Blood Alcohol Calculator (BAC)
Estimate your blood alcohol content (BAC) from standard drinks, body weight, sex, and time using the Widmark formula. A rough guide only — never for deciding to drive. Free, no signup.
Rough estimate only. This is an educational tool, not a breathalyzer or medical device. Never use it to decide whether to drive — the only safe level behind the wheel is 0.00. If you've been drinking, get a ride or wait.
Relaxation and lowered inhibition; reaction time already slipping.
At ~0.015 %/h until BAC reaches 0.00.
3 standard drinks × 14 g.
Estimates use the Widmark formula and an average 0.015 %/h elimination rate. Your real BAC can differ substantially. This is not medical or legal advice and must never be used to decide whether to drive.
How the Widmark BAC formula works
Blood alcohol content is a measure of how much pure ethanol is circulating in your bloodstream, expressed as a percentage by volume. The Widmark formula, developed by Swedish scientist Erik Widmark in the 1930s, is still the standard way to estimate it. It starts with the grams of pure alcohol you drank — about 14 g per US standard drink — and spreads that alcohol across the water in your body:
BAC% = (alcohol grams ÷ (body weight in grams × r)) × 100 − (0.015 × hours)
The factor r reflects the fraction of your body that behaves as water — on average about 0.68 for men and 0.55 for women because of typical differences in body composition. More body water means the same alcohol is more diluted, giving a lower peak. The final term subtracts elimination: the liver clears alcohol at a near-constant ~0.015 BAC points per hour, which is why only time lowers your level — coffee, food, and cold showers do not. Because absorption, food, tolerance, and metabolism all vary, treat the output as a rough planning estimate, not a blood reading.
Why you can't rely on a BAC estimate to drive
This is the part that matters most: never use a calculator to decide whether you are safe to drive. The Widmark model assumes an average body, instant and complete absorption, and a single elimination rate — assumptions that rarely hold for a real person on a real night. Food in your stomach, the speed you drank, medications, hydration, and individual metabolism can push your actual BAC well above or below the estimate, and a police breathalyzer may read higher than this number. Impairment also starts long before the 0.08 legal limit, so a “legal” number is not a safe one. The only blood alcohol level that is safe for driving is 0.00. If you have been drinking, plan ahead: arrange a designated driver, call a rideshare, or simply wait — this tool's “hours until roughly sober” figure is a floor, not a green light.
Building a healthier relationship with alcohol
If you are running the numbers because you want to drink more mindfully, the most useful habit is often the simplest: notice the days you don't drink at all. Tracking alcohol-free or mindful-drinking days turns a vague intention into visible progress, and a short streak is a surprisingly strong nudge. You can log a one-tap “alcohol-free today” check-in in HabitBox, a free habit tracker that runs fully on-device (no account) on iOS and Android, and watch the pattern build over weeks. For app ideas built specifically around this, see our roundup of the best sober tracker apps.
Important: this calculator provides a rough educational estimate only. It is not medical advice and must never be used to decide whether it is safe to drive or operate machinery. Individual results vary widely. If you are concerned about your drinking, please talk to a doctor or a qualified support service.
Frequently asked questions
How is blood alcohol content (BAC) calculated?+
This tool uses the Widmark formula, the standard estimate taught in toxicology. It divides the grams of pure alcohol you drank by your body water (your weight times a distribution factor, r), converts to a percentage, then subtracts what your liver has already cleared at about 0.015 per hour. The formula is BAC% = (alcohol grams ÷ (weight in grams × r)) × 100 − (0.015 × hours).
What is the legal BAC limit for driving?+
In most US states the per-se limit is 0.08 % for drivers 21 and over — at or above it you are legally impaired regardless of how you feel. Utah is stricter at 0.05 %, commercial drivers are usually held to 0.04 %, and many states have near-zero limits for drivers under 21. Impairment begins well below any legal number, so the limit is a ceiling, not a target.
How long does alcohol stay in your system?+
Your body eliminates alcohol at a fairly steady rate of roughly 0.015 BAC points per hour, and nothing reliably speeds it up — not coffee, cold showers, food, or exercise. So a BAC of 0.08 takes about 5–6 hours to reach zero, and only time lowers it. This calculator estimates the hours until you would be roughly sober from your current level.
What affects your BAC the most?+
The biggest drivers are how much pure alcohol you drank (number and strength of drinks), your body weight and composition, your biological sex (via the distribution factor), and how much time has passed. Food in your stomach slows absorption, and factors like hydration, medications, and liver health shift the real number. Because so many variables interact, any estimate is approximate.
Is this calculator accurate enough to decide whether to drive?+
No. Never use this or any calculator to decide whether to drive. The Widmark estimate can be off by a wide margin because it cannot account for your food intake, absorption rate, medications, tolerance, or measurement error — and it may read lower than a real breathalyzer. The only safe blood alcohol level for driving is 0.00. If you have been drinking, arrange a ride, use a rideshare, or wait it out.
What counts as one standard drink?+
In the US a standard drink contains about 14 grams of pure alcohol, which is roughly a 12 oz (355 ml) beer at 5 %, a 5 oz (150 ml) glass of wine at 12 %, or a 1.5 oz (44 ml) shot of 40 % spirits. Many cocktails, craft beers, and generous home pours count as more than one, so enter your best honest estimate of standard drinks.
Track the days you don't drink
Curiosity about your BAC often means you'd like to drink a little more mindfully. HabitBox makes an 'alcohol-free today' check-in a single tap, builds a streak you can see, and keeps every log on your device — free, no account, on iOS and Android.
Free to start · No account required · No ads