What Is Blood Alcohol Content (BAC)?

What BAC actually measures

BAC expresses how much alcohol is circulating in your blood relative to the total volume of blood. In the United States it is written as a percentage, such as 0.05% or 0.08%. Alcohol is absorbed through the stomach and small intestine, enters the bloodstream, and is then broken down mostly by the liver at a fairly steady rate.

Because the liver clears alcohol slowly, drinking faster than your body can process raises BAC. Only time lowers it — coffee, cold showers, and food after drinking do not speed up alcohol elimination, though eating before or while drinking slows absorption.

The Widmark formula

The Widmark formula estimates BAC from the grams of alcohol consumed, divided by body weight multiplied by a distribution factor (r), which reflects how much of the body is water. This factor is roughly 0.68 for men and 0.55 for women on average, because body-water proportion differs. A subtraction is then applied for the alcohol already metabolized over time.

For example, a 154 lb person who drinks two standard drinks will reach a higher BAC than a 220 lb person drinking the same amount, because the alcohol is diluted in less body water. This is why the calculator asks for your weight and sex — but every result is an estimate, not a measurement.

Why legal limits vary by country

Driving limits are set by national law, not biology, so they differ widely. Many US states use 0.08%, while much of Europe uses 0.05% (0.5 g/L), and some countries apply zero-tolerance rules for new or professional drivers. A BAC that is legal in one place may be over the limit in another.

The safest choice is not to drive after drinking at all, since alcohol impairs judgment, reaction time, and coordination well before any legal limit is reached. Use a BAC estimate only for general awareness — never to decide whether you are fit to drive.

Frequently asked questions

How long does it take for BAC to reach zero?

On average the body clears roughly 0.015% BAC per hour, but this varies by person. If your BAC is 0.08%, it could take around five hours or more to reach zero — and nothing safely speeds this up except time.

Is a BAC estimate accurate?

No calculator can precisely measure your BAC. Estimates ignore factors like food, medications, hydration, and individual metabolism. Only a certified breathalyzer or blood test gives a reliable reading.

Does body weight affect BAC?

Yes. A heavier person generally reaches a lower BAC from the same number of drinks because alcohol is distributed through more body water. Sex and body composition also matter, which is why the formula uses a distribution factor.

What counts as one standard drink?

A US standard drink contains about 0.6 fl oz (14 grams) of pure alcohol — roughly a 12 fl oz beer, a 5 fl oz glass of wine, or a 1.5 fl oz shot of spirits. Definitions differ by country, so always check local guidance.