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How to Calculate Blood Alcohol Concentration (BAC)

This guide explains the math behind BAC estimates so you can better understand what online calculators can and cannot tell you.

What does BAC measure?

Blood alcohol concentration (BAC) is usually expressed as a percentage that represents grams of alcohol per 100 mL of blood. For example, a BAC of 0.08% means 0.08 grams of alcohol per 100 mL of blood.

The basic BAC estimation formula

Many online calculators use a Widmark-style formula. It estimates BAC by dividing the amount of pure alcohol consumed by an estimate of the water in your body, then subtracting an hourly elimination rate for the time since your first drink.

Step-by-step example

A simple example might assume a certain number of drinks, each with a defined volume and ABV, convert that into grams of pure alcohol, divide by an estimated distribution factor based on body weight and sex, and then subtract a fixed amount per hour since drinking began.

Why these calculations are only estimates

Real human bodies do not follow a single formula. Metabolism varies with genetics, liver health, food, medications, hormones, stress, and more. Even with perfect inputs, Widmark-style formulas can only produce an approximate BAC. If you want to know when this BAC will drop to zero, try our Time to Zero Calculator.

Questions about BAC formulas

Different calculators may use slightly different assumptions about body water, elimination rate, or standard drink sizes. Even with the same formula, rounding differences and input options can produce different results.
No. Law enforcement and medical settings typically use calibrated breathalyzers or blood tests that directly measure alcohol, not formulas based on self-reported drinking.
No. Knowing the math does not remove the uncertainty in your real BAC or the risk of impairment. The safest approach is still to avoid driving after drinking.

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