Calculate blood alcohol

Estimate a Widmark blood-alcohol percentage from UK alcohol units while showing, but not treating as a safety threshold, the two UK blood limits.

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ToolBlood Alcohol Calculator
Input
Output
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Preview

How it works

Each UK unit becomes 8 grams of ethanol, which is divided by body weight and a 0.68 or 0.55 Widmark distribution factor. A fixed 0.015 percentage point is removed for every elapsed hour and the value stops at zero; it is then compared with 0.08% for England, Wales, and Northern Ireland and 0.05% for Scotland.

Formula
estimated BAC % = alcohol grams ÷ (10 × distribution factor × body weight kg) − 0.015 × hours
  • Elapsed time defaults to 0, so no elimination is subtracted unless a duration is entered.
  • No countdown to sobriety or a legal threshold is returned because the fixed model cannot determine when someone is safe to drive.

Worked example

3 units, 80kg male, no time elapsed
A quick estimate straight after drinking
Input
											Alcohol units: 3
Body weight kg: 80
Sex: male
Hours elapsed: 0
Output
												Blood alcohol percent: 0.044
Uk drink drive limit percent: 0.08
Scottish drink drive limit percent: 0.05
Over uk limit: no
Over scottish limit: no

When to use this

Alcohol education converts UK units to ethanol grams, forensic toxicology attaches uncertainty to Widmark reconstructions, and UK road-safety records compare measured blood samples with the separate statutory limits.

Edge cases

  • Alcohol still being absorbed is nevertheless treated as fully distributed while the fixed elimination subtraction starts at the beginning of the session.
  • Food, age, medicines, body composition, and individual metabolism receive no adjustment; the selected distribution factor and 0.015 rate remain fixed.
  • An estimate below a displayed limit does not establish fitness to drive, so the page deliberately provides no time-until-sober or time-until-limit answer.

References