BAC Calculator (Blood Alcohol Concentration)

No. The result is a rough estimate from averages, not a measurement. A 2020 forensic analysis put the uncertainty of this kind of calculation at roughly 22 to 37 percent. Impairment also starts below the legal limit: NIAAA notes that even small increases in BAC can reduce coordination and cloud judgment, which raises the risk of a crash. Nothing on this page can show that driving after drinking is safe or legal.

With the Widmark formula: grams of alcohol divided by body weight times a factor r (0.68 for men, 0.55 for women), times 100 to give percent, minus about 0.015 for every hour since the first drink. A 180 lb man with 4 beers (56 g of alcohol) has a theoretical peak of 0.101% and an estimated 0.071% two hours after his first drink.

Not very. Body water, absorption, food, medications and clearing speed all differ from person to person, and the formula uses averages. Forensic guidelines ask experts to report a range, not one value, for example 0.010 to 0.025 per hour for clearing. The same drinks can leave one person well above this estimate and another well below it. Only a breath or blood test measures BAC.

NIAAA says a typical adult reaches a BAC of 0.08% after about 4 drinks for a woman or 5 for a man in roughly 2 hours. The formula in this calculator, which subtracts what has cleared, gives 3.1 drinks for a 160 lb woman and 3.9 for a 160 lb man two hours after the first drink. Heavier people need more: a 200 lb man needs 4.8. Enter your own numbers to see yours.

By this estimate the body clears about 0.015 percentage points of BAC an hour, so one standard drink takes roughly 1.7 hours to clear for a 180 lb man and 2.7 hours for a 140 lb woman. Real rates run from about 0.010 to 0.025 an hour. NIAAA says the body metabolizes alcohol at a steady rate regardless of attempts to sober up with caffeine or other means.

Alcohol spreads through body water, and pound for pound women have proportionally less water in their bodies than men do, as NIAAA explains. The Widmark factor reflects this: 0.55 for women against 0.68 for men. At the same weight and with the same drinks, that makes the peak about 24 percent higher for a woman.

In the United States one standard drink contains about 14 grams of pure alcohol: 12 oz of 5% beer, 5 oz of 12% wine or 1.5 oz of 40% spirits (80 proof). Strength matters. NIAAA notes that a 12 oz beer at 10% is two standard drinks. For other drinks, fluid ounces × alcohol percent ÷ 60 gives the number of standard drinks.

It is the highest BAC the drinks could produce: all the alcohol absorbed at once with none cleared, which forensic toxicology guidelines call the theoretical maximum. Real BAC rises gradually and clears while you drink, so it normally peaks below this number, but it can keep rising for up to 2 hours after the last drink while alcohol is still being absorbed.

Yes, although this calculator does not ask about it. NIAAA says drinking on an empty stomach speeds absorption and gives a higher blood alcohol level than drinking on a full stomach. A review of clearing rates also found 0.010 to 0.015 per hour after drinking on an empty stomach and 0.015 to 0.020 when not fasting. The formula uses 0.015 in both cases.

For drivers 21 and older the usual limit is 0.08%. Each state writes its own law, and federal law (23 U.S.C. 163) withholds part of a state’s federal highway funds unless driving with a BAC of 0.08% or more is an offense in itself (a per se offense). Federal rules bar commercial drivers from safety-sensitive duty at 0.04% or more (49 CFR 382.201), and states that lack a 0.02% limit for drivers under 21 lose some highway funds (23 U.S.C. 161). Staying under a limit is not the same as being fit to drive, and this page is not legal advice.