BAC calculator (estimator)
Free educational BAC calculator: enter your weight, drinks consumed, and hours since first drink. Widmark-formula estimate, not a way to decide whether to drive.
A BAC calculator uses the Widmark formula to estimate your blood alcohol concentration from body weight, drinks consumed, and time since your first drink. This one is educational only, real BAC depends on food, hydration, medication, and individual metabolism, and even a low estimate is not a reason to drive.
This BAC calculator uses the Widmark formula to give a rough educational estimate of your blood alcohol concentration. It is explicitly not a substitute for a breathalyser, not a tool for deciding whether you are clear to drive, and not advice. Real BACs can differ substantially from an estimate for the same inputs.
How this calculator works
The Widmark formula:
BAC = A / (W × 10 × r) − β × t, where A is grams of
pure alcohol consumed, W is body weight in kg, r is a sex-based
distribution constant (0.68 / 0.55, averages, not your specific
biology), β is the elimination rate (we use 0.015 g/dL per hour,
the conventional value), and t is hours since drinking began. We
treat one US standard drink as 14 g of pure alcohol per NIAAA.
What the formula misses, in plain English: it assumes a single dose absorbed instantly, doesn't model food in the stomach, uses population averages for distribution and elimination, and ignores medications and hydration. The 1932 paper acknowledged most of these limitations; modern research has only added to the list.
Full formulas and citations live on the methodology page.
BAC calculator: questions people ask
What is BAC?
Blood alcohol concentration, the amount of alcohol in your bloodstream, usually expressed as grams of alcohol per 100 mL of blood (g/dL, or equivalently as a percentage). It rises while you are drinking and falls as your liver processes the alcohol.
How does this BAC calculator work?
It uses the Widmark formula from 1932, which is still the basis for most BAC estimators. The formula takes your body weight, sex, the amount of pure alcohol consumed, and the hours since you started drinking, and outputs a rough concentration. It does not account for food, medications, hydration, individual metabolism, or drink composition.
Why are BAC estimates unreliable?
Because real biology is much messier than the formula. Absorption depends on whether you ate, how concentrated the drinks were, and your stomach contents. Elimination varies between people by a factor of two or more. Stated inputs (drinks, time, weight) have their own error. Two people with identical inputs can have substantively different actual BACs.
Should I use this calculator to decide whether to drive?
No. This estimator is for general education only. It is not a measurement, it cannot account for the variables that matter most, and the error bar around any single estimate is wide. If you have had any alcohol, do not drive.
How long does it take alcohol to leave your system?
On average, your liver processes about 0.015 g/dL of BAC per hour , roughly one US standard drink an hour, though that varies. A BAC of 0.08 takes about 5 to 6 hours to fall to 0.00. Sleep and time are the only reliable mechanisms; coffee, cold showers, and food do not speed elimination.
Why does the calculator ask about sex?
The Widmark formula uses a sex-based distribution constant (about 0.68 for men, 0.55 for women) to approximate average differences in body water composition. This is a known oversimplification - individual biology varies more than the binary suggests. Treat the output as a rough estimate either way.