Alcohol BAC Estimator (Widmark Formula)
Estimate blood alcohol concentration after a drink session using the Widmark equation, with body weight, sex, and elapsed time. Educational use only โ not for impairment decisions.
Widmark equation: BAC (g/dL = %) = 0.1 ร ethanol(g) / (W_kg ร r) โ ฮฒ ร t. r per Watson PE et al., Br J Addict 1981 (0.68 men, 0.55 women). Elimination ฮฒ = 0.015 %/h moderate, 0.018 %/h chronic. US per-se DUI thresholds: 0.08% all states (Utah 0.05%), CDL 0.04%, under-21 zero tolerance. Individual variation ยฑ30% from food, body composition, ALDH2 genetics. Educational use only โ do not use to decide if you can drive. NIAAA Standard Drinks Guide.
General calculation reads
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About this tool
The Widmark formula (Widmark EMP, Die theoretischen Grundlagen, 1932) is the standard forensic and clinical equation for estimating blood alcohol concentration: BAC = (A ร 100 ร 0.806) / (W ร r) โ ฮฒ ร t. A = total grams of pure ethanol consumed; 0.806 is ethanol density (g/mL); W = body weight in kg; r = Widmark constant (volume of distribution ratio) โ average 0.68 for men, 0.55 for women per Watson PE et al., Br J Addict 1981, reflecting lower water content of female bodies; ฮฒ = average alcohol elimination rate, 0.015 % BAC per hour for moderate drinkers, 0.018 for chronic heavy drinkers; t = hours since first drink. Standard US drink sizes (NIAAA): 12 fl oz beer at 5% ABV, 5 fl oz wine at 12% ABV, or 1.5 fl oz spirits at 40% ABV โ each contains 14 g of pure ethanol. US per-se DUI threshold is 0.08% BAC in all states (Utah 0.05%); commercial drivers 0.04%; zero-tolerance for drivers under 21. Widmark is an approximation โ real BAC depends on absorption (food in stomach delays peak 30โ90 min, fat tissue lowers r, genetic ALDH2 variants slow metabolism), and individual variation is ยฑ30%. Never use this calculator to decide if you can drive.
How to use it
- Enter your weight, sex, and the start time of drinking.
- Log each drink (size + ABV) and the time consumed.
- See current BAC estimate, peak BAC, and hours until below 0.00.
- Treat all numbers as approximate โ actual BAC varies ยฑ30% between individuals.
Frequently asked questions
- How accurate is Widmark?
- Approximate. The formula assumes complete absorption and steady elimination, which is roughly true 30โ90 min after drinking stops. Peak BAC varies by ยฑ30% between individuals due to absorption rate, body composition, and genetic enzyme variants.
- Why is r different for men and women?
- r is the ratio of body water content ร density. Average male body is ~58% water, female ~49% (women carry more body fat, which has no alcohol). So a man and a woman of identical weight reach different BAC from the same drink.
- Does food reduce BAC?
- Food in the stomach slows gastric emptying and absorption, which lowers the peak BAC by 20โ30% and delays peak by 30โ90 min โ but the total area under the curve (total alcohol absorbed) is unchanged.
- Why is the elimination rate constant?
- Above ~0.01% BAC, alcohol metabolism is zero-order (saturated enzyme kinetics) โ eliminated at a constant rate regardless of concentration. Below 0.01% it transitions to first-order but that is rarely relevant.
- Can I use this to drive?
- No. Use this for general education only. Even at 0.05% BAC measurable impairment exists, and individual variation can put your actual BAC well above the calculator estimate. The only safe driving BAC is 0.00.