Blood alcohol content
Blood alcohol content (BAC) or blood alcohol concentration is the concentration of alcohol in blood. It is usually measured as mass per volume. For example, a BAC of 0.02% means 0.02 grams of alcohol per 100 grams of individual's blood, or 0.2 grams of alcohol per 1000 grams of blood.
Blood alcohol concentration is measured in so many different units that it can be extremely confusing. The following formula may be of some help.
Blood alcohol levels
1 g/kg = 1 g kg-1 = 1 g/L = 100mg/dL = 1 mg/cc = 100 mg%  = 1 decigrams% = 0.1 g% = 0.1%
In many countries, the BAC is measured and reported as grams of alcohol per 1000 millilitres (1 litre) of blood (g/1000 mL). Because the specific gravity of blood is very close to the specific gravity of water (its main component), the numerical values for BAC (%, percent) and (g/1000 mL, permille) do not differ to any consequential degree other than the placement of the decimal point.
In the UK, BAC is reported as milligrams of alcohol per 100 millilitres of blood. For example, a BAC of 0.08% is legally given as a limit of 80 mg per 100 ml . It is also reported in grams per Litre, which is an equivalent measurement .
The number of drinks consumed is a very poor measure of intoxication largely because of variation in physiology and individual alcohol tolerance. However, it is generally accepted that the consumption from sober of two standard drinks (containing a total of 20 grams) of alcohol will increase the average person's BAC roughly 0.05% (a single standard drink consumed each hour after the first two will keep the BAC at approximately 0.05%), but there is much variation according to body weight, gender, and body fat percentage. Furthermore, neither BAC nor the number of drinks consumed are necessarily accurate indicators of the level of impairment. Tolerance to alcohol varies from one person to another, and can be affected by such factors as genetics, adaptation to chronic alcohol use, and synergistic effects of drugs.
Alcohol content in blood can be directly measured by a hospital laboratory. More commonly in law enforcement investigations, BAC is estimated from breath alcohol concentration (BrAC) measured with a machine commonly referred to as a Breathalyzer which is a genericized trademark.

# Effects at different levels
Unless a person has developed a high tolerance, a BAC rating of 0.20 represents very serious intoxication (most first-time drinkers would be unconscious by about 0.15), and 0.35 represents potentially fatal alcohol poisoning. 0.40 is the accepted LD50, or lethal dose for 50% of adult humans. For a long-time, heavy drinker, those numbers can at least double. In extreme cases, individuals have survived BAC ratings as high as 0.914.
To use a simple Blood Alcohol Content calculator from the Wisconsin Department of Public Safety, refer to the following link: Blood Alcohol Calculator

# Units of measurement
There are several different units in use around the world for defining blood alcohol concentration. Each is defined as either a mass of alcohol per volume of blood or a mass of alcohol per mass of blood (never a volume per volume). 1 milliliter of blood is approximately equivalent to 1 gram of blood, 1.06 grams to be exact. Because of this, units by volume are similar but not identical to units by mass.

# Legal limits
For purposes of law enforcement, BAC is used to define intoxication and provides a rough measure of impairment. Although degree of impairment may vary among individuals with the same BAC, it can be measured objectively and is therefore legally useful and difficult to contest in court. Most countries disallow operation of motor vehicles and heavy machinery above prescribed levels of BAC. Operation of boats and aircraft are also regulated.

## Limits by country (BAC: Blood Alcohol Content)
The alcohol level at which a person is considered to be legally impaired varies by country. The list below gives limits by country. These are typically BAC (blood alcohol content) limits for the operation of a vehicle.

## Limits by country (BrAC: Breath Alcohol Content)
In certain countries, alcohol limits are determined by the Breath Alcohol Content (BrAC), not to be confused with BAC.
- In Greece, the BrAC limit is 25 micrograms of alcohol per 1000 millilitres of breath. The limit in blood is 0,50 g/l).
- In The Netherlands and Finland, the BrAC limit is 220 micrograms of alcohol per litre of breath (g/l, colloquially known as "Ugl").

## Other limitation schemes
- For South Korea, the penalties for different BAC levels include
0.010.049 = No Penalty
0.050.09 = 100 days of license cancellation
0.10 = Cancellation of car license.
0.36 = Arrest
Getting caught driving while drunk 3 times in 5 years; or 2 times in 3 years results in arrest.
- 0.010.049 = No Penalty
- 0.050.09 = 100 days of license cancellation
- >0.10 = Cancellation of car license.
- >0.36 = Arrest
- Getting caught driving while drunk 3 times in 5 years; or 2 times in 3 years results in arrest.

# Test assumptions
Blood alcohol tests assume the individual being tested is average in various ways. For example, on average the ratio of BAC to breath alcohol content (the partition ratio) is 2100 to 1. In other words, there are 2100 parts of alcohol in the blood for every part in the breath. However, the actual ratio in any given individual can vary from 1300:1 to 3100:1, or even more widely. This ratio varies not only from person to person, but within one person from moment to moment. Thus a person with a true blood alcohol level of .08 but a partition ratio of 1700:1 at the time of testing would have a .10 reading on a Breathalyzer calibrated for the average 2100:1 ratio.
A similar assumption is made in urinalysis. When urine is analyzed for alcohol, the assumption is that there are 1.3 parts of alcohol in the urine for every 1 part in the blood, even though the actual ratio can vary greatly.
Breath alcohol testing further assumes that the test is post-absorptivethat is, that the absorption of alcohol in the subject's body is complete. If the subject is still actively absorbing alcohol, his body has not reached a state of equilibrium where the concentration of alcohol is uniform throughout the body. Most forensic alcohol experts reject test results during this period as the amounts of alcohol in the breath will not accurately reflect a true concentration in the blood.

