BAC calculator

Almost every BAC calculator uses the Widmark equation. Take the grams of pure alcohol you drank, divide by your body weight multiplied by a distribution ratio that represents how much of you is water, and subtract the alcohol your liver has cleared since: roughly 0.015 percentage points per hour. In symbols, BAC = A / (r × W) − β × t. The inputs are drinks, weight, sex, and time, which is why every calculator asks for those four things and no more.

Not accurate enough to act on. The two constants in the formula vary widely between people: the distribution ratio runs from about 0.49 to 0.76 across healthy adults and the elimination rate from 0.012 to 0.020 percentage points per hour. Propagated honestly, that puts the edges of a BAC estimate roughly 25 percent apart, and the edges of a sober-by time a factor of two apart. On top of that the formula cannot know your stomach contents, your liver, your medications, or what the bar actually poured. A calculator that answers with a single number has not solved that spread; it has hidden it.

For many adults, three to four US standard drinks over a couple of hours will do it, and for a smaller person two can be enough. But the honest answer is a range rather than a count: at 60 kg, three standard drinks put a woman’s estimate somewhere around 0.11 to 0.14 percent, while in a 100 kg man the same three drinks land near 0.06 to 0.07. Body size, sex, timing, and how quickly you drank all move it, which is exactly why counting drinks is a poor way to judge whether you are over a limit.

Your liver clears alcohol at a near-constant rate of roughly 0.015 percentage points of BAC per hour, close to one US standard drink per hour, with a published range of 0.012 to 0.020. Four standard drinks in a 70 kg woman peak somewhere around 0.13 to 0.16 percent and take roughly 7 to 14 hours from the last drink to reach zero. Nothing shortens that: the rate is set by enzymes, not by effort.

Take your estimated peak and divide by the elimination rate. At the average 0.015 percentage points per hour, a peak of 0.12 takes about eight hours to reach zero from the last drink, and the honest range across the published rates of 0.012 to 0.020 puts it somewhere between six and ten. The calculator above does this for your own night and shows the time to zero as a window rather than a single clock time. Nothing shortens it, and the count starts at your last drink rather than at bedtime.

Food eaten before or while drinking slows absorption and lowers the peak, which is real and worth doing. Nothing lowers BAC once the alcohol is in your blood. Water helps the hangover and not the number, coffee makes you an alert person with the same blood alcohol, and cold showers make you a cold one. Only time removes alcohol, at a rate you do not control.

No, and neither can any other calculator. An estimate built from population averages cannot measure your blood, the margin of error straddles the legal limit for most realistic nights, and impairment of reaction time and judgment is measurable well below every legal limit in the world. Never drive after drinking. If you are wondering whether you are sober enough to drive, the answer is no.

Because the underlying science is a range. The Widmark equation needs two constants that differ from person to person, and published research gives both as intervals rather than values. Propagating those intervals honestly produces a band, and the width of that band is the most useful thing on the screen: it tells you how much the estimate could be wrong by. The high edge is the one to plan around.

It is the same measurement in a different unit. The US writes BAC as a percentage by weight, so 0.08 percent means 0.08 grams of alcohol per 100 millilitres of blood. Continental Europe uses promille, grams per litre, which is exactly ten times the US figure: the Dutch limit of 0.5 promille is a BAC of 0.05. The UK quotes milligrams per 100 millilitres, so its 80 mg limit is again 0.08.