Blood Alcohol Content (BAC) Calculator
Estimate blood alcohol content based on drinks, body weight, sex, and time. Includes sober-time estimate.
What this calculates
Blood alcohol content (BAC) measures the percentage of alcohol in your bloodstream. The legal driving limit in most of the US is 0.08 %; many other countries use 0.05 % or 0.02 %. This estimator uses the Widmark formula — useful for understanding rough effects, but never a substitute for actually waiting before driving.
Formula & how it works
BAC = (alcohol_grams ÷ (weight_kg × r)) × 100 − (elapsed_hours × 0.015). r is the Widmark factor: 0.68 men, 0.55 women (women have less body water proportionally). Standard drink ≈ 14 g alcohol (US): 12 oz 5% beer, 5 oz 12% wine, 1.5 oz 40% spirits. Metabolism averages 0.015 % BAC per hour.
Worked example
70 kg man, 3 standard drinks over 2 hours. Alcohol = 42 g. Raw BAC = 42 ÷ (70 × 0.68) × 100 ≈ 0.088 %. Subtract 2 × 0.015 = 0.058 %. Still over 0.05 % limit in many countries, near 0.08 % in the US. Time to sober (BAC < 0.01): about 4 more hours.
Frequently asked questions
Is this legally reliable?
No. BAC depends on too many factors (recent food, hydration, body composition, metabolism speed) that estimators can't see. Police use breathalyzers and blood tests. Never drive based on a calculator.
What's a 'standard drink'?
Definitions vary. US: 14 g pure alcohol = 12 oz 5% beer = 5 oz wine = 1.5 oz 80-proof spirit. UK uses 8 g units; Japan uses 10 g; Australia 10 g. Adjust counts when traveling.
Does food slow alcohol absorption?
Significantly. Drinking on an empty stomach can produce 30 %+ higher peak BAC than the same drinks with a full meal. Eat before and during drinking.
Can coffee sober me up?
No. Caffeine masks tiredness but doesn't speed alcohol metabolism. You feel more alert but BAC remains the same — and that combination is more dangerous than alcohol alone.
Disclaimer: Estimate only. Never drive after drinking. Call a ride. If unsure, wait many hours.