BAC Calculator

For a 150 lb man with 3 standard drinks over 2 hours, the Widmark formula estimates a blood alcohol concentration (BAC) near 0.061% — calculated by dividing the grams of alcohol consumed by body weight and a distribution ratio, then subtracting alcohol eliminated over time. Individual results vary widely, and no calculator can tell you whether it is safe to drive.

Tip: “Copy with settings” shares a link that opens this calculator with your numbers already filled in.

How this calculator works

This calculator applies the classic Widmark formula, a century-old estimation method still used as a rough teaching and reference tool. It takes your sex, body weight, number of US standard drinks, and hours since you started drinking, and estimates blood alcohol concentration as a percentage.

The result is a mathematical estimate, not a measurement. Widmark's constants (the distribution ratio r and the elimination rate) are population averages from decades-old studies. Real BAC depends on far more than this formula captures: food in your stomach, how quickly you drank, medications, liver function, hydration, and genuine individual metabolic variation. Two people with identical inputs can have meaningfully different actual BAC.

This tool exists for education, not for deciding whether to drive, operate machinery, or do anything else that requires being unimpaired. Never drive after drinking, regardless of what any calculator shows. If there is any doubt, do not drive — call a taxi, a rideshare, or a sober friend instead.

How is BAC calculated?

BAC% = (alcohol grams ÷ (body weight in grams × r)) × 100 − (elimination rate × hours)Widmark r (distribution ratio, classic averages): 0.68 for men, 0.55 for womenElimination rate (classic average): about 0.015% per hourUS standard drink = 14 grams of pure alcohol

The r constant approximates the fraction of body weight that alcohol distributes into (roughly total body water); it is a population average, not a personal measurement, and the same is true of the elimination rate.

Worked example: 150 lb man, 3 drinks, 2 hours

Convert weight: 150 lb × 0.45359237 = 68.04 kg = 68,039 g.
Alcohol consumed: 3 standard drinks × 14 g = 42 g.
Raw BAC: 42 ÷ (68,039 × 0.68) × 100 = 42 ÷ 46,266 × 100 ≈ 0.0908%.
Subtract elimination over 2 hours: 0.0908% − (0.015 × 2) = 0.0908% − 0.03% ≈ 0.061% estimated BAC.

Frequently asked questions

How accurate is the Widmark formula?

It is a rough estimate, not a measurement. Studies comparing Widmark estimates to breath and blood tests show meaningful individual variation in both directions. It was designed as a forensic approximation, not a precision tool.

Can this tell me if I'm okay to drive?

No. Impairment does not begin at a fixed number, and this formula cannot account for your food intake, drinking speed, tolerance, medications, or fatigue. The only safe choice after drinking is not to drive.

What is the legal limit where I live?

Commonly used legal limits vary by country and, within the US, by state — this calculator does not display or apply any specific legal threshold. Check your local traffic authority for the limit that applies to you.

Why do men and women get different results for the same drinks and weight?

The classic Widmark r constants (0.68 for men, 0.55 for women) reflect average differences in body water distribution between the sexes in the original studies. They are population averages, not a statement about any individual.

Does the elimination rate ever go negative?

No — the calculator floors the result at 0%. Once enough time has passed that the formula's elimination term exceeds the alcohol term, it reports 0% rather than a negative number.