BAC Calculator

What is a BAC calculator?

A BAC (Blood Alcohol Content) calculator estimates the percentage of alcohol in your bloodstream based on how much you have had to drink, your body weight, your sex, and how long ago you started drinking. BAC is usually expressed as a percentage by volume — for example, a BAC of 0.08% is the legal driving limit in many places.

This calculator uses the classic Widmark formula, the same approach forensic and educational tools have relied on for nearly a century. It is intended for learning how the variables interact, not for making decisions about whether it is safe or legal to drive.

Important: this is an estimate for education only. Your real BAC depends on many factors this tool does not model — food in your stomach, drinking speed, medications, body composition, liver function, hydration, and individual metabolism. Never use this number to decide whether you are fit to drive. If you have been drinking, do not drive.

How does it work?

The Widmark model treats alcohol as if it distributes evenly through the body’s water. You enter:

• Body weight in pounds.
• Sex, which sets the Widmark distribution ratio $r$ (roughly $0.55$ for women and $0.68$ for men, reflecting average differences in body water content).
• Number of standard drinks. One US standard drink contains about 14 grams of pure alcohol — a 12 oz (355 ml) beer at 5%, a 5 oz (148 ml) glass of wine at 12%, or a 1.5 oz (44 ml) shot of 40% spirits.
• Hours since your first drink, used to subtract the alcohol your body has already metabolized.

The body eliminates alcohol at a fairly steady rate of about $0.015\%$ BAC per hour, which is why time lowers the estimate.

Formula

$BAC = \frac{A}{W \cdot r} \times 100 - 0.015\,t$

Where:

• $A$ = grams of pure alcohol consumed (standard drinks $\times\ 14$)
• $W$ = body weight in grams (pounds $\times\ 453.592$)
• $r$ = Widmark distribution ratio ($0.55$ female, $0.68$ male)
• $t$ = hours elapsed since the first drink

The result is clamped at zero — once metabolism has cleared the alcohol, the estimate cannot go negative.

Worked examples

Example 1 — male, 176 lb, 4 drinks, 2 hours.

First convert weight to grams: $176 \times 453.592 = 79{,}832$ g. The alcohol consumed is $4 \times 14 = 56$ g, and $r = 0.68$.

$BAC = \frac{56}{79{,}832 \cdot 0.68} \times 100 - 0.015 \times 2 \approx 0.1032 - 0.03 = 0.073\%$

Example 2 — female, 130 lb, 2 drinks, 1 hour.

Weight in grams: $130 \times 453.592 = 58{,}967$ g. Alcohol: $2 \times 14 = 28$ g, with $r = 0.55$.

$BAC = \frac{28}{58{,}967 \cdot 0.55} \times 100 - 0.015 \times 1 \approx 0.0863 - 0.015 = 0.071\%$

In both cases the estimate lands close to the common 0.08% legal threshold, which shows how quickly a few drinks add up.

Practical notes

• Peak vs. current BAC. Alcohol takes 30–90 minutes to fully absorb, so your true peak may be higher than a simple time-since-first-drink estimate suggests.
• Standard drinks vary. A large cocktail or a generous home pour can contain two or more standard drinks. Undercounting drinks is the most common source of error.
• Sex is a simplification. The $0.55$/$0.68$ ratios are population averages; individual body water content varies.
• Elimination is not perfectly linear. The $0.015\%$/hour rate is a typical average, not a guarantee.

To plan around alcohol and overall health, you may also find our BMI calculator, calorie calculator, and daily water intake calculator useful — alcohol contributes calories and dehydrates the body.

FAQ

Is this calculator accurate enough to decide if I can drive? No. It is an educational estimate with wide error margins. Do not drive after drinking, regardless of what any calculator shows.

Why does the BAC go down over time? Your liver metabolizes alcohol at roughly $0.015\%$ BAC per hour, so the longer it has been since your first drink, the lower the estimate.

What counts as one standard drink? In the US, about 14 grams of pure alcohol: a 12 oz beer (5%), a 5 oz glass of wine (12%), or a 1.5 oz shot of 40% spirits.