Use a calculator.

Specifically: use a units-aware calculator.

I've been doing a lot of calculations based on energy and related terms, and am finding GNU Units (and yes, distinctly specified as an alternative to, say, BSD Units, which has a small subset of features) a phenomenally useful tool.

It turns out that even SI units get confusing and non-base-ten-rational pretty quickly -- pretty much the moment you go from watts to watt-hours (and yes, there's an alternate unit of energy: Joules, but watt-hours turn out to be a really useful concept).

It turns out that units-based measurements are complicated, and keeping tabs manually is a PITA, and using a units-aware calculator is all kinds of awesome. I wrote a little paen to it a couple of years ago here: https://www.reddit.com/r/dredmorbius/comments/1x9u0f/gnu_uni...

What's powerful is that GNU Units doesn't just do straightforward converstions (e.g., what's 180 cm in inches), but can do calcuations of multiple quantities: energy per unit area times area to give energy for a given size of land, etc. Since you can specify the units of the result, there's also a built-in sanity check that your units correspond (a common source of error in physics calculations).

Where GNU Units really shines is in being extensible. You can define constants, values, or units that you want to have readily available. I've plugged in "hiroshima" (the equivalent energy content of the ~15 kt atomic bomb dropped over Hiroshima, Japan), as well as values for the land areas of earth, the various continents, and several nations and states, as well as other area and other metrics for planets and moons within the Solar System. This makes a few relative comparisons fairly straightforward.

I'm aware that metric is "more logical" by certain ways of thinking, but old-school units had their own logic:

Units of length were based off of body parts or could be rapidly assessed. An inch is roughly a thumb's width. A foot, obvious. A yard is roughly a man's pace. A cubit, the length from elbow to fingertips. A fathom, that width of a man's arms, outstretched, as you'd find them whilst measuring out a sounding line shipboard. An acre is the amount of land a man could plough in a day (the German for this recognises the origin explicitly: tagwerk, or "days work"). An acre-foot seems silly until you realise that knowing the area of a reservoir, you can calculate its capacity quickly by simply noting water depth in feet. A british thermal unit is handy if you're sourcing coal for a steam engine -- how much water are you going to heat and boil each day? A mile ("mille") was one thousand double paces, from a slightly shorter-than-typical-today Roman soldier. A furlong was, literally, "one furrow long".

Volume measures: "bushel" derives from "bosta", a handful. Cup is obvious, from a typical drinking vessel. "Gallon" originates from "galleta", latin for bucket or pail.

Measures of weight are ancient -- "pound" comes nearly directly from latin, "libra pondo", with the same meaning. Ton is derived from volume -- the weight of a tun cask. Grain is another obvious one.

What these have in common is that for a quick and dirty in-the-field (often literally) assessment, these measures offered an immediate and sensible-in-context quantification. Measuring land area in acres gave a sense of how much work was required for that land -- how much one man might farm in a year, or how many men and horses you'd need to work it. Egyptian stonemasons could measure out their work by arm, sailors water depths, soldiers distance, farmers grain, hydraulic engineers stored water, without any significant calculations. There's something to be said for that.

I'm not saying the units make more sense now. I am saying that failing to recognise the logic by which the units were derived misses some highly salient points.