Blood Alcohol Content BAC Formula

If you were arrested for DUI and the police gave you a DUI breath test, they recorded your blood alcohol content, or BAC. In all states, the maximum BAC level is 0.08% if you are over 21. The BAC level for drivers under 21 can be as low as 0.00%, which means zero tolerance. Call 844-832-6384 at any hour and talk to a top DUI lawyer in your area about proven ways to win your DUI case!

Widmark’s basic formula for calculating a person’s estimated Blood Alcohol Content (BAC) at a particular time may be expressed as follows:

BAC Formula Widmark

% BAC = (A x 5.14 / W x r) – .015 x H

The Variables in Widmark’s Formula

A = liquid ounces of alcohol consumed
W = a person’s weight in pounds
r = a gender constant of alcohol distribution (.73 for men and .66 for women)*
H = hours elapsed since drinking commenced

Finding A in the BAC Formula Calculator

To find A in the Widmark formula above, you multiply the number of liquid ounces of alcoholic beverages consumed by the percentage of alcohol in the beverage. For example, a typical bottle of regular beer contains contains 12 ounces of liquid and is 5% alcohol, so it contains .60 liquid ounces of alcohol.

The Numbers in the Widmark Formula Calculator

The 5.14 appearing in the formula is a conversion factor of .823 x 100/16, wherein .823 is used to convert liquid ounces to ounces of weight, 100 is used to convert the final figure to a percentage, and 16 is used to convert pounds to ounces. The .015 figure appearing in the formula is the average alcohol elimination rate.

Examples

The use of the above formula can best be illustrated by employing it to calculate BAC in practical examples. In the examples below, the alcohol distribution ratios used are .73 for men and .66 for women. The alcohol content was determined from the Alcohol Content of Common Beverages list, and it is assumed that all beverages were consumed on an empty stomach. In using the BAC formula, it should be remembered that the body begins to eliminate alcohol immediately upon absorption and continues to eliminate it until it has been completely removed from the body.

BAC Formula Example 1:

A 170-pound man consumed seven 12-ounce bottles of regular beer between 8 PM and midnight. What was his BAC at 12:30 AM?

A = 7 beers x 12 oz. x 5% = 4.2 oz.
W = 170 lbs. H = 4.5 hours
BAC = (4.2 x 5.14/170 x .73) – .015 x 4.5 = .174 – .068 = .106 percent

BAC Formula Example 2:

A 115-pound woman consumed four 1 1/4-ounce shots of bourbon between 9 PM and 11 PM. What was her BAC at midnight?

A = 4 shots x 1.25 oz. x .40% = 2.0 oz.
W = 115 lbs. H = 3.0 hours
BAC = (2.0 x 5.14/115 x .66) – .015 x 3.0 = .135 – .045 = .09 percent

BAC Formula Example 3:

A 205-pound man consumed six 12-ounce bottles of light beer and three 5-ounce glasses of wine between 7 PM and 10 PM, and was found by a breath testing instrument to have a BAC of .16 at 11:15 PM. Is the test result consistent with the amount of alcohol consumed?

A = 6 beers x 12 oz. x 4% + 3 wines x 5 oz. x 12% = 2.88 + 1.80 = 4.68 oz.
W = 205 lbs. H = 4.25 hours
BAC = (4.68 x 5.14/205 x .73) – .015 x 4.25 = .161 – .064 = .097 percent

The test result is inconsistent with the amount of alcohol consumed. Therefore, if the amount of alcohol consumed and the times of consumption can be established by credible evidence, the BAC calculations may be used to discredit the reported chemical test result.

 BAC Calculator Example 4:

A 160-pound man who had consumed an unknown quantity of alcohol was involved in an automobile accident at 9 PM. After the accident, the man consumed 4 bottles of regular beer beginning at 9:30 PM. At 11 PM. He was arrested and was given a breath test that indicated a BAC of .14 percent. What was his BAC at the time of the accident?

In this example, the BAC caused by the alcohol consumed after the accident must be calculated and subtracted from the BAC indicated by the breath test. It is important to note that, in this calculation, the period of elimination (H) starts with the time of the accident (9:00 P.M.) rather than the time of commencement of post-accident drinking (9:30 P.M.). The reason for this is to account for the pre-accident alcohol that was eliminated between 9:00 P.M. and 9:30 P.M.

A (post-accident beers) = 4 beers x 12 oz. x 5% = 2.4 oz.

W = 160 lbs. H = .5 hours + 1.5 hours = 2.0 hours
BAC (from post accident beers) = (2.4 x 5.14/160 x .73) – .015 x 2.0 = .106 – .030 = .076 percent
BAC (at 9 PM) = .140 – .076 = .064 percent

In the above examples, it was assumed that all consumed alcohol was fully absorbed at the time for which the person’s BAC was calculated. This assumption may be valid in situations where the alcohol was consumed on an empty stomach and an hour or more had elapsed between the end of drinking and the time of calculation. However, if food was present in the stomach at the time of drinking, or if any other condition existed that would have slowed the rate of alcohol absorption, the assumption is almost certain to be invalid.

If absorption was incomplete at the time for which the calculation is made, the calculated BAC will be falsely high because the amount of alcohol absorbed into the bloodstream was less than the amount assumed in the calculation. In other words, the calculation can incorrectly assumes the presence in the bloodstream of unabsorbed alcohol.

For more information on the gender constant of alcohol distribution, read our page about Blood Alcohol Content (BAC) Calculation & Distribution.