BAC Calculator
A rough educational estimate of blood alcohol content using the Widmark formula. This is not a tool for deciding whether it is safe or legal to drive — the only safe amount before driving is zero.
⚠ Never use this to decide whether to drive. If you have been drinking, do not drive.
The Widmark formula
r ≈ 0.68 (men), 0.55 (women); one standard drink ≈ 14 g alcohol
The formula ignores food in your stomach, drinking speed, medications, liver health, hydration, and the actual strength of your drinks — all of which move real BAC substantially. Two people with the same inputs can have very different actual levels. Metabolism averages ~0.015%/hour but varies person to person.
Same weight, same drinks, same elapsed time — different result, because of the r constant. A 65 kg person, 2 standard drinks, 1 hour elapsed:
Female (r = 0.55): peak BAC = (28 g ÷ (65,000 g × 0.55)) × 100 ≈ 0.078%, minus 0.015 for the hour elapsed ≈ 0.063%.
Male (r = 0.68): peak BAC = (28 g ÷ (65,000 g × 0.68)) × 100 ≈ 0.063%, minus 0.015 ≈ 0.048%.
The same intake produces a meaningfully higher estimate for the female input, purely because the formula's r constant assumes a lower proportion of body water — it's a population average baked into the Widmark method, not a personalized measurement, and real individual results vary well beyond what sex alone predicts.
What BAC levels mean
| BAC | Typical effects |
|---|---|
| 0.02% | Mild relaxation; some judgment/attention decline |
| 0.05% | Lowered alertness, impaired coordination |
| 0.08% | Legal driving limit (U.S.); clear impairment |
| 0.15%+ | Major impairment; risk of injury |
| 0.30%+ | Potentially life-threatening |
Impairment starts below the legal limit, and only time lowers BAC — coffee, water, food, and cold showers do not speed it up. If you have been drinking, use a taxi, rideshare, or designated driver. In an emergency involving heavy drinking (confusion, vomiting, slow breathing, unconsciousness), call emergency services.
What counts as one standard drink
| Drink type | Standard serving | Typical strength |
|---|---|---|
| Regular beer | 12 oz (355 ml) | ~5% ABV |
| Wine | 5 oz (148 ml) | ~12% ABV |
| Malt liquor | 8–9 oz (237–266 ml) | ~7% ABV |
| Distilled spirits (shot) | 1.5 oz (44 ml) | ~40% ABV |
Each of these contains roughly the same amount of pure alcohol (about 14 grams), which is why they're all counted as "1 standard drink" despite very different serving sizes. A restaurant or home pour is frequently larger than these reference sizes, especially for wine and mixed spirits.
Common BAC estimation mistakes
- Assuming a poured drink matches a standard drink. As the table above shows, many pours run larger than the reference size, so actual alcohol intake is often higher than a simple drink count suggests.
- Believing coffee, cold showers, or exercise speed up sobering. None of them do — only time reduces BAC, at roughly the 0.015%-per-hour average this formula uses.
- Using any BAC number, including this estimate, to decide whether it's safe to drive. No formula can account for the individual factors that determine real impairment — see the safety notice throughout this page.
Frequently asked questions
What is BAC?
The percentage of alcohol in your blood. 0.08% is the U.S. driving limit, but impairment starts lower.
How is BAC estimated?
The Widmark formula, minus ~0.015%/hour for metabolism — an approximation only.
Can I use this to decide if I can drive?
No, never. It can be badly wrong. The only safe amount before driving is none.
Why does the formula use a different constant for men and women?
The r constant approximates the fraction of body weight that is water, since alcohol distributes through it — the formula uses population averages of about 0.68 for men and 0.55 for women, not a personal measurement.
Does a bar pour always match a "standard drink"?
Often not — many wine and mixed-spirit pours run larger than the reference serving sizes, so actual alcohol intake is frequently higher than a simple drink count suggests.
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Important safety & medical disclaimer: This calculator provides a rough educational estimate only and must never be used to determine whether you are fit or legally permitted to drive or operate machinery. Actual blood alcohol content depends on many individual factors this tool cannot measure and can differ greatly from the estimate. If you have consumed any alcohol, do not drive. This is not medical or legal advice. Formula: Widmark. Last reviewed: September 2026.