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BAC Calculator
Use this free BAC calculator to estimate your blood alcohol content from your weight, the drinks you had, and the time since your first drink. It uses the Widmark formula, the standard method taught in alcohol education, and it also estimates how long it will take for your BAC to fall back to zero.
Count standard drinks: one 12 oz beer (5%), one 5 oz glass of wine (12%), or one 1.5 oz shot of spirits (40%) each count as one drink. A pint of strong beer or a double shot counts as two.
| Total alcohol | 56 g |
| Legal limit (most states, 21+) | 0.08% |
| Your BAC vs. the limit | Below the limit — still do not drive |
| Safety message | Do not drive. Get a ride. |
How it works
BAC is the share of alcohol in your blood, measured as a percentage. A BAC of 0.08% means 0.08 grams of alcohol in every 100 milliliters of blood. Your body absorbs alcohol from the stomach and small intestine, and the liver then breaks it down at a fairly steady rate of about 0.015% per hour. That rate is why time — not coffee, food, or a cold shower — is the only thing that brings BAC down.
This calculator uses the Widmark formula, the standard method for estimating BAC. First, it counts your total alcohol: each standard drink holds about 14 grams of pure alcohol, so 4 drinks hold 56 grams. Next, it divides that alcohol by your body weight in grams, multiplied by a distribution factor — 0.68 for men and 0.55 for women, because alcohol spreads through body water, and women on average have a lower share of body water. That gives the peak BAC before any alcohol has been cleared. Finally, it subtracts 0.015% for every hour since your first drink, because your liver has been clearing alcohol all along.
Eating food slows alcohol absorption, so a full stomach lowers the peak BAC by roughly 10% compared with drinking on an empty stomach. This calculator applies that as a simple 10% reduction in the alcohol that reaches the blood when you say you ate. It is a rough adjustment, not a precise measurement — a heavy meal slows things more than a few crackers, and food never makes drinking safe for driving.
Weight matters because the same amount of alcohol is more concentrated in a smaller body. Sex matters because of the body-water difference in the Widmark factor. Time matters twice: drinking slowly over several hours keeps BAC lower than the same drinks finished quickly, because the liver clears alcohol while you are still drinking. That is why the calculator asks for hours since your first drink, not just the drink count.
Once it has your BAC, the calculator divides it by 0.015% per hour to estimate how long until your BAC reaches zero. A BAC of 0.071% takes about 4 hours and 44 minutes to clear. Remember that this is an estimate: illness, medicines, and how fast you drank can all shift the real number, so always allow extra time and never use the estimate to decide you are fit to drive.
Worked example
A man weighing 180 pounds has 2 beers, 1 glass of wine, and 1 shot of spirits over 2 hours, with no food.
Step 1: Count the drinks — 2 + 1 + 1 = 4 standard drinks.
Step 2: Total alcohol — 4 × 14 grams = 56 grams.
Step 3: Body weight in grams — 180 lb × 453.592 = 81,647 grams.
Step 4: Peak BAC — (56 ÷ (81,647 × 0.68)) × 100 = (56 ÷ 55,520) × 100 = 0.101%.
Step 5: Alcohol cleared — 0.015% × 2 hours = 0.030%.
Step 6: Estimated BAC now — 0.101% − 0.030% = 0.071%.
Step 7: Time until sober — 0.071% ÷ 0.015% per hour = 4.73 hours, about 4 hours and 44 minutes.
The tool shows the same result: an estimated BAC of 0.071%, which is below the 0.08% limit in most states — but still impaired, and still not safe to drive. He should wait about 4 hours and 44 minutes for his BAC to reach zero, and get a ride home in the meantime.
FAQ
What is BAC?
BAC stands for blood alcohol content. It is the percentage of alcohol in your bloodstream, written as a decimal like 0.08%. A BAC of 0.08% means there are 0.08 grams of alcohol in every 100 milliliters of blood. It rises as you drink and falls slowly as your liver breaks the alcohol down, at roughly 0.015% per hour.
What is the legal BAC limit for driving?
In most states the legal limit for drivers aged 21 and over is 0.08%. Some states set lower limits for drivers under 21 and for commercial drivers, and a few states have stricter rules for repeat offenders. This is general information only, not legal advice, and the safe choice is never to drive after drinking at all, whatever your BAC.
How accurate is this BAC calculator?
It is an estimate only. The Widmark formula this calculator uses is the standard textbook method, but real BAC varies with food, hydration, medicines, health, and how fast you drank. Two people with the same weight and the same drinks can have different BACs. Never use an estimate to decide whether you are safe to drive — if you have been drinking, get a ride.
What counts as one standard drink?
One standard drink holds about 14 grams of pure alcohol. That is a 12-ounce beer at 5% alcohol, a 5-ounce glass of wine at 12% alcohol, or a 1.5-ounce shot of 80-proof spirits at 40% alcohol. A pint of strong beer, a large glass of wine, or a double shot can hold two standard drinks or more, so count the alcohol, not the glasses.
How long does it take for BAC to reach zero?
Your body clears alcohol at roughly 0.015% per hour, and nothing speeds that up — not coffee, cold air, food, or a shower. Time is the only thing that lowers BAC. A BAC of 0.09% takes about 6 hours to reach zero, and a BAC of 0.15% takes about 10 hours. This calculator shows your estimated time to zero based on your result.