Blood alcohol calculator (BAC)
Estimate approximate blood alcohol content using the standard Widmark formula.
Safety notice: This calculator provides a rough estimate only and must never be used to decide whether it is safe to drive or perform other activities. Actual BAC varies significantly based on individual metabolism, food intake, medication, and other factors this calculator cannot account for. The only safe choice is not to drive after drinking any amount of alcohol. If you or someone else may have a drinking problem, contact SAMHSA's National Helpline at 1-800-662-4357.
One standard US drink = 0.6 fl oz pure alcohol, roughly one 12 oz beer, one 5 oz glass of wine, or one 1.5 oz shot of spirits.
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How BAC is estimated (the Widmark formula)
This calculator uses the Widmark formula, a well-established method for estimating blood alcohol content that has been used in medical and some legal contexts for decades. In plain terms, it takes the total grams of pure alcohol consumed, divides that by your body weight in grams multiplied by a distribution ratio, converts the result to a percentage, and then subtracts an amount for the alcohol your body has already metabolized since you started drinking.
The distribution ratio, known as the Widmark r value, accounts for the fact that alcohol distributes through body water, and body composition differs between sexes on average: 0.68 for males and 0.55 for females are the standard textbook constants this calculator uses. Alcohol grams are calculated from standard US drinks at 14 grams of pure alcohol each, body weight is converted from pounds to grams, and the elimination rate is set at the standard average of 0.015% BAC per hour, though this table notes that elimination rate varies meaningfully from person to person based on metabolism, liver function and other individual factors.
Because the formula relies on population averages rather than your specific physiology, it is best understood as a reasonable approximation used for general estimation, not as a substitute for an actual chemical test.
Why this number is an estimate, not a measurement
The only accurate way to know your actual BAC at any given moment is a breathalyzer test or a blood test administered by a professional. This calculator, like any Widmark-based estimate, can meaningfully over- or under-estimate a given individual's real BAC, because it cannot account for factors such as how much food was in your stomach, the pace at which you drank, medications you may have taken, your hydration level, or natural variation in how your body processes alcohol.
Two people of the same weight and sex who drink the same amount can end up with noticeably different actual BAC levels. Treat any number this calculator produces as a rough ballpark for general awareness, never as a precise reading of your condition.
Legal BAC limits
In the United States, the legal limit for driving is generally 0.08% BAC for drivers 21 and older. Lower limits typically apply to commercial drivers and to drivers under 21 in most states, often at or near a 0.00-0.02% threshold under so-called zero-tolerance laws. Exact thresholds, penalties and enforcement details vary by state and are not fully covered here, so check your specific state's law if you need precise legal information.
Frequently asked questions
How is BAC estimated (the Widmark formula)?
This calculator uses the Widmark formula: BAC% equals alcohol consumed in grams divided by body weight in grams times a distribution ratio, times 100, minus 0.015% for each hour elapsed since the first drink. The distribution ratio is 0.68 for males and 0.55 for females, standard textbook values reflecting typical body water distribution.
Why is this number an estimate, not a measurement?
The Widmark formula uses average constants that do not account for your individual metabolism, food intake, medication, hydration or how quickly you drank. The only accurate way to know your actual BAC is a breathalyzer or blood test administered by a professional.
What are the legal BAC limits?
In the United States, the legal driving limit is generally 0.08% for drivers 21 and over. Commercial drivers and drivers under 21 face lower limits in most states. Exact laws vary by state, so this is general information, not a substitute for checking your local law.
Should I drive if this shows I'm under the legal limit?
No. Never rely on an estimated BAC number to decide whether it's safe to drive. This calculator can meaningfully over- or under-estimate your actual BAC, and impairment can begin well below the legal limit. The only truly safe choice is not to drive after drinking any amount of alcohol.
Does food or medication affect BAC?
Yes, significantly. Food in your stomach slows alcohol absorption and can lower peak BAC, while certain medications can intensify alcohol's effects independent of BAC itself. Neither factor is accounted for in this calculator's estimate.
This calculator provides a rough estimate only and must never be used to decide whether it is safe to drive or perform other activities. Actual BAC varies significantly based on individual metabolism, food intake, medication, and other factors this calculator cannot account for. The only safe choice is not to drive after drinking any amount of alcohol. If you or someone else may have a drinking problem, contact SAMHSA's National Helpline at 1-800-662-4357.