How BAC Estimates Work (And Why They Are Not a Safety Tool)
Start with the part that matters: no calculator can tell you whether you are safe or legal to drive. If you have been drinking, do not drive. The BAC Estimator exists to show how the arithmetic works and how wide its error is - that is all it is for.
The Widmark formula
Published by Erik Widmark in 1932 and still the basis of most estimates. It takes the mass of alcohol consumed, divides by body weight multiplied by a distribution factor, and subtracts an elimination term for time elapsed.
The distribution factor reflects that alcohol spreads through body water rather than through fat - conventionally about 0.68 for men and 0.55 for women, because body composition differs on average. Elimination is treated as roughly 0.015 per cent per hour.
So the inputs are: how much alcohol, how much you weigh, your sex, and how long since you started. That is all the formula knows about you.
What it cannot see
This is why the output is so approximate.
Food. Drinking on an empty stomach produces a far higher peak than the same drinks with a meal, because the stomach empties faster. The formula has no input for it.
Elimination rate, which varies between individuals by a factor of roughly two, with genetics, liver function, medication and regular drinking all contributing. The standard 0.015 is a population average.
Body composition beyond a sex-based constant. Two people of the same weight with different body fat have different distribution volumes.
What you actually drank. A home-poured spirit is routinely one and a half to two measures, and wine glasses vary enormously. Misjudging the drinks is often the biggest error in the whole calculation.
Medication and illness, some of which change alcohol handling substantially.
Put together, a realistic error band is wide enough that the same inputs could describe someone over the limit and someone under it. That is not a flaw to be refined away; it is inherent to estimating from four inputs.
Why time is the only thing that helps
Alcohol leaves at roughly a fixed rate, and nothing speeds it up. Coffee, cold showers, food after the fact, exercise and water do not change elimination - they change how awake you feel, which is worse than useless, because it separates feeling sober from being sober.
The practical consequence people underestimate is the morning after. A heavy evening ending at 1am can leave measurable alcohol at 9am, which is how people are over the limit on the way to work without feeling drunk at all.
Common questions
How long until I am sober?
Roughly one hour per standard drink, and that is an average with wide variation. Only time works.
Does the limit differ by country?
Considerably - 0.08 per cent in England and the US, 0.05 in Scotland and much of Europe, near zero in others and for new drivers. Check where you are.
Can I rely on a breathalyser app?
No. Phone apps do not measure anything; they estimate the same way the formula does.
What if I only had one drink?
Then you are probably fine and you still cannot know from a calculator. If driving is involved, the only reliable answer is not to.
Want to see how the arithmetic works?