BAC Calculator
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About BAC
Blood alcohol concentration — the percentage of alcohol in your bloodstream — is what breathalyzers estimate, laws regulate, and impairment tracks. 0.08% is the driving limit in most US states (0.05% in Utah and much of the world), but impairment measurably begins around 0.02–0.05%, well under every legal line.
Enter your sex, weight, standard drinks, and hours since the first one for the textbook estimate with its working. One critical definition: a “standard drink” is 14 g of alcohol — a 12-oz regular beer, 5-oz wine, or 1.5-oz spirits — and generous pours, craft beers, and cocktails routinely count as 1.5–2 drinks each.
Alcohol also carries ~7 kcal per gram — see where it fits your day with the Calorie Calculator
The Widmark Formula
The century-old standard, still what most estimators run:
BAC% = (alcohol grams ÷ (weight grams × r)) × 100 − 0.015 × hours r (body-water constant): ~0.68 men · ~0.55 women Standard drink = 14 g alcohol (NIAAA)
Worked example: a 180-lb man, four standard drinks, two hours in: 56 g ÷ (81,600 × 0.68) × 100 − 0.03 = ~0.071% — under the 0.08 line yet measurably impaired, which is precisely the zone where people make bad driving decisions.
Drinks vs Estimated BAC
Widmark estimates two hours after the first drink (elimination included) — illustrative bodies, not yours:
| Standard drinks | Man, 180 lb | Woman, 140 lb |
|---|---|---|
| 1 | 0.00% | 0.01% |
| 2 | 0.02% | 0.05% |
| 3 | 0.05% | 0.09% |
| 4 | 0.07% | 0.13% |
| 5 | 0.10% | 0.17% |
| 6 | 0.12% | 0.21% |
Read the columns' gap: the same drinks put a smaller, lower-body-water drinker dramatically higher — one of several reasons drink-for-drink matching is a bad idea.
What Moves Real BAC Off the Estimate
Upward surprises: drinking fast (absorption outpaces the hourly elimination the formula assumes), empty stomach (food slows absorption substantially), carbonated mixers, higher-ABV pours than the “standard” assumes, and medications that impair alcohol metabolism. Downward: none you can rely on — coffee, cold showers, and “feeling fine” change alertness perception, not blood chemistry. Only time eliminates alcohol.
The legal fine print worth knowing: 0.08% is the per-se limit for regular US drivers, but commercial drivers face 0.04%, under-21 drivers face zero-tolerance thresholds (0.00–0.02%), and impairment charges below 0.08 exist everywhere. Breathalyzer and blood results — not calculators — are what courts use; nothing here has evidentiary value in either direction.
Frequently Asked Questions
How many drinks is 0.08 BAC?
Roughly 4 standard drinks in two hours for a 180-lb man, or about 2–3 for a 140-lb woman — with wide individual swing. There's no safe universal count; the same person hits different numbers on different days, which is the whole argument against calculating your way to the limit.
How long until my BAC returns to zero?
Elimination runs a fairly steady ~0.015%/hour — no shortcuts. A 0.10% peak needs about 7 hours to clear, which is why morning-after driving after a heavy night can still be legally impaired driving.
What counts as one standard drink?
14 g of alcohol (NIAAA): a 12-oz 5% beer, 5-oz 12% wine, or 1.5-oz 80-proof shot. A 9% IPA in a pint glass is ~2 standard drinks; a generous cocktail often more — undercounting drinks is the #1 way estimates run falsely low.
Does food lower BAC?
Food slows absorption — flattening the peak — but doesn't reduce the alcohol or speed elimination. Eating before drinking is genuinely protective against sharp spikes; it is not a mechanism for sobering up afterward.
Can I use this to know if I'm okay to drive?
No — and that's a design decision, not a disclaimer ritual. The formula's constants are population averages; your real BAC can sit meaningfully above the estimate, and impairment starts below legal limits anyway. If you're calculating, you've answered the question: arrange a ride.
Why do men and women get different results?
The body-water constant: alcohol distributes in water, and average body-water fraction differs (~0.68 vs ~0.55) — same drinks, smaller distribution volume, higher concentration. Body composition matters beyond sex, one more source of individual swing the formula can't see.
Sources & References
- [1]What Is a Standard Drink? — National Institute on Alcohol Abuse and Alcoholism (NIAAA / NIH)
Methodology. This calculator uses formulas and health categories recognized by the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC). It is reviewed and maintained by the Vast Calculators editorial team.
Last updated · July 11, 2026
Disclaimer. This tool provides estimates for general informational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider before making decisions about your health.