How Do Online BAC Calculators Work?

Most online Blood Alcohol Content (BAC) calculators require a person to enter four data points: weight, gender, the number of drinks consumed, and the amount of time they’ve been drinking. Once the data is submitted, a formula is used to calculate the person’s estimated BAC.

Many calculators employ a formula provided by the National Highway Traffic Safety Administration, which takes into account how alcohol is absorbed by the body and metabolized in the liver.

While online BAC calculators provide an approximation of a person’s BAC, it’s important to understand the number is just an estimate; the calculators should not be used it to decide whether or not it’s safe to drive.

To prove it, take a look following assumptions used in most online BAC calculators:

  • The majority of BAC calculators have predefined values for both alcohol serving size and alcohol content. For instance, a beer is 12 oz and 4.5% alcohol content, a glass of wine is 4 oz and 15% alcohol content and a shot is 1.5 oz and 40% alcohol content. If you're consuming a drink with a larger serving size and/or one that’s stronger than average, the calculator will underestimate your BAC.
  • BAC calculators do not take into account the amount of food in your stomach. A person’s BAC rises faster when they consume alcohol on an empty stomach since there’s no food to dilute the alcohol.
  • BAC calculators assume your alcohol consumption is consistent over a specified period of time. For instance, when you drink two drinks in two hours, the calculator assumes you have consumed one drink per hour. If drinks are consumed more quickly, your estimated BAC will be miscalculated.
  • BAC calculators use a formula based on average values – the average amount of water in the body and in the blood, and the average metabolism rate. Individual factors that affect these values and are not calculated in a BAC estimate.
On the other hand, a breathalyzer will provide a much more accurate estimate of a person’s BAC because they provide values based on a person’s deep lung breath sample, rather than making assumptions.