Blood Alcohol Content (BAC) Calculator
Estimate your blood alcohol content (BAC) based on drinks consumed, body weight, and time elapsed. Shows how your estimated BAC compares to UK drink-drive limits.
How Blood Alcohol Content (BAC) Calculator works
The Widmark formula
The Widmark formula estimates blood alcohol content from the mass of alcohol consumed, body weight, and a sex-specific body water constant (0.68 for males, 0.55 for females). These constants reflect differences in body composition — men have proportionally more water-containing tissue. The formula is: BAC = (alcohol grams ÷ (weight kg × 1000 × body water constant)) × 100. Alcohol metabolism then reduces BAC at approximately 0.015% per hour.
UK drink-drive limits
In England, Wales, and Northern Ireland, the legal limit is 80 mg of alcohol per 100 ml of blood (0.08%). Scotland has a lower limit of 50 mg per 100 ml (0.05%). These are legal thresholds, not safety thresholds — cognitive impairment begins at lower levels. The maximum safe BAC for driving is zero.
Why this is only an estimate
Actual BAC depends on many factors this calculator cannot account for: food consumption (eating slows alcohol absorption significantly), liver health, medications, genetic variation in alcohol metabolism enzymes, body fat distribution, and whether you are hydrated. Individual metabolism rates vary by a factor of three or more between people. These estimates should never be used to make decisions about driving or other safety-critical activities.
How to get alcohol out of your system
The only reliable way to reduce BAC is time. Coffee, food, water, exercise, and sleep do not speed metabolism of alcohol already in the bloodstream — they may make you feel more alert but do not lower BAC. If you have been drinking, the only safe approach is to wait the necessary hours before driving or operating machinery.
Frequently asked questions
What is the UK drink-drive limit?
80 mg of alcohol per 100 ml of blood in England, Wales, and Northern Ireland. Scotland has a lower limit of 50 mg per 100 ml. These are legal limits — driving with any alcohol in your system increases risk. Many road safety organisations recommend not drinking at all if you intend to drive.
How long does alcohol stay in your system?
Your body metabolises alcohol at roughly 0.015% BAC per hour on average, but this varies significantly between people. A pint of beer (approximately 2–3 units) might take 2–3 hours to clear. After a heavy night's drinking, alcohol can remain in your system until the following morning or longer.
Can I use this calculator to decide whether to drive?
No. This calculator is for educational purposes only. Individual metabolism varies enormously. You could have a lower BAC than predicted and still be impaired. Or your BAC could be higher. The only safe policy is to not drive after drinking any alcohol.
What are a standard "unit" of alcohol?
One UK unit is 10 ml (8g) of pure alcohol. A pint of 4% beer contains about 2.3 units; a 175ml glass of 13% wine contains about 2.3 units; a 25ml single measure of 40% spirit contains 1 unit. The NHS recommends not regularly drinking more than 14 units per week.
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This is a rough estimate only. Never use this to decide whether to drive. Do not drive if you have been drinking. This calculator provides estimates for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.