Blood alcohol content rises and falls according to consistent rules — but the rate depends on your weight, sex, and what you drank. Here's the science and a practical guide for common scenarios.
Understanding how your body processes alcohol has practical value well beyond drink-driving avoidance. It explains the morning-after feeling, why "sleeping it off" isn't always enough, and why some people are affected so much more than others.
The liver metabolises alcohol at a roughly fixed rate: approximately one unit per hour for most adults. One UK unit is 10ml (8g) of pure alcohol. This rate doesn't change based on coffee, food eaten after drinking, sleep, or exercise — the only thing that reduces BAC is time.
BAC is the percentage of alcohol in your bloodstream. The UK drink-drive limit is 80mg per 100ml blood (0.08% BAC), though Scotland's limit is lower at 50mg/100ml (0.05%).
This is an estimate, not a reliable drink-drive calculator. Individual variation in alcohol metabolism is significant.
At the same weight and number of drinks, women reach a higher BAC than men because:
This means the same drink hits women's bloodstream harder and faster — not due to lower tolerance in a colloquial sense, but basic physiology.
These are approximate and assume drinking ended before sleep. They show why finishing a bottle of wine at midnight means you could still have significant alcohol in your system at 10am — even after 8 hours of sleep.
Many drink-drive offences occur the morning after — not on the night itself. The "one hour per unit" rule only applies while the liver is actively metabolising; factors like whether you drank on an empty stomach affect absorption speed but not metabolism speed. A responsible approach after heavy drinking is to avoid driving until at least the afternoon the next day, and if in doubt, not at all.
Alcohol significantly disrupts sleep architecture even in small amounts. While it helps you fall asleep faster, it suppresses REM sleep and causes more fragmented sleep in the second half of the night. Two drinks at dinner measurably reduces sleep quality — a consideration worth knowing for anyone who values sleep performance.