How the BAC estimate is calculated
The Widmark formula uses alcohol mass and an average body-water factor. It subtracts an estimated clearance of 0.015 percentage points per hour.
Alcohol grams = volume in ml × ABV ÷ 100 × 0.789 × count; BAC % = max(0, alcohol grams ÷ (weight in grams × r) × 100 − 0.015 × hours); r = 0.68 for male or 0.55 for female.
- Convert body weight to kilograms, then grams. One pound is exactly 0.45359237 kg.
- Multiply standard drinks by their grams of alcohol. In volume mode, multiply each drink's ml by its ABV fraction, 0.789 g/ml and the count.
- Divide alcohol grams by weight in grams times the body-water factor, then multiply by 100.
- Subtract 0.015 times hours since the first drink. Floor a negative estimate at zero.
- Divide the remaining BAC percentage by 0.015 to estimate additional hours until about zero, assuming no further drinking. This is not a driving clearance time.
Worked example: A male adult weighing 70 kg consumes 4 standard drinks of 10 g each. After 1 hour, the estimate is 40 ÷ (70,000 × 0.68) × 100 − 0.015 = 0.0690%. The model estimates about 4 h 36 min more until about zero, but this does not mean driving will be appropriate then.
Example BAC estimates for adults
| Sex for the formula | Weight (kg) | Drinks | Alcohol per drink | Elapsed hours | Estimated BAC | Total alcohol | Estimated time until about 0.00% |
|---|---|---|---|---|---|---|---|
| Male | 80 kg | 2 | 10 g (Australia/NZ) | 1 h | 0.0218% | 20 g | about 1 h 27 min |
| Female | 60 kg | 2 | 10 g (Australia/NZ) | 1 h | 0.0456% | 20 g | about 3 h 2 min |
| Male | 70 kg | 4 | 14 g (US) | 2 h | 0.0876% | 56 g | about 5 h 51 min |
| Female | 55 kg | 1 | 14 g (US) | 0 h | 0.0463% | 14 g | about 3 h 5 min |
| Male | 90 kg | 6 | 10 g (Australia/NZ) | 3 h | 0.0530% | 60 g | about 3 h 32 min |
Swipe the table sideways to see the rest.
All weights are in kg. These are model estimates, not measured BAC or permission to drive.
Things to consider
- A serving is not necessarily one standard drink. Check the alcohol grams or volume and ABV.
- For mixed drink types, add their alcohol grams and use custom grams with a count of 1.
- BAC may still be rising after the last drink. A displayed zero does not prove alcohol is absent.
- Coffee, showers and water do not speed up alcohol clearance.
- Call local emergency services for suspected alcohol poisoning, regardless of this estimate.
Frequently asked questions
How is BAC calculated using Widmark?
The formula divides alcohol mass by body mass times an average distribution factor, then accounts for elapsed time. This page uses r = 0.68 for male and 0.55 for female, and subtracts 0.015 percentage points per hour. It assumes a simplified distribution and clearance process, so it cannot capture absorption delays or individual differences.
Why do men and women have different estimates?
The formula uses different average body-water distribution factors. The same alcohol amount and weight therefore give different estimates. These averages do not predict an individual's body composition, metabolism or BAC.
How long does alcohol take to leave the body?
This model uses about 0.015% BAC per hour, but actual clearance varies. The time shown is additional time from the current estimate, assuming no more drinking. Coffee, showers and water do not speed it up. Neither the countdown nor an estimated zero can establish fitness to drive.
What is a standard drink in different countries?
It measures pure alcohol, not a glass size. A UK unit is 8 g; an Australia/NZ standard drink is 10 g; a US standard drink is 14 g. Other definitions include 12 g. Choose the definition matching your alcohol count, or enter custom grams. Fluid ounces also differ: a US fl oz is 29.5735295625 ml and an imperial fl oz is 28.4130625 ml.
Do legal BAC limits vary by country?
Common legal driving limits range from 0.00% to 0.08% depending on country and licence type. Some drivers must have zero alcohol. Check current local rules, but do not use this estimate to decide whether to drive.
Why can't this tell me if I am fit to drive?
Food, medication, health, drinking speed and individual metabolism affect actual BAC and impairment. The model does not measure blood alcohol, and impairment can occur below a legal threshold. If you've been drinking, don't drive. Arrange another way home.
Results are estimates for general information only, not financial, medical or professional advice.