Blood Alcohol Content Calculator
An estimate only. Individual variation is large enough that no calculator can tell you whether you are safe or legal to drive.
The Widmark formula
Erik Widmark published this relationship in 1932 and it remains the basis of forensic alcohol calculation:
The r factor is the volume of distribution: alcohol distributes into body water, not into fat. Women average a higher proportion of body fat and less body water, so the same quantity of alcohol reaches a higher concentration — which is the physiological reason behind different drinking guidelines, not a social convention.
The 0.015 % per hour is the average elimination rate. The liver metabolises alcohol at a roughly constant rate through alcohol dehydrogenase, which is why nothing speeds it up. Coffee, cold showers, exercise and food all affect how you feel; none affects the rate.
Why the estimate can be badly wrong
Widmark's formula was derived from population averages, and the individual variation around them is large. Factors that shift the real figure and that no calculator can see:
- Food. Drinking on an empty stomach can raise peak BAC by 30 % or more, because gastric emptying controls how fast alcohol reaches the bloodstream.
- Body composition. The r factor assumes average build. A lean, muscular person has more body water than the formula assumes; someone with high body fat has less.
- Elimination rate. Real rates range from about 0.010 to 0.020 % per hour, and habituated drinkers clear faster. That two-fold spread alone can double or halve the time to zero.
- Genetics, medication, illness and liver function all matter, sometimes a great deal.
- Drink sizes. A “glass of wine” poured at home is frequently two units.
Legal limits and the only safe rule
| Jurisdiction | Limit (BAC %) |
|---|---|
| UK (England, Wales, Northern Ireland) | 0.08 |
| Scotland, France, Germany, Italy, Spain, Australia | 0.05 |
| United States, Canada | 0.08 |
| Sweden, Norway, Poland | 0.02 |
| Czechia, Hungary, Romania | 0.00 |
| Novice and professional drivers (most countries) | 0.00 – 0.02 |
Do not use this to decide whether to drive. The estimate carries an error margin wide enough to place you either side of a legal limit, and impairment of reaction time and judgement begins well below every limit in the table. If you have been drinking, do not drive. That is the only advice this page will give.
Frequently asked questions
Can anything speed up sobering up?
No. Elimination proceeds at a roughly fixed rate set by liver enzyme capacity. Coffee, cold showers, food, exercise and sleep change how alert you feel while leaving blood alcohol exactly where it was — which makes them actively dangerous, because they create a false impression of sobriety.
Why can I still be over the limit the next morning?
Simple arithmetic. Ten units finished at 1am produces a BAC around 0.15 % in an average man; at 0.015 % per hour that takes roughly ten hours to clear, so 11am. Morning-after drink-driving convictions are common for exactly this reason.
What counts as one standard drink?
It varies by country, which is a frequent source of error. This calculator uses 10 g of pure alcohol, the UK and Australian unit. The US standard drink is 14 g and Japan's is 20 g. A 175 ml glass of 13 % wine is about 2.3 UK units, not one.