Blood Alcohol Calculator UK

Why the UK Needs Its Own BAC Calculator

Most blood alcohol tools are built around the American "standard drink" of 14g of pure alcohol. The UK measures alcohol differently: in units, where one unit is 10ml or 8 grams of pure ethanol. A UK-specific calculator matters because using the wrong drink definition throws off the whole estimate. This tool takes your weight, sex, the number of UK units you have drunk and the time elapsed, then applies the Widmark formula to estimate your blood alcohol level in both percentage and the UK's official mg per 100ml figure.

Understanding UK Units

Units sound simple but trip people up because drink strengths have crept upward. The formula is units = (ABV% × volume in ml) / 1000.

• A pint (568ml) of 4% lager is (4 × 568) / 1000 = 2.3 units.
• A large 250ml glass of 12% wine is (12 × 250) / 1000 = 3 units.
• A single 25ml shot of 40% spirits is (40 × 25) / 1000 = 1 unit. So "just a couple of pints and a glass of wine" can quietly add up to roughly 7.6 units — far more alcohol than many people assume.

The Two UK Legal Limits

There is no single UK limit. England, Wales and Northern Ireland sit at 80mg per 100ml (0.08%), while Scotland is stricter at 50mg per 100ml (0.05%). A driver who is legal in Carlisle can be committing an offence a few miles north in Gretna. The calculator flags your estimate against both limits so cross-border drivers can see the difference instantly.

The Widmark Formula, UK-Style

The engine behind the result is the forensic Widmark equation: BAC (g/L) = grams of alcohol / (weight in kg × r) then subtract elimination × hours. Here r is the volume-of-distribution constant — 0.68 for men, 0.55 for women — which is why women tend to reach a higher BAC from the same drinks. Elimination is treated as a fixed ~15mg per 100ml per hour, because alcohol follows zero-order kinetics: your liver works at a steady pace no matter how much you drink.

Worked Example

Imagine an 80kg man who drinks 6 units over 3 hours at a pub in Edinburgh.

1. Alcohol consumed: 6 × 8 = 48g.
2. Widmark: 48 / (80 × 0.68) = 0.882 g/L of blood.
3. Subtract elimination: 0.882 − (3 × 0.15) = 0.432 g/L.
4. Convert: that is roughly 43mg per 100ml, or 0.043%.

He is under the 80mg English limit and under Scotland's 50mg limit — but only just, and one more drink would push him over the Scottish line. The same drinks consumed by a 60kg woman would produce a far higher figure, around 80mg, putting her over both limits. This is exactly the kind of difference the tool surfaces. For the science of how the body burns alcohol, see our blood alcohol content calculator, and to plan a recovery day afterwards try the water intake and sleep calculator.

A Serious Word on Safety

This calculator is an educational estimate only. Real blood alcohol depends on food, hydration, medication, drink strength variation and your individual physiology — all of which can make your true level higher than the maths suggests. It cannot and must not be used to decide whether you are fit to drive. The only genuinely safe amount of alcohol before getting behind the wheel is none. If you have been drinking, plan a taxi, public transport or a sober driver. Never drink and drive.