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Health & Body
Blood Alcohol (BAC) Calculator — Widmark Estimate by Drinks & Weight
A rough Widmark-formula estimate of your blood alcohol content — never a green light to drive
Your blood alcohol content (BAC) is the percentage of alcohol in your bloodstream, and in the U.S. it's the number that defines legally drunk: 0.08% is the per-se limit for drivers 21 and older in every state, with 0.04% for commercial drivers and effectively 0.00–0.02% under zero-tolerance laws for anyone under 21. This calculator uses the Widmark formula, the same basic model courts and toxicologists have relied on for decades, to give you a ballpark BAC from four inputs: your sex, your body weight in pounds, the number of standard drinks, and the hours since your first drink.
A U.S. standard drink contains about 0.6 fluid ounces (14 grams) of pure alcohol — that's roughly one 12 oz beer at 5% ABV, one 5 oz glass of wine at 12%, or one 1.5 oz shot of 80-proof liquor. A tall craft IPA, a generous home pour, or a mixed drink can easily be 1.5–2 standard drinks, so count honestly.
The Widmark math works like this: BAC = (drinks × 14 ÷ (weight in grams × r)) × 100 − 0.015 × hours, where r is the body-water constant — about 0.68 for men and 0.55 for women — and 0.015 is the average rate your body burns off alcohol per hour.
Worked example: a 180 lb man has 4 standard drinks over 2 hours. His weight in grams is 180 × 453.592 = 81,647 g. Alcohol consumed = 4 × 14 = 56 g. Peak BAC = (56 ÷ (81,647 × 0.68)) × 100 = 0.1008%. Subtract metabolism: 0.015 × 2 = 0.03. Estimated BAC ≈ 0.071% — under 0.08, but close enough that he should absolutely not drive.
The most common mistake is underestimating drink size: people count a 16 oz double pour or a 22 oz craft beer as "one drink" and badly undershoot their real BAC. Pour size, ABV, food, medications, hydration and individual metabolism all shift the true number.
This tool is informational only — it is not medical or legal advice and is not a way to decide whether you're safe to drive. Impairment begins well below 0.08%, the formula can't see your unique body, and the only truly safe BAC behind the wheel is 0.00%. If you've been drinking, get a ride.
📖 See also: Body Fat Percentage: How to Measure It at Home
Calculator
Fill in the fields and click "Calculate" for instant results.
📰 Formula
• Pure alcohol per standard drink = 14 g (0.6 fl oz) • Weight in grams = weight(lb) × 453.592 • r = 0.68 (men) or 0.55 (women) — body-water constant • Peak BAC % = (drinks × 14 ÷ (weight_g × r)) × 100 • Metabolized = 0.015 × hours since first drink • Estimated BAC % = Peak BAC − Metabolized (never below 0) • Hours to 0.00 ≈ Estimated BAC ÷ 0.015
📰 Formula
• Pure alcohol per standard drink = 14 g (0.6 fl oz) • Weight in grams = weight(lb) × 453.592 • r = 0.68 (men) or 0.55 (women) — body-water constant • Peak BAC % = (drinks × 14 ÷ (weight_g × r)) × 100 • Metabolized = 0.015 × hours since first drink • Estimated BAC % = Peak BAC − Metabolized (never below 0) • Hours to 0.00 ≈ Estimated BAC ÷ 0.015
🧪 Worked examples
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Example 2
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Example 3
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Example 4
⚠️ Common mistakes
- Counting a large craft beer, double pour or strong cocktail as a single standard drink.
- Using the men's r value (0.68) for a woman — it understates her BAC.
- Assuming the formula tells you it's legal or safe to drive (it does not).
- Forgetting that drinks consumed in the last 30–60 minutes haven't fully absorbed yet.
💡 Tips
- One U.S. standard drink = 12 oz beer (5%) = 5 oz wine (12%) = 1.5 oz spirits (40%).
- Your body clears alcohol at a fixed ~0.015% per hour — coffee, water and a cold shower don't speed it up.
- Treat any positive BAC as a reason not to drive; impairment starts well before 0.08%.
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❓ Frequently asked questions
What is a standard drink in the United States?
A U.S. standard drink contains 0.6 fl oz (14 grams) of pure alcohol. That's about one 12 oz beer at 5% ABV, one 5 oz glass of wine at 12%, or one 1.5 oz shot of 80-proof (40%) liquor.
What BAC is legally drunk in the US?
For drivers 21 and over, 0.08% is the per-se limit in every U.S. state (0.05% in Utah). Commercial drivers are limited to 0.04%, and under-21 drivers face zero-tolerance laws of roughly 0.00–0.02%.
How accurate is the Widmark BAC formula?
It's a population-average estimate. Real BAC varies with body fat, food in your stomach, medications, hydration, liver health and how fast you drank. Treat the result as a rough ballpark, not a measurement — only a breathalyzer or blood test is accurate.
Why do women reach a higher BAC than men from the same drinks?
Women generally have a lower proportion of body water (a lower r value, ~0.55 vs ~0.68 for men), so the same amount of alcohol is less diluted and produces a higher BAC at equal body weight.
How long does it take for alcohol to leave your system?
Your body clears alcohol at about 0.015% BAC per hour. From a BAC of 0.08%, that's roughly 5.3 hours to reach 0.00%. Nothing reliably speeds this up — only time lowers BAC.
Can coffee, water or a cold shower lower my BAC?
No. They might make you feel more alert, but your liver metabolizes alcohol at a roughly fixed rate. Only time reduces your actual BAC — a 'sober-feeling' drunk driver is still impaired.
Does eating food before drinking change my BAC?
Food slows how fast alcohol is absorbed, lowering and delaying your peak BAC, but it does not change the total amount of alcohol your body must process. You still reach a similar level — just more slowly.
Is it safe to drive if this calculator shows under 0.08%?
No. This is a rough estimate, not a legal clearance, and impairment of reaction time and judgment begins well below 0.08%. If you've had any alcohol, the safe choice is to not drive and arrange a ride.
Is this BAC calculator medical or legal advice?
No. It is for general information and education only. It cannot account for your individual physiology and should never be used to decide whether you're fit to drive or operate machinery. When in doubt, don't drink and drive.