What this estimator does, and what it cannot do
This estimator applies the Widmark equation to show, in textbook terms, how a number of units relates to blood alcohol concentration and how that falls over time.
It cannot tell you whether you are safe or legal to drive. Individual variation in absorption, distribution and elimination is very wide, the equation ignores food, timing and most of the things that matter, and no calculation can establish where you sit against a legal limit. If you have been drinking, do not drive.
Enter your details
Select Calculate to see where an estimate falls against the UK limits.
- Below both UK limits 0 to 50
- Above the Scottish limit 50 to 80
- Above every limit in the UK 80 to 150
Before you read the result
- Calculate your units first with the Alcohol Unit Calculator. Volume and strength both matter.
- The body water factor is part of the equation, not a statement about you — it reflects average differences in body composition.
- The result assumes all the alcohol has been absorbed, which takes 30 to 90 minutes and is slower with food.
- Elimination is assumed at a fixed 15 mg/100ml per hour. Real rates range roughly from 10 to 20.
Coffee, cold showers, food, exercise and fresh air change how alert you feel and do nothing to how fast alcohol leaves the blood. The liver processes it at a broadly fixed rate. Feeling more awake while still over a limit is the specific danger this misconception creates.
How this calculator works
The Widmark equation, with a linear elimination term:
Grams of alcohol = units × 8
Peak BAC (mg/100ml) = grams ÷ (weight in kg × r) × 100
Current BAC = peak − (hours × 15)
where r is 0.68 for men and 0.55 for womenThe r factor is the proportion of body mass in which alcohol distributes, which is essentially body water. It differs between sexes on average because lean tissue holds more water than fat tissue.
The model treats absorption as instantaneous and elimination as perfectly linear. Neither is true. Real curves rise over 30 to 90 minutes, and elimination varies with liver function, regular drinking, medication and genetics.
Worked example: 4 units at 75 kg
Using the default figures — 4 units, 75 kg, a body water factor of 0.68, two hours elapsed:
- Grams of alcohol: 4 × 8 = 32 g
- Peak: 32 ÷ (75 × 0.68) × 100 = 63 mg/100ml
- After 2 hours: 63 − 30 = 33 mg/100ml
- Hours to reach zero from peak: 4.2
For context, the drink-drive limit is 80 mg/100ml in England, Wales and Northern Ireland, and 50 mg/100ml in Scotland. The figures above sit below both — and that is precisely the reasoning this page exists to discourage. Change the body water factor to 0.55 and the peak becomes 78 mg/100ml, a whisker under the England limit and well over the Scottish one, from identical drinks. A model this sensitive to one input cannot clear anyone to drive.
Common mistakes
- Using it to decide whether to drive. It cannot answer that question, and treating it as though it can is dangerous.
- Assuming elimination continues overnight at a convenient rate. Morning-after driving is a common cause of drink-drive offences.
- Entering drinks instead of units. A large glass of wine is over two units.
- Ignoring absorption time. BAC continues to rise for up to 90 minutes after the last drink.
- Treating the equation as personal. It is a population average with very wide individual variation.
Frequently asked questions
Why can this not tell me if I am under the limit?
Because the inputs it lacks matter more than the ones it has. Food in the stomach can halve peak concentration. Elimination rates vary by roughly a factor of two between individuals. Absorption is still happening for up to 90 minutes after the last drink. Medication, illness, sex, age and regular drinking all shift the picture. The only reliable measurement is an evidential breath, blood or urine test, and the only safe approach is not to drive after drinking.
What are the UK drink-drive limits?
In England, Wales and Northern Ireland the limit is 80 mg of alcohol per 100 ml of blood, 35 micrograms per 100 ml of breath, or 107 mg per 100 ml of urine. Scotland has lower limits of 50 mg blood, 22 micrograms breath and 67 mg urine. Being under a limit is not the same as being safe to drive; impairment begins well below it.
How long until I am clear the next morning?
Longer than most people assume. At roughly one unit an hour, ten units finishing at midnight would not clear until around 10am, and that is from the point absorption completed rather than from the last drink. Morning-after offences are common precisely because sleeping feels like it should have resolved it.
Does the type of drink matter?
Only through its unit content. A unit of alcohol is a unit whether it arrives as beer, wine or spirits. Carbonated drinks may be absorbed slightly faster, and drinking on an empty stomach raises peak concentration considerably, but the total alcohol is what drives the arithmetic.
Where can I get advice about my drinking?
Drinkaware runs a free confidential helpline and self-assessment tools. The NHS website can direct you to local alcohol services and a GP can refer you. If you drink heavily every day, seek medical advice before stopping — withdrawal can be dangerous without support.
Related tools
References
- GOV.UK — the drink-drive limits for England, Wales, Northern Ireland and Scotland
- NHS — alcohol guidance, self-assessment and support services
Sources are checked at publication and can change — how I choose and check references.