Blood alcohol estimate

A rough idea of how much alcohol is in your blood and how slowly it leaves. There is no reliable way to know when you are under the limit, so if you are driving, the safest choice is no alcohol.

Your details

Units drunk
Your weight
Sex
Hours since your first drink
Hours spent drinking (optional, for the hour by hour curve)
Age (optional)
Height (optional)

Not sure of the units? Work them out first with the alcohol units calculator.

6.0 units, 3 h after your first drink

43 mg

per 100 ml of blood, a rough estimate: alcohol is still in your blood

  • Rough peak if all drunk at once88 mg per 100 ml
  • Taken away by your body so far (about 15 mg an hour)45 mg per 100 ml
  • Rough time still to go until the estimate reaches zero2 h 54 min
  • Body water factor used (Widmark)0.68 (male)
  • A more personal body water figureadd your age and height
  • Hour by houradd how long you spent drinking to see the curve

This is a textbook Widmark estimate. Real levels can be much higher: food, how fast you drank, medicines, your liver and your body make a big difference, and people clear alcohol at very different speeds. Never drink and drive. How fast alcohol clears varies a lot from person to person, so there is no reliable way to know you are under the limit, and you can still be over it the morning after. Scotland's limit is lower than the rest of the UK's. If you will drive, the safest choice is no alcohol at all.

Worried about your drinking, or someone else's? Your GP can help, and Drinkaware and Drinkline (0300 123 1110, free and confidential) are there too. Call 999 if someone cannot be woken, is breathing slowly or is fitting after drinking.

Hour by hour

Blood alcohol over time

Add how long you spent drinking and the result also spreads the drinks evenly over that time in 15 minute steps, clearing about 15 mg per 100 ml an hour, to show a rough curve: when it peaks and how long it lingers. It is there to show how slowly alcohol leaves, never to time driving.

The working

6 units, 12 st 8 lb, 3 hours later

Six units is 48 g of alcohol. Widmark divides that by body weight (79.8 kg) times a body water factor of 0.68 for men, which gives a rough peak of 88 mg per 100 ml of blood if it was all drunk at once. The body takes away about 15 mg an hour, so after 3 hours the estimate is about 43 mg, and it needs nearly 3 more hours to reach zero. Real people vary a lot either side of that.

A more personal figure

Body water from your height and age

The textbook factor of 0.68 for men and 0.55 for women is an average. Watson's 1980 equations estimate your own body water from age, height and weight. A man of 30 at 178 cm and 80 kg has about 45.6 litres, a factor of 0.71, so 6 units peak at about 84 rather than 88 mg per 100 ml. Shorter, older or lighter people often get a higher figure than the textbook one; we always point you to the higher of the two. Neither can tell you whether you are under a driving limit.

The UK limits

Why the number is not a target

MeasureEngland, Wales and NIScotland
Breath (micrograms per 100 ml)3522
Blood (mg per 100 ml)8050
Urine (mg per 100 ml)10767

GOV.UK says it is impossible to turn these into a number of drinks, because weight, sex, food, stress and metabolism all change the answer. Your driving is affected well below them. That is why this page never compares your estimate with a limit.

What changes it

Same drinks, different blood

  • Food slows absorption, so an empty stomach means a higher, earlier peak
  • Smaller bodies and less body water mean a higher level from the same drink
  • Some people clear alcohol at 10 mg an hour, not 15, which adds hours
  • Nothing speeds clearance up: not coffee, a shower or sleep

Read with care

What this cannot tell you

This is a textbook estimate for curiosity and caution only.

  • Tell you that you can driveIt never will. If you are driving, do not drink.
  • Measure anythingOnly a breath or blood test measures alcohol, and those are for the police and hospitals.
  • Know your bodyMedicines, liver health, food and how fast you drank can make the real figure much higher.

Questions

Common questions

How accurate is a blood alcohol calculator?

Not very. The Widmark formula is a classroom average, and real levels can be far higher or lower. Use it to understand how long alcohol lingers, never to decide anything risky.

How long does one unit take to leave the body?

About an hour on average, according to the NHS and Drinkaware, but it varies a lot from person to person.

Can this tell me if I am safe to drive?

No, and no calculator can. Alcohol affects everyone differently and you can be unsafe even under the legal limit. How fast alcohol clears varies a lot, you can still be over the limit the next morning, and Scotland's limit is lower than the rest of the UK's. If you are going to drive, the safest choice is no alcohol at all.

Why does weight matter?

The same alcohol spreads through more body water in a bigger body, so the concentration is lower. Women usually have less body water for their weight, which is why the factor is 0.55 rather than 0.68.

Sources

Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.

If drinking is on your mind

Your GP can help in confidence. Drinkline is free on 0300 123 1110, and Drinkaware has tools to help you cut down. If you drink heavily every day, do not stop suddenly without advice. Call 999 if someone cannot be woken, is breathing slowly or is fitting after drinking.

Related

Related calculators