BAC Calculator
Estimate blood alcohol content from what you drank and when, using the Widmark equation with absorption and elimination modelled minute by minute. Educational only — never a green light to drive.
Never use this to decide whether to drive
The Widmark equation is a population average. Real blood alcohol varies by 20–30% between people with identical inputs, and no calculator can measure yours. If you have been drinking, do not drive — regardless of what any number on this page says.
Alcohol consumed
28 g
2.0 US standard · 3.5 UK units
Estimated peak BAC
0.031%
Mildly affected
Back under 0.03%
21:50
1 h 50 min after the first drink
Back to 0.00%
23:50
3 h 50 min after the first drink
At the peak (21:44) — Mildly affected
Relaxation and slight loss of judgement. Already illegal in some countries.
Drinks
Widmark factor r = 0.68
Stomach contents
0.015 is the usual average. Occasional drinkers sit nearer 0.012, heavy drinkers nearer 0.020.
What this calculator can and cannot tell you
It can show you the shape of alcohol in the body: how fast it rises, how long the peak lasts, and the fact that clearing it takes far longer than most people expect. That shape is genuinely useful for planning an evening, and it is the reason the "back to 0.00%" figure is usually the most surprising number on the page.
It cannot tell you your actual blood alcohol. Studies comparing Widmark estimates against measured BAC routinely find errors of 20–30% in either direction, and outliers well beyond that. Two people of identical sex and weight, drinking the same thing at the same time, can land at meaningfully different levels.
So treat every number here as an illustration, not a measurement. There is exactly one safe rule after drinking, and it does not involve arithmetic: do not drive.
The Widmark equation
Erik Widmark published the model in 1932 and it is still the basis of forensic BAC estimation. The idea is simple: alcohol distributes into the water in your body, so concentration is the dose divided by the volume it spreads through, minus whatever the liver has already removed.
- A — grams of pure alcohol consumed. Volume × ABV × 0.789, the density of ethanol.
- W — body weight in grams.
- r — the distribution ratio, the fraction of body mass that behaves like water. Conventionally 0.68 for men and 0.55 for women.
- β — the elimination rate, about 0.015 percentage points per hour.
- t — hours since drinking started.
This page does not apply the formula in one shot. Because elimination is a flat rate rather than a percentage, overlapping drinks cannot be summed in closed form — so the curve is integrated a minute at a time, with each drink absorbing gradually and the liver subtracting its fixed share throughout.
Why elimination is the slow part
Most substances leave the body by first-order kinetics — a fixed fraction per hour, which produces a half-life. Alcohol does not. Above very low concentrations the enzyme that breaks it down, alcohol dehydrogenase, is fully saturated, so the liver clears a fixed amount per hour no matter how much is there. That is zero-order kinetics, and it is why the downslope on the chart is a straight line rather than a curve.
The practical consequence: about one standard drink per hour, and no way to speed it up. Coffee, cold showers, exercise, and water change how awake you feel while changing your BAC by nothing at all. A 0.16% peak takes roughly eleven hours to reach zero — which is how people end up still over the limit on the morning commute.
The rate is not identical for everyone. Occasional drinkers sit nearer 0.012 %/h, while regular heavy drinkers induce the enzyme system and can reach 0.020 %/h or more. Sex, liver health, and genetics all move it, which is why the slider exists.
What the model leaves out
- Body composition, not just weight. The r factor assumes an average build. Alcohol does not dissolve in fat, so at the same weight a leaner person has more distribution volume and a lower peak. Sex differences in BAC are mostly this, not a difference in metabolism.
- Food changes the peak, not the total. A full stomach slows gastric emptying and can cut peak BAC by 20–30%, but the same alcohol still has to be cleared. The stomach setting here stretches absorption; it does not remove anything.
- First-pass metabolism. Some alcohol is broken down in the stomach wall before it reaches the blood. It is more pronounced in men and with slow drinking, and Widmark ignores it entirely.
- Medication and health. Many drugs interact with alcohol metabolism or multiply its effects. Liver disease can change clearance dramatically.
- Real pour sizes. A "glass of wine" poured at home averages substantially more than 150 ml, and a home spirit measure is typically well over a bar shot. Under-reporting the pour is the single largest source of error in practice.
Standard drinks and legal limits differ by country
One "standard drink"
- United States — 14 g of pure alcohol
- United Kingdom — 8 g (one "unit")
- Australia — 10 g
- Japan — 20 g (one "drink")
- Ethanol density: 0.789 g/mL
Driving limits (BAC %)
- 0.08 — US, England & Wales, Canada (criminal)
- 0.05 — most of the EU, Australia, Scotland
- 0.03 — South Korea, Japan
- 0.02 — Sweden, Norway, China, Poland
- 0.00 — Czechia, Hungary, Brazil, and for novice drivers almost everywhere
Note that impairment starts well below every one of these numbers. Measurable declines in reaction time and divided attention show up from around 0.02% — the limit is a legal threshold for prosecution, not a line where driving becomes safe.
This page is an educational model of the Widmark equation. It is not medical, legal, or safety advice, and it must never be used to judge fitness to drive or operate machinery. If someone is unresponsive, breathing irregularly, or cannot be roused after drinking, treat it as alcohol poisoning and call emergency services.