How to Calculate an Approximate BAC (Blood Alcohol Concentration)

You will need

• Pencil and paper

OR

• A graphing calculator

Step 1

First is to just learn the basic formula and understand it. The equation is (150/body weight)(% alcohol/50)(ounces consumed)(0.025). Remember that you are multiplying; that's what the parenthesis are for!

Step 2

The first variable, body weight, is self-explanatory; insert the person's body weight! Then you can divide 150 by that body weight. Hold onto that number; you'll be needing it to plug into the equation.

This person weighs 125 pounds, so (150/125)(% alcohol/50)(ounces consumed)(0.025).

Step 3

The % alcohol is more complicated, though. Below is the information on a beer, a glass of wine, and a shot of hard liquor. You see the % of alcohol? Take that, but wait! You aren't done with it yet! You have to consider how many hours the person has been drinking. Use the chart below. Subtract the corresponding number from the % of alcohol (which you will keep as a whole number, not convert into a decimal). Then you can plug this number into the equation.

1 Beer - 12 ounces - 4% alcohol
1 Glass of Wine - 5 ounces - 12% alcohol
1 Shot of Hard Liquor - 1.25 ounces - 40% alcohol

Hours Since First Drink                     # to Subtract from Alcohol %
1                                                   .015
2                                                   .030
3                                                   .045
4                                                   .060
5                                                   .075
6                                                   .090

This person consumed two beers over three hours.

4% - .045 = 3.955

(150/120)(3.955/50)(ounces consumed)(0.025)

Step 4

The next variable, ounces consumed, is also fairly straightforward. Look at the chart below to see how many ounces are in a beer, a glass of wine, and a shot of hard liquor, on average (or use your own number). Don't forget that if the person had more than one, to multiply it by the number of drinks consumed.

1 Beer - 12 ounces - 4% alcohol
1 Glass of Wine - 5 ounces - 12% alcohol
1 Shot of Hard Liquor - 1.25 ounces - 40% alcohol

Remember how this person had two beers? The three hour time frame has nothing to do with any of the rest of the equation, so ignore it. Beers are 12 oz. apiece, and she had two, so 2 beers * 12 oz. = 24 oz.

(150/125)(3.955/50)(24)(0.025)

Step 5

Now that you've plugged in the numbers, all you have to do is solve the equation. First come 150/body weight and % alcohol/50. Divide those, and insert those numbers, then multiply straight across. This should give you a decimal. If it gives you a number that is unreasonably high, try doing the calculations over again; chances are, you hit something wrong!

(150/125)(3.955/50)(24)(0.025)
(1.2)(0.08)(24)(0.025)
(0.096)(24)(0.025)
(2.304)(0.025)
0.057 or 0.06

This means that the person's alcohol level is under the legal limit for driving. Although it would be better for them to find someone else to drive them home, they are not "driving drunk".

Tips and Warnings

• It's always best to have a designated driver! If you are just having a glass of wine at a restaurant, for example, try to wait until your body has metabolized it and you therefore have no more alcohol in your system (see above chart).