Tutorial on Calculating Blood Alcohol Levels

Alcohol Concentration

    Blood Alcohol Level is expressed in terms of concentration, i.e., the amount of alcohol in a certain amount of blood.  Because alcohol is dissolved evenly throughout most of your body water, and blood is mostly water, the concentration in your body will be the same as the concentration in your blood and vice versa. Thus, if you can figure out the concentration in body water, that should be the same as the concentration in the blood.

    What this means is that if you know the amount of alcohol consumed, and you know that weight of your body, you can divide one into the other to find the alcohol concentration in the body in general which is the same as the concentration in the blood:

BAL= alcohol consumed/body weight

Of course, things are seldom that simple. There are always complications that have to be accommodated. Here are a few:
 

Unit of measurement

You can express this concentration in terms of the units you use to measure alcohol and body weight. For example, you could measure alcohol
in terms of beers,  and body weight in pounds. So if you drank 6 beers and weighed 200 pounds, your BAL would be:

6 beers/200 pounds  = .003 beers/pound

Clearly, beers/pound is not a useful measure. Alcohol is consumed in many beverages other than beer, the size of a beer may be different and the concentration of alcohol in different beers may be different. Therefore it makes sense to express alcohol in terms of actual alcohol consumed (the table below will help). This is always done in terms in terms of mg (milligrams) of alcohol because the metric system is used universally in science. For this reason as well, body weight is never expressed in terms of pounds. In fact, the unit used is mls (milliliters). It is common to see body size expressed in 100 ml units. Thus BAL is often expressed as mg of alcohol in 100 ml of blood

BAL= mg alcohol/100 ml blood

Body composition

    Your body can be thought of as being made up of two different materials, water and fat. Alcohol dissolves mostly in water and so, any alcohol you drink will end up in your body water, not your body fat. If your body weighs 100 lbs and a great deal of that is fat, you will have a small amount of body water. Because alcohol ends up in body water, it will reach a high concentration. On the other hand, if you have a small amount of body fat, you will have a lot of body water, the alcohol will be more diluted and not reach as high a concentration.

    Therefore, in order to calculate BAL, you must estimate how much of your body is water. This can be easy or it can be complicated, depending on how accurate you want to be. There are several factors that can be used to predict body water, but by far the most important one is sex, and for practical, purposes, that's all you have to know. Fortunately, this does not present a challenge for most people.

    Early attempts to calculate BAL were done by Widmark, and his estimates of body water were known as the Widmark constant. For men, the Widmark constant is usually about .75 (75% of the body weight of an average man is water), and for women ut is .66 (66% of the body of an average woman is water).

    Thus, when calculating BAL, the amount of alcohol consumed is divided by body mass times the Widmark constant. A different equation applies to men and to women because each had a different Widmark constant.

BAL (men) = alcohol/ 75% of body weight
BAL (women) = alcohol/ 66% of body weight.
 
    There are other factors that can determine the percent of body water. A 150 pound person who is 6 feet high will have less body fat than a 150 pound person who is 4 feet high, thus height can make a difference. In addition, males tend to have a higher percent of body fat as they get older (this happens to women too, but not to the same extent). It is possible to make a better estimate of body water when in addition to sex, you know height, and age in the case of males. But for most people, simply knowing the sex and applying the Widmark constant will give a good estimate. The increase in accuracy achieved by using additional information is useful for people who are at the extremes of the population, i.e., people who are very tall or very short.
 

Rate of Metabolism

    All the calculations above assume that all the alcohol you drink is absorbed and distributed around the body instantly as soon as you drink it. This, of course, is not true. It takes some time for the alcohol to get out of the stomach and distribute itself throughout all of your body. This means it takes a period of time after you start drinking for alcohol to reach that the BAL you calculate. The amount of time depends on a number of factors. These include whether you have recently eaten and the type of beverage you drink, but we will ignore this for now and assume that total absorption takes 45 minutes from the time you drink.

Now throughout that 45 minutes, the alcohol you drank is being destroyed by an enzyme called alcohol dehydrogenase. In 45 minutes, a significant amount of alcohol is destroyed. This should be taken into account when predicting BAL. To take this into consideration, you need to know the rate that the body can get rid of the alcohol.

The equation now is:

BAL (at x minutes after drinking) = BAL minus the amount of alcohol destroyed in x minutes
 
  So, how fast does the alcohol dehydorgenase get rid of the alcohol? That can vary between individuals, but for most people, it is between 10 and 20 ml of alcohol per 100 ml blood per hour. For practical purposes you can assume it to be 15.
 

Summary

1. Determine the amount of alcohol you consumed in mls. As a guide, you can use this table:

 
BEVERAGE
Alcohol content
Percent (volume)
Alcohol content
in mgs
Spirits (1 fl oz)
 
 
(100 proof)
50
11,200
(89 proof)
43
9,600
(80 proof)
40
8,900
Beer (12 fl oz.)
5
13,500
Wine (2.5 fl oz)
12
8,400
 Example You drink 3 glasses of wine. According to the table, that is a total of 25,200 mls of alcohol (8,400X3 = 25,200)
 

    2. Calculate your total body water in Kg.    One  kg is 2.2 lbs, so divide your weight in lbs by 2.2 and multiply by the Widmark constant (.75 for males and .66 for females).
 

Example You are a 200 lb male. Convert to lbs to Kg by dividing by 2.2 (200/2.2 = 90.9kg), 200 lbs = 90.9 Kg. Then determine the body water by multiplying the Widmark constant (90.9X.75 = 68.2 kg). Because l litre of water weighs 1 kg, this means that your total body water is  68.2 litres.
 

    3. Calculate the concentration of alcohol in the  blood. Do this by dividing the alcohol by the number of 100 ml units of body water in the body. There are ten 100 ml units in 1 litre of body water.
 

Example   Three glasses of wine in a 200 lb male would produce a concentration of  369.5 mg alcohol per litre (25,200/68.2  =  369.5). Now, this number will have to be converted to the more conventional unit of ml "alcohol /100 ml of blood". This is done by dividing by 10. So the BAL that 3 glasses of wine can produce in a 200 lb male is (369.5/10 = 36.9) 36.9 mg alcohol/100 ml blood.
 

    4. Account for metabolism. Do this by subtracting 15 from the BAL for every hour that has passed since drinking began.

Example At one hour, this would be (36.9-15 = 21.95) 21.95.
 
 The Very Easy Method

    Just click here and go to the BAL Calculator

This page is http://dogsbody.psych.mun.ca/Tools/BAL/
W. A. McKim, 1998.