BAC Calculator guide
BAC Calculator: Blood Alcohol Estimate and UK Units
Use our BAC calculator to estimate your blood alcohol level and count UK units, and find out why you should never use a calculator to decide whether to drive.
By the AllCalculator editorial teamUpdated 16 October 20262 min read
How much alcohol is in your blood depends on what you've drunk, your size, your sex and how long it's been. Our BAC Calculator gives a rough estimate and counts your units. It's for learning about alcohol, not for deciding whether you're safe to drive.
How to use the calculator (BAC Calculator)
- Choose your sex and enter your weight.
- Enter what you've drunk: pints, glasses of wine, single spirits or other units.
- Enter the hours since your first drink.
You'll see your estimated blood alcohol level, the units you've drunk and roughly how long until the alcohol is gone.
How it works
The calculator uses the Widmark formula:
- It counts your units. A UK unit is 10ml, or 8g, of pure alcohol.
- It spreads the alcohol through your body using the Widmark factor, 0.68 for men and 0.55 for women, which reflects how much of your body is water.
- It takes off about 15mg per 100ml for every hour since you started drinking.
UK drink-drive limits
| Measure | England, Wales and NI | Scotland |
|---|---|---|
| Blood (mg per 100ml) | 80 | 50 |
| Breath (µg per 100ml) | 35 | 22 |
| Urine (mg per 100ml) | 107 | 67 |
Why you can't rely on it to drive
There's no reliable way to know whether you're under the limit. Food, medicines, tiredness, health and how fast you drank all affect your level, and even small amounts slow your reactions. The only safe option is not to drink at all if you're driving. You can also still be over the limit the morning after.
A worked example
With the calculator's default settings, a woman weighing 70kg who has drunk 2 pints of 4.5% beer (about 5.1 units) over 2 hours has an estimated level of about 76mg per 100ml. That's already over the Scottish limit, and close to the limit elsewhere.
Frequently asked questions
How many units are in my drink?
A pint of 4% beer is about 2.3 units, a 175ml glass of 13% wine is about 2.3 units and a 25ml single of 40% spirit is 1 unit.
Can coffee or a cold shower sober me up?
No. Only time works. Your liver clears about one unit an hour.
How much is too much?
The UK Chief Medical Officers advise not drinking more than 14 units a week, spread over 3 or more days. If you're worried about your drinking, the NHS and Drinkline (0300 123 1110) can help.
Sources and further reading
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.