# Metabolism and excretion
Alcohol is removed from the bloodstream by a combination of metabolism, excretion, and evaporation. The relative proportion disposed of in each way varies from person to person, but typically about 90 to 98% is metabolised, 1 to 3% is excreted in urine, and 1 to 5% evaporates through the breath. A very small proportion (less than 0.5%) is also excreted in the sweat, tears, etc. Excretion into urine typically begins after about 40 minutes, whereas metabolisation commences as soon as the alcohol is absorbed, and even before alcohol levels have risen in the brain. (In fact, in some males, alcohol dehydrogenase levels in the stomach are high enough that some metabolization occurs even before the alcohol is absorbed.)
Metabolism is mainly by the group of six enzymes collectively called alcohol dehydrogenase. These convert the ethanol into acetaldehyde (an intermediate that is actually more toxic than ethanol). The enzyme acetaldehyde dehydrogenase then converts the acetaldehyde into non-toxic acetyl-CoA.
Many physiologically active materials are removed from the bloodstream (whether by metabolism or excretion) at a rate proportional to the current concentration, so that they exhibit exponential decay with a characteristic halflife (see pharmacokinetics). This is not true for alcohol, however. Typical doses of alcohol actually saturate the enzymes' capacity, so that alcohol is removed from the bloodstream at an approximately constant rate. This rate varies considerably between individuals; experienced male drinkers with a high body mass may process up to 30 grams (38 mL) per hour, but a more typical figure is 10 grams (12.7 mL) per hour. Persons below the age of 25, women, persons of certain ethnicities, and persons with liver disease may process alcohol more slowly. Many East Asians (e.g. about half of Japanese) have impaired acetaldehyde dehydrogenase; this causes acetaldehyde levels to peak higher, producing more severe hangovers and other effects such as flushing and tachycardia. Conversely, members of certain ethnicities that traditionally did not brew alcoholic beverages have lower levels of alcohol dehydrogenases and thus "sober up" very slowly, but reach lower aldehyde concentrations and have milder hangovers. Rate of detoxification of alcohol can also be slowed by certain drugs which interfere with the action of alcohol dehydrogenases, notably aspirin, furfural (which may be found in fusel oil), fumes of certain solvents, many heavy metals, and some pyrazole compounds. Also suspected of having this effect are cimetidine (Tagamet), ranitidine (Zantac), and acetaminophen (Tylenol).
There are currently no known drugs or other ingestible agents which will accelerate alcohol metabolism. Alcohol ingestion can be slowed by ingesting alcohol on a full stomach. Alcohol in non-carbonated beverages is absorbed more slowly than alcohol in carbonated drinks.

## Retrograde extrapolation
Retrograde extrapolation is the mathematical process by which someone's blood alcohol concentration at the time of driving is estimated by projecting backwards from a later chemical test. This involves estimating the absorption and elimination of alcohol in the interim between driving and testing. The rate of elimination in the average person is commonly estimated at .015 to .020 percent per hour, although again this can vary from person to person and in a given person from one moment to another. Metabolism can be affected by numerous factors, including such things as body temperature, the type of alcoholic beverage consumed, and the amount and type of food consumed.
In an increasing number of states, laws have been enacted to facilitate this speculative task: the BAC at the time of driving is legally presumed to be the same as when later tested. There are usually time limits put on this presumption, commonly two or three hours, and the defendant is permitted to offer evidence to rebut this presumption.
Forward extrapolation can also be attempted. If the amount of alcohol consumed is known, along with such variables as the weight and sex of the subject and period and rate of consumption, the blood alcohol level can be estimated by extrapolating forward. Although subject to the same infirmities as retrograde extrapolationguessing based upon averages and unknown variablesthis can be relevant in estimating BAC when driving and/or corroborating or contradicting the results of a later chemical test.

# Blood alcohol content calculation
BAC can be roughly estimated using a mathematical approach. While a mathematical BAC estimation is not as accurate as a breathalyzer, it can be useful for calculating a BAC level that is not currently testable, or a level that may be present in the future. While there are several ways to calculate a BAC, one of the most effective ways is to simply measure the total amount of alcohol consumed divided by the total amount of water in the bodyeffectively giving the percent alcohol per volume water in the blood.
The total water weight of an individual can be calculated by multiplying their body weight by their percent water. For example, a 150 pound woman would have a total amount of water of 73.5 pounds (150 x .49). For easiest calculations, this weight should be in kilograms, which can be easily converted by dividing the total pounds by 2.205. 73.5 pounds of water is equivalent to 29.4 kilograms of water. 29.4 kilograms of water is equivalent to 29,400 mL of water (1 L of water weighs 1 kg, and 1 L = 1000 mL).
Gender plays an important role in the total amount of water that a person has. In general, men have a higher percent of water per pound (58%) than women (49%). This fact alone strongly contributes to the generalization that men require more alcohol than women to achieve the same BAC level. Additionally, men are, on average, heavier than women. The more water a person has, the more alcohol is required to achieve the same alcohol:blood ratio, or BAC level. Further, studies have shown that women's alcohol metabolism varies from that of men due to such biochemical factors as different levels of acetaldehyde dehydrogenase (the enzyme which breaks down alcohol) and the effects of oral contraceptives.