Blood Alcohol Calculator
Estimate blood alcohol content
How It Works
Overview
Blood Alcohol Content (BAC) is the percentage of alcohol in your bloodstream by weight — a 0.08% reading means 0.08 grams of ethanol per 100 mL of blood. This calculator uses the Widmark formula, the same equation that underpins most online BAC tools and many forensic estimates, to project a peak BAC from how much you've drunk, your body weight, sex, and how much time has passed.
Important: this is an estimate only and is not legal, medical, or safety advice. Real BAC varies with food intake, drink concentration, drinking speed, hydration, medications, and individual physiology — easily shifting results by 0.02–0.03% in either direction. Never use a calculator output to decide whether to drive; the only safe BAC for driving is zero.
The Formula
The Widmark formula expressed in BAC percentage:
- A = total alcohol consumed in grams (1 standard US drink ≈ 14 g)
- W = body weight in grams
- r = Widmark factor — roughly 0.68 for men, 0.55 for women — reflecting the fraction of body weight that is water
- t = hours elapsed since the first drink
- 0.015 = average alcohol elimination rate in BAC% per hour
Time is subtracted because the liver metabolizes alcohol at a steady ~0.015%/hour. The formula assumes alcohol is fully absorbed, which usually takes 30–90 minutes after your last drink.
Worked Example
A 180 lb (81.6 kg, or ~81,647 g) male has 3 standard beers over 2 hours:
- Alcohol consumed: 3 × 14 g = 42 g
- W × r = 81,647 × 0.68 ≈ 55,520
- Raw BAC: (42 / 55,520) × 100 ≈ 0.0757%
- Metabolism over 2 hours: 2 × 0.015 = 0.030%
- Estimated BAC: 0.0757 − 0.030 ≈ 0.045%
- Time to reach 0.00%: 0.045 ÷ 0.015 = 3 more hours
A 140 lb female drinking the same amount over the same time would land near 0.075% — close to the 0.08% legal driving limit — illustrating why pound-for-pound identical consumption produces very different BACs.
When to Use This
- Educational planning before an event — to see how a few drinks over an evening typically scale.
- Pacing yourself at home — comparing a beer-only night vs. a cocktail night for similar peak BAC.
- Understanding morning-after impairment — high BACs at bedtime can still register as 0.04%+ at 7 a.m.
- Comparing standard vs. oversized drinks — a 9% craft IPA in a 16 oz can is roughly 2.4 standard drinks, not one.
Common Mistakes to Avoid
- Counting every drink as "one." A double, a strong cocktail, or a craft beer often counts as 1.5–3 standard drinks.
- Using it to decide if you can drive. Estimation error alone can put you legally over while the calculator says you're under.
- Ignoring absorption time. Peak BAC often arrives 30–90 minutes after the last drink — finishing a shot 10 minutes ago doesn't reflect peak yet.
- Believing food "absorbs" alcohol. Food slows absorption (lower peak), but doesn't reduce total alcohol — you still process the same grams.
- Trying to speed up sobering. Coffee, exercise, cold showers, and water do not raise the 0.015%/hour metabolism rate; only time does.
Frequently Asked Questions
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