Written by attorney David Ashley Haenel

How is blood alcohol level calculated?

Over one hundred years ago, a researcher by the name of Widmark developed the formula for calculating a person's blood alcohol level. This formula is helpful to those who want to know how much alcohol they can consume before their blood alcohol content tests above what is allowed within their state. Widmark's formula for computing blood alcohol levels should only be used as a guide because the results are not 100% accurate, as with a urine or blood test.

Here is Widmark's formula for calculating a person's blood alcohol level:

%BAC = (A x 5.14/W x r) - .015 x H

In order to understand how to solve the formula, it's important to first understand how it's broken down.

  • A = alcohol measured in liquid ounces (the total amount consumed by a person).
  • 5.14 = this number is the conversion of .823 x 100/16 (.823 converts the liquid ounces into weight ounces, and the 100 converts the final number into a percentage, and the number 16 converts pounds to ounces).
  • 0.15 = the average elimination rate for alcohol.
  • W = how much the person weighs in pounds.
  • r = this is the ratio for alcohol distribution, wherein a woman is .66 and a man is .73.
  • H = how many hours that have passed since alcohol is eliminated from the body; this is typically the number of hours between when the person stops drinking and the blood alcohol test is given.

The Formula is Not Fool Proof

Drivers need to be aware that this formula is well-known by law enforcement officials and it is not meant to be a foolproof test for avoiding a DUI. Widmark, though being famous in his line of work as a proven researcher, has made it clear that this formula is meant only as a guide for calculating a person's blood alcohol level. The best results that can be received regarding a person's blood alcohol level are achieved through chemical testing.

The Simplicity of Chemical Testing

Because alcohol is quickly absorbed into the blood stream, it is simple for law enforcement officials to conduct chemical testing in order to determine someone's blood alcohol level. The blood test only determines how much alcohol is found in the blood at the time of the test. It does not indicate how long the individual has been drinking, how many drinks they have had, or if there is a problem with alcohol present. The test does indicate, though, if a person has reached or gone over the legal limit if they are driving.

What Other Types of Testing Are Available?

Additional testing includes having a urine test or taking a breath test. A breath test is notorious for not being as reliable as blood or urine tests. A urine test also has a reputation for not being as reliable a blood alcohol test. When the DUI defense attorney examines the step by step processes used during blood alcohol testing, they can pick up on mistakes made and build a defense case. Even though a blood test is considered the most accurate, it still carries some level of risk like the others. However, there is a greater risk in refusing to take any of the tests law enforcement officials require of an individual. Penalties could be enforced, especially if this is the person's second or third refusal to take a test. Therefore, it's better to take a blood alcohol test when asked than to refuse.

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