What the Alcohol BAC Calculator does
This calculator estimates your blood alcohol concentration (BAC) with the Widmark formula from what you drank, its strength, your body weight and the hours since your first drink, and shows roughly how long it will take to fall to zero. It is an estimate from population averages - never a test - and it must never be used to decide whether you can drive. If you have been drinking, do not drive.
Every assumption is on the page: the Widmark factor r (0.68 for men and 0.55 for women, Widmark's own averages), an elimination rate of 0.015% per hour, and ethanol's density of 0.789 g per ml. You can change r if you know a better value. Your weight and drinks stay in your browser; nothing is remembered unless you tick the box.
How to use it
- Enter your body weight in kilograms or pounds and choose the body type the formula should use.
- Enter the hours since you started drinking.
- Add each kind of drink: pick a common size such as a pint, a can, a glass of wine or a peg, or choose Other and type the size in ml and the strength in % ABV from the label. Set how many you had.
- Read the estimated BAC, the rough time until it reaches zero, and how many standard drinks and units you had.
- Treat the time to zero as the earliest it could happen. Many people take longer.
Reading the results
BAC is shown as a percentage (grams of alcohol per 100 ml of blood), the US convention, and also in milligrams per 100 ml (the UK and India) and grams per litre (much of Europe). 0.08% is the same as 80 mg per 100 ml and 0.8 g/L.
The "peak" is what the formula gives if all the alcohol were absorbed instantly. In reality absorption takes 30 to 90 minutes and is slower after food, so soon after drinking the real level may still be rising.
The level description is a general guide to typical effects. Impairment of judgement and reaction time begins well below any legal limit, and some people are affected far more than the average.
Worked example: three pints of 4% beer, an 80 kg man, two hours after the first
A UK pint is 568 ml. At 4% ABV that is 22.72 ml of ethanol, and at 0.789 g per ml, 17.93 g of alcohol. Three pints are 53.78 g.
Widmark: 53.78 / (80,000 g x 0.68) x 100 = 0.0989%. Two hours of elimination at 0.015% per hour removes 0.030%, leaving an estimated 0.069% - about 69 mg per 100 ml.
At 0.015% per hour it takes 0.069 / 0.015 = 4.6 more hours to reach roughly zero, and that is the earliest it could happen. The three pints are 6.7 UK units by mass, or about 3.8 US standard drinks of 14 g.
Formulas and scoring rules
- Grams of alcohol
A = volume (ml) x ABV / 100 x 0.7890.789 g/ml is the density of ethanol at 20 C.- Widmark estimate
BAC (%) = A / (body weight in g x r) x 100 - 0.015 x hoursr = 0.68 (male) or 0.55 (female) by default; never below zero.- Time to roughly zero
hours = BAC now / 0.015- Unit conversions
mg per 100 ml = % x 1,000; g/L (per mille) = % x 10- Standard drinks
US = A / 14 g; UK units = A / 8 g; Australia = A / 10 g
Why your real blood alcohol can differ a lot
The Widmark factor r varies from person to person with body fat and water: published values range from about 0.5 to 0.9. Elimination varies too, roughly 0.010% to 0.025% per hour, and is affected by liver health, medication and how often you drink. Eating slows absorption; drinking quickly, fizzy mixers and an empty stomach speed it up.
Put together, two people of the same weight drinking the same amount can have very different levels, and the same person can differ from one evening to the next. That is why this page gives an estimate and the warning to never drive after drinking - not a number to plan around.
Drink-drive limits differ - and they are not a safe level
Legal limits vary: 0.08% (80 mg per 100 ml) in most of the United States and in England and Wales, 0.05% in Scotland and much of Europe and Australia, and 30 mg per 100 ml in India under the Motor Vehicles Act. Many countries set lower or zero limits for new or professional drivers. Being under a limit does not mean you are unimpaired, and police and courts rely on breath or blood tests, never on a formula.
Limitations: what the result does not prove
- It is not medical advice and not a breathalyser. It cannot tell you that you are sober, safe or under any limit.
- It assumes all alcohol is absorbed at once and eliminated at a constant average rate. Real absorption and elimination differ between people and occasions.
- Pregnancy, medication, illness, age and alcohol tolerance are not modelled.
- Very high estimates mean danger, not accuracy: if someone is confused, vomiting, breathing slowly or cannot be woken, call emergency services.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- NHTSA - Drunk driving and blood alcohol concentration - checked 19 Sep 2026
- NIAAA - What is a standard drink?
- NHS - Alcohol units
- GOV.UK - The drink drive limit
- Searle J. - Alcohol calculations and their uncertainty (Medicine, Science and the Law, 2015)
Frequently asked questions
How long does it take for alcohol to leave your body?
On average the body removes about 0.015% BAC per hour, roughly one UK unit an hour for many adults, but the rate varies widely. The calculator divides your estimated BAC by 0.015 to give the earliest likely time to reach zero; many people take longer.
How is blood alcohol calculated with the Widmark formula?
Grams of alcohol are divided by body weight in grams times the Widmark factor r, multiplied by 100, and the alcohol eliminated since the first drink is subtracted. All the constants used are shown under the result.
How many units are in a pint of beer?
A UK pint of 4% beer contains 22.7 ml of pure alcohol, which is 2.3 units by the 10 ml definition. Stronger beers carry more: a pint at 5.2% is about 3 units. The calculator also shows the total in US standard drinks of 14 g.
Can this calculator tell me if I am safe to drive?
No. It is a rough estimate from average values and your real level can be much higher. Impairment starts below legal limits, and only a breath or blood test measures BAC. If you have been drinking, do not drive.
Why do women reach a higher BAC than men from the same drinks?
On average women have a higher share of body fat and less body water at the same weight, so alcohol is more concentrated. Widmark's averages reflect this with r = 0.55 for women and 0.68 for men; you can enter your own value.
Does eating food lower my blood alcohol?
Food slows absorption, so the peak is lower and later, but it does not remove alcohol you have already drunk; elimination still happens at roughly the same rate. The formula here ignores food, which is one reason it is only an estimate.
What does a BAC of 0.08% mean?
It means 0.08 grams of alcohol per 100 ml of blood, the same as 80 mg per 100 ml or 0.8 grams per litre. It is the legal driving limit in many US states and in England and Wales; lower limits apply in many other places.
Last reviewed by the A2Z.Tools team against the sources listed above.
