How blood alcohol is estimated, and why it is only a rough guess
This article is general information, not medical advice. It is not a substitute for a doctor, dietitian or other qualified professional who knows your circumstances. If you have a medical condition, are pregnant, or are changing your diet or exercise significantly, talk to one first.
Blood alcohol content (BAC) is the amount of alcohol in your blood, usually written as a percentage: 0.08% means 0.08 grams of alcohol in every 100 millilitres of blood, or 80 mg per 100 ml. Calculators estimate it with a formula published by the Swedish scientist Erik Widmark in the 1930s, using how much alcohol you drank, your body weight, your sex and how long ago you started. The answer is a rough average, not a measurement. The blood alcohol calculator uses the same formula, and this guide explains what goes into it and where it falls short.
The Widmark formula
The idea is simple. Alcohol spreads through the water in your body, so the same drink gives a higher concentration in a smaller body, or in one with less water. The formula has three parts:
- Grams of alcohol drunk. Volume × strength (ABV) × 0.789, the density of ethanol. A 175 ml glass of 13% wine holds about 18 g of alcohol; a pint of 4.5% beer about 20 g.
- Body weight × a distribution ratio. The ratio, often written r, reflects how much of your body the alcohol can dissolve into. Typical values are about 0.68 for men and 0.55 for women, because at the same weight women on average have more body fat and less body water.
- Elimination over time. The liver removes alcohol at a fairly steady rate, on average about 0.015% BAC an hour, and the formula subtracts that for every hour since drinking started.
Put together: peak BAC (%) = grams of alcohol ÷ (body weight in grams × r) × 100, minus 0.015 for each hour. The calculator also turns your total into UK units (8 g each) and US standard drinks (14 g each), the same sum the alcohol units calculator does for a single drink.
A worked example
Take a woman who weighs 60 kg and has two 175 ml glasses of 13% wine, starting two hours ago. That is about 36 g of alcohol. Divided by 60,000 g × 0.55, it gives a theoretical peak of roughly 0.11%; subtracting two hours at 0.015% leaves an estimate of about 0.08%. A 90 kg man drinking the same two glasses gets a peak of about 0.06% and an estimate near 0.03% after two hours. Same drinks, very different numbers. The calculator also shows roughly how many more hours it would take to reach zero: for the woman here, more than five.
Why real levels differ
The formula treats everyone as an average body, which is why the calculator warns that real levels can be up to about 50% higher or lower than the estimate. The main reasons:
- Food. On an empty stomach alcohol reaches the small intestine quickly, giving a higher, earlier peak. A meal slows absorption and lowers the peak.
- Absorption time. The formula assumes all the alcohol is absorbed at once. In reality BAC keeps rising for a while after the last drink, often for 30 to 90 minutes and longer after food, so you can feel fine when you stop and be more impaired an hour later.
- Body composition. The fixed ratios are averages; a very muscular person and a person with more body fat at the same weight distribute alcohol differently.
- Elimination rate. 0.015% an hour is a typical figure, but the real rate varies considerably between people, with genetics, liver health, regular heavy drinking and some medicines all playing a part.
- Drink size. Home pours and strong craft beers often hold more than the standard sizes.
How long alcohol takes to clear
Because the liver works at a roughly fixed pace, time is the only thing that lowers BAC. Coffee, cold showers, water and sleep do not speed it up. Caffeine can make you feel more alert without making you any less impaired, and the caffeine calculator shows how long that coffee stays in your system too. A useful rule from the UK unit system is that the average adult processes about one unit, 8 g of alcohol, an hour, though individuals vary. After a heavy evening it is common to still have alcohol in your blood the next morning. Alcohol also disrupts the second half of the night's sleep, which is one reason you feel worse than the hours in bed suggest; Sleep cycles, and waking up less awful covers how sleep is structured.
Legal limits and driving
Legal driving limits are set as blood or breath concentrations and differ by country: 0.08% in most US states and in England, Wales and Northern Ireland; 0.05% in Scotland, Australia and most of Europe; and lower or zero for new or professional drivers and in many other countries. In England and Wales the same limit is also set as 35 micrograms of alcohol per 100 ml of breath.
None of this means it is safe to drive below a limit. Reaction time and judgement are affected at lower levels, an estimate can be badly wrong in either direction, and you cannot reliably feel your own BAC. That is why the calculator shows a warning on every result, and why the only safe approach is not to drink at all if you are going to drive, including the morning after. Use an estimate to understand how alcohol behaves and how quickly drinks add up, never to decide whether you are fit to drive. If someone is very drunk and confused, vomiting, hard to wake or breathing slowly, call emergency services: these can be signs of alcohol poisoning.
Sources and further reading
The claims in this guide rest on these references, which were checked when the guide was last updated. Spotted an error? The contact page says how to report it.