This BAC calculator estimates blood alcohol concentration from what you drank, your body weight and elapsed time, using the Widmark formula — the standard estimator used in health education. It produces an estimate for understanding alcohol’s effects, never a verdict on fitness to drive: the only safe driving BAC is 0.00.

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The Widmark Formula

BAC% ≈ (grams of alcohol ÷ (body weight in g × r)) × 100 − 0.015 × hours, where r (the share of body mass that distributes alcohol) averages 0.68 for men and 0.55 for women, and 0.015/hour is the average elimination rate. A US standard drink carries 14 g of pure alcohol: 12 oz of 5% beer, 5 oz of 12% wine, or 1.5 oz of 40% spirits — restaurant pours and craft beers routinely exceed these, which is one of several reasons real BAC can land well above any estimate. Part of our Health Calculators collection.

Worked Example (Verified)

  • 80 kg man, 4 standard drinks, 1 hour: 56 g ÷ (80,000 × 0.68) × 100 = 0.103%, minus 0.015 = ≈ 0.088% — above the 0.08 limit
  • Same drinks, 60 kg woman: 56 ÷ (60,000 × 0.55) × 100 = 0.170% − 0.015 = ≈ 0.155% — nearly double, from body composition alone. Identical drinks are not identical doses.
  • Time is the only sobering agent: at ~0.015/hour, clearing 0.08 takes over five hours. Coffee, cold air and willpower change feelings, not chemistry.

What the Estimate Cannot Know

  • Food slows absorption (peak lower and later); carbonation and empty stomachs speed it.
  • Pour reality: a "glass of wine" at home is often 7–9 oz of 14% — nearly two standard drinks wearing a trench coat.
  • Individual r factors and elimination rates vary ±20%+; medications and fatigue multiply impairment at any given BAC.
  • Laws: 0.08 g/dL is the US per se standard (0.05 in Utah and most of Europe; lower or zero for commercial and under-21 drivers) — and impairment-based charges apply below it.

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Frequently Asked Questions

How is BAC calculated?

By the Widmark formula: alcohol grams divided by (body weight × distribution factor r), less ~0.015% per hour of metabolism. This calculator uses r = 0.68 (men) / 0.55 (women) and 14 g per standard drink.

How many drinks does it take to reach 0.08?

Roughly 4 standard drinks in an hour puts an average 80 kg man near 0.09; for a 60 kg woman, about 2.5–3 drinks. Body weight and sex swing the answer enormously — the examples above differ by nearly 2× on identical drinks.

How long until my BAC is zero?

From 0.08%, about 5–6 hours at the average elimination rate of 0.015/hour — "sleeping it off" for a few hours does not zero anyone who was genuinely drunk.

Can I trust a BAC calculator to decide if I can drive?

No. Estimates vary ±0.02 or more from pour sizes, food and personal metabolism — the entire margin around the legal limit. Every responsible source, this one included, says the same: if you have been drinking, do not drive. The calculator exists to build intuition, not permission slips.

What is a standard drink?

14 grams of pure alcohol: 12 oz regular beer (~5%), 5 oz wine (~12%), or 1.5 oz spirits (~40%). Craft beers at 8%, generous wine pours and doubles count as 1.5–2 drinks each.

Is this BAC calculator free?

Yes — free, instant, no sign-up. And again: estimates only, never a driving decision.