BAC Calculator
Roughly estimate blood alcohol content from drinks, body weight and time with the Widmark formula.
How it works
BAC% = alcohol(g) ÷ (r × body weight(g)) × 100 − 0.015 × hours (r = 0.68 men, 0.55 women)
The Widmark equation is the classic back-of-envelope estimate of blood alcohol. It spreads the alcohol you drank (about 14 g per standard drink) through the water in your body — captured by the factor r, which averages 0.68 for men and 0.55 for women — then subtracts roughly 0.015% per hour as your liver clears it. It’s an average model and ignores food, medications, drink strength and how fast you drank.
Worked example
A 170-lb man who has had 3 standard drinks over 2 hours: peak ≈ (42 g ÷ (0.68 × 77,111 g)) × 100 ≈ 0.080%, minus 0.015 × 2 for metabolism, leaving about 0.050%.
Frequently asked questions
How accurate is this?
It’s a rough average only. Real BAC swings with food in your stomach, hydration, medications, liver health, drink strength and timing — individuals can land well above or below the estimate. Never treat it as precise.
Can I use this to decide if I can drive?
No. Never use any calculator to decide whether to drive. Impairment begins before the legal limit, the estimate can be wrong in either direction, and the only safe amount before driving is none.
How long until my BAC reaches zero?
At roughly 0.015% cleared per hour, a peak of 0.08% takes over five hours to fully clear — and only time does it. Coffee, food, water and cold showers do not speed it up.
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This is a rough estimate only, not medical or legal advice. Never use it to decide whether it’s safe to drive — food, medications, health, how fast you drank and body differences all change real blood alcohol, and impairment can begin before any legal limit. If you’ve been drinking, do not drive.