What is the Widmark formula?

A 1932 formula by E.M.P. Widmark relating alcohol consumed, body weight, a sex-based body-water distribution constant, and time elapsed to estimated blood alcohol content — still widely referenced in forensic toxicology, though modern estimates often use somewhat different constants.

Why does biological sex affect the calculation?

Widmark's r constant reflects average differences in body water percentage between men and women, since alcohol distributes through body water — women's typically lower body-water percentage means the same amount of alcohol is more concentrated.

How accurate is this estimate?

Not accurate enough to rely on for any real decision — real BAC can vary by 20-30% or more from this population-average estimate depending on food intake, hydration, medications, and individual genetics.

What counts as one 'standard drink'?

In the US, one standard drink is about 14 grams of pure alcohol — roughly a 12oz beer at 5% ABV, a 5oz glass of wine at 12% ABV, or 1.5oz of 80-proof spirits.

Is 0.08% the legal limit everywhere?

It's the typical US adult limit, but many US states use 0.04% for commercial drivers and 0.00-0.02% "zero tolerance" limits for drivers under 21; other countries set different, often lower, limits this tool doesn't look up.

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Health & Fitness

BAC Estimator

Rough blood alcohol content estimate via the Widmark formula — never a measurement of actual impairment.

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Last reviewed 2026-08-30

About the BAC Estimator

Estimates blood alcohol content (BAC) using the Widmark formula, a population-average model from body weight, biological sex, drinks consumed, and time elapsed — a rough illustration only, never a substitute for a real breathalyzer or blood test, and never a basis for deciding whether it's safe to drive.

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How to use it

Enter your body weight and biological sex (affects the Widmark body-water constant).
Enter the number of standard drinks consumed and hours since your first drink.
Read the estimated current and peak BAC, and time until BAC returns to zero or below the typical legal limit.

Formula

Peak BAC (%) = (alcohol grams × 100) ÷ (body weight in grams × Widmark r constant), where alcohol grams = standard drinks × 14g (the US standard drink). Current BAC = peak BAC − (0.015 %/hour × hours elapsed), floored at 0. Widmark r ≈ 0.68 for men, 0.55 for women (original 1932 Widmark constants, reproduced in forensic-toxicology references).

Worked example

A 175lb male having 3 standard drinks over 2 hours: an estimated peak BAC around 0.075%, declining to roughly 0.045% after the 2 hours elapsed — illustrative only, not a measurement.

US BAC Legal Limits by Driver Category

US BAC Legal Limits by Driver Category
Driver Category	Legal Limit
Standard adult driver (21+)	0.08%
Commercial driver	0.04%
Driver under 21 ("zero tolerance")	0.00-0.02%

Interpreting your result

This is a rough, population-average illustration using original 1932 Widmark formula constants — not a personal or legal measurement. Real BAC varies by roughly ±20-30% or more depending on food intake, hydration, medications, liver function, drink strength, and individual genetics; more recent forensic literature reports somewhat higher average r constants than Widmark's original figures. The formula also assumes alcohol is already fully absorbed and evenly distributed, ignoring absorption timing. Legal limits vary by jurisdiction (0.08% is the typical US adult limit; lower for commercial drivers and drivers under 21). Never use this number to decide whether it's safe to drive — the only safe choice after any drinking is not to drive.

Recommendations

• Food, hydration, and absorption timing are not modeled at all — a real BAC can differ substantially (in either direction) from this estimate for the same inputs.
• "Time until BAC ≈ 0" is not the same as "safe to drive" — impairment can persist even as calculated BAC approaches zero, and this tool cannot determine actual impairment for any individual.
• Legal limits differ by context — many US states use a stricter 0.04% limit for commercial drivers and 0.00-0.02% "zero tolerance" for drivers under 21.

Frequently asked questions

No — never. This is a rough population-average estimate, not a measurement of your actual BAC, and impairment doesn't disappear the instant a calculated number hits zero. If you've been drinking, don't drive.