calcs.vodka /uk/
Nation Scotland's limit is lower — 50 mg/100mL blood (22 µg/100mL breath) versus 80 mg/100mL (35 µg/100mL breath) in England, Wales and Northern Ireland.
United KingdomTier 3

BAC calculator

Weight, sex, alcohol units and time elapsed in. An estimated blood alcohol content out, checked against your nation's legal limit.

Free·No sign-up·Last checked 2026-08-27
kg
Sex
Sets the Widmark distribution ratio, r
8 g ethanol each — see the alcohol units calculator
hr
Estimated BAC
81.7mg/100mL blood
Over the 80 mg/100mL limit
0 mg/100mL80 mg/100mL limit200 mg/100mL
Breath limit (this nation): 35 µg/100mL breath
Novice: No separate novice limit
Commercial: 20 mg/100mL (rail/transport workers)
Hours until 0 mg/100mL
5.4hr
Hours until under the limit
0.1hr
Estimate only
Estimate only. Many factors affect BAC. Do not use to decide whether to drive. Not legal or medical advice.
Formula — Widmark
Alcohol (g) = units × 8 BAC (mg/100mL) = (alcohol_g ÷ (weight_g × r)) × 100,000 − (15 × hours) weight_g = weight_kg × 1000. r = 0.68 (male) or 0.55 (female).
Worked example. 73 kg male, 6 units, 1 hour → 48 g alcohol ÷ (73,000 g × 0.68) × 100,000 − 15 = 81.7 mg/100mL.
Method
Widmark formula, population-average distribution ratio
r (male)
0.68
r (female)
0.55
Elimination rate
15 mg/100mL per hour
These are the values from Widmark's original research. Several other published calculators use 0.73/0.66 instead — a real disagreement in the literature, not a typo. Either constant set gives an estimate, not a measurement. The breath figures shown are the statutory pairs set in law, not calculated from the blood estimate.
Sources
  1. 01GOV.UK — The drink-drive limit
  2. 02NHS — Alcohol units
  3. 03Drinkaware — Drink driving and the legal limit

Questions

It's a population-average model. Food in the stomach, medications, individual metabolism, drink strength errors and the exact timing of drinks can all shift real BAC well away from the estimate in either direction.
Several r constants exist in the literature. This calculator uses 0.68 for men and 0.55 for women, values used by the original Widmark research; some other calculators publish 0.73/0.66 from a different source. The difference changes the estimate by several points, one reason to treat any single number as approximate.
Food slows how fast alcohol is absorbed, which can lower the peak BAC and spread it over more time, but it does not change the total amount of alcohol your body has to process.
Scotland lowered its drink-drive limit to 50 mg/100mL blood in 2014, while England, Wales and Northern Ireland kept the older 80 mg/100mL limit. Both use the same breath-testing principle but publish different statutory blood and breath figures.
No. It gives an estimate for reference only. The only reliable way to know your BAC is a breath or blood test, and the only way to guarantee you're under any limit is not to drive after drinking.