Important: No calculator can tell you it is safe or legal to drive after drinking. Alcohol affects everyone differently and any amount impairs driving. The only safe choice is not to drive after drinking. This tool gives a rough estimate of how long alcohol may stay in your system, mainly to show how easy it is to still be over the limit the morning after.
Advanced options
What The Drink Drive Calculator Works Out
- Alcohol likely cleared by: the simple rule of thumb that the body clears roughly one unit an hour, plus an hour for absorption, added to your finish time. Finish a 12-unit night at midnight and you may not be clear until lunchtime; thousands of drivers fail breath tests before 9am every year. Nothing speeds this up: not coffee, a shower, sleep or a fried breakfast.
- Est. peak level: the Widmark formula used in forensic science: grams of alcohol (8g per UK unit) divided by body weight times a distribution factor (0.68 for men, 0.55 for women), expressed in mg per 100ml of blood. It has a very wide margin of error for any one person.
- Legal limits: 80mg per 100ml of blood (35 micrograms per 100ml of breath) in England, Wales and Northern Ireland; 50mg (22 micrograms) in Scotland. There is no way to convert these into a safe number of drinks, which is why every road safety body gives the same advice: if you are driving, do not drink at all.
- Add drinks (advanced): counts common UK servings onto the units you typed: a pint of 5% lager, beer or cider is 2.8 units, a large 250ml glass of 13% wine is 3.3 and a single 25ml spirit is 1. Stronger craft beers and home-poured measures are usually more.
- Hours spent drinking (advanced): alcohol is burned off while you drink, so a session spread over five hours leaves a lower level at closing time than the same drinks downed in one. The "level when you stopped drinking" card shows the estimate after that burn-off.
- Food with the drinks (advanced): a meal slows absorption and lowers the peak, trimmed here by about 10% for a snack or 20% for a full meal. It does not reduce the total alcohol you must clear, only how high the level climbs.
- Elimination rate (advanced): the default of 15mg per 100ml per hour is a rough population average; real rates run from about 10 for occasional drinkers to 20 or more for heavy regular drinkers. Lower it to see a more cautious timeline.
- Peak in breath terms (advanced): the same peak converted to micrograms per 100ml of breath using the legal 80 to 35 ratio, which is the figure a roadside breathalyser would show.
- Under the 80mg and 50mg limits (advanced): the clock time at which the estimate falls below each UK limit, working down from the level when you stopped at the elimination rate. Being just under the limit is not being safe or legal with any confidence.
- Back to zero (advanced): the Widmark estimate of when no alcohol remains, usually later than the one-unit-an-hour rule for big nights. A conviction means a minimum 12-month ban, an unlimited fine, up to six months in prison and a criminal record. General information only, not legal or medical advice.
Source and further reading: GOV.UK: the drink drive limit.