BAC Calculator
Estimate blood alcohol content using the Widmark formula.
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About the BAC Calculator
Blood Alcohol Concentration (BAC) measures the proportion of alcohol in your blood, expressed as grams of ethanol per 100 millilitres of blood (g/dL), often written as a percentage. A BAC of 0.08% means 0.08 grams of alcohol per 100 ml of blood. The Widmark formula — developed by Swedish physiologist Erik Widmark in the 1930s and still the foundation of forensic BAC estimation — calculates this from the amount of alcohol consumed, body weight, biological sex, and elapsed time. The formula is the same used in courtrooms and by police breath-test calibration.
Alcohol is metabolised primarily by the liver at a roughly constant rate of 0.015% BAC per hour (about one standard UK unit per hour in a 70 kg person). This rate varies between individuals but is largely unaffected by coffee, water, food eaten after drinking, exercise, or cold showers — common myths notwithstanding. Food eaten before or during drinking does slow absorption from the stomach into the bloodstream, which delays the BAC peak and can reduce peak BAC. But once alcohol is in the blood, only time removes it.
The UK drink-drive limit is 80 mg of alcohol per 100 ml of blood (0.08% BAC) in England, Wales, and Northern Ireland — one of the highest legal limits in Europe. Scotland lowered its limit to 50 mg/100 ml (0.05%) in 2014, aligning with most of Europe. Many road safety experts advocate lowering the UK limit to match Scotland and Europe, citing research showing measurably increased crash risk at 0.05%+. This calculator is provided for educational awareness only — the only safe approach to driving is zero alcohol.
How it works
BAC = (drinks × 14 × 0.6) / (weight_g × r) − (0.015 × hours) r = 0.68 for males, 0.55 for females BAC is expressed as grams per 100 mL (percentage)
Where
drinksNumber of standard drinks consumed (1 drink = 14g pure alcohol = 1 pint of 4% beer or 1 medium glass of wine)14 × 0.614g alcohol per drink × 0.6 ethanol conversion factorweight_gBody weight in grams (body weight in kg × 1000)rWidmark distribution factor: 0.68 for males (more body water), 0.55 for females0.015 × hoursAlcohol metabolised: approximately 0.015% BAC per hourWorked example
Person: 75 kg male, consumed 4 pints of 4% beer over 3 hours.
Standard drinks: 4 pints × 4% / standard 4% = 4 drinks (each pint of 4% beer ≈ 1 standard drink).
BAC at 3 hours = (4 × 14 × 0.6) / (75,000 × 0.68) − (0.015 × 3)
= 33.6 / 51,000 − 0.045
= 0.000659 − 0.045 → this gives BAC in g/mL, multiply by 100 for percentage.
Correctly: = (4 × 14 × 0.6) / (75 × 0.68) − (0.015 × 3) [weight in kg, result in g/100mL]
= 33.6 / 51 − 0.045 = 0.659 − 0.045 = 0.614% — well over the UK limit of 0.08%.
Time to reach 0.08%: (0.614 − 0.08) / 0.015 ≈ 35.6 more hours after the 3-hour mark.
Tips to improve your result
- 1.
The "morning after" risk is real and widely underestimated. A heavy night of drinking (10+ units) that ends at midnight may leave you over the drive limit until well into the following afternoon. If you drank heavily, do not drive the next morning without calculating a conservative estimate — or simply don't drive.
- 2.
The UK unit system: 1 unit = 10 ml (8g) of pure alcohol. A pint of 4% beer = 2.3 units; a 250 ml glass of 13% wine = 3.25 units; a 25 ml measure of 40% spirit = 1 unit. The NHS recommends no more than 14 units per week for adults, spread over at least 3 days.
- 3.
Women typically reach higher BAC than men of the same weight for the same amount of alcohol because of the lower Widmark r factor (0.55 vs 0.68). This reflects the higher proportion of body fat vs body water in females on average — alcohol distributes in body water, not fat.
- 4.
This calculator assumes average metabolism. Actual rates vary by 20–30% due to genetic differences in alcohol dehydrogenase enzyme activity, liver health, medications (many drugs inhibit alcohol metabolism), and habitual drinking patterns (heavy drinkers develop tolerance through enzyme upregulation but not through faster metabolism).
- 5.
Breath analysers measure alcohol in exhaled air and convert to estimated blood BAC using a fixed blood-to-breath ratio of 2,300:1. Actual ratios vary between individuals (1,700:1 to 2,800:1), which means some individuals are systematically over- or under-estimated by breath tests — a source of legal challenge in drink-drive cases.