retailers ask you to measure yourself and give the centimetre value behind sizes used by certain brands.
* Distances at sea and air are still measured as nautical miles (= 1852 m exactly) and speeds as knots (nautical miles per hour). At inland waterways, the CEVNI rules use kilometres and km/h, though. For conversion purposes, 1 m/s is very nearly 2 kn.
* Angles are likewise measured in degrees, while the absolute unit of angle, radian, has very few practical applications. (Milliradian, its subdivision, is extremely handy on measuring skinny triangles, such as in surveying or acting as a sniper. All non-American scope sights are equipped with milliradian scales.)
* It's much simpler to use Gaussian units rather than SI ones in electrodynamics.
===Common phrases===
As these examples show, the metric system makes expressing simple statements complicated:
*I'd walk 1.609 km for a Camel
*(Sings) "2.54 centimetre worm, 2.54 centimetre worm, measuring the marigold..." 
**"I'd walk 1,609,344 km/For one of your smiles..."
**"I would walk 804.674 km/And I would walk 804.674 more/Just to be the man who walked 1609.348 km to fall down at your door."
*28.35 g of prevention is worth 0.453 kg of cure – except if you're talking Troy weights, in which case 31.1 g of prevention is worth 0.373 kg of cure
*0.473 L is 0.454 kg the world around (Er, well actually only in the USA)
*Give them 2.54 cm and they'll take 1609.344 metres!
*96,561 Kilometres Under the Sea by Jules VerneThe original French title is Vingt mil lieues sous les mers: French lieue is exactly 4 km, making the original journey 80,000 km; twice the circumference of Earth.
*How many teaspoons in a cup, again? (50) (48 in the U.S.)
*"But I have promises to keep, and multiples of 1.609 km to go before I sleep" - Robert Frost, "Stopping by Woods on a Snowy Evening" 
*The longest word in the English language is "s1.609kms," because there are 1.609 km between the first and last letters
*Texans can now wear 37.8-litre hats!
*
*"The 453.592 grams of flesh, which I demand of him..." - Shakespeare, The Merchant of Venice
==External links==
* are a set of alternative systems of measurements used in physics based on fundamental natural constants.
*It's time for the US to use the metric system, Vox
*A guide to metric units from our lord and savior xkcd
*Are Imperial Measurements Outdated? – Number Hub with Matt Parker (Youtube)
*A fun take on measuring systems.
==Notes==
==References==
Category:Mathematics
Category:Moral panics
Category:Science