Blood alcohol content calculator

How many drinks are really in your glass

The calculator asks for drinks, where one drink is 10 g of pure alcohol. To find the grams in a drink: millilitres × strength in percent ÷ 100 × 0.789 (the density of ethanol). Then divide the result by 10.

Approximate values for common drinks show that one glass is almost never exactly one drink:

• Beer 0.5 l, 5.2%: about 20.5 g, nearly 2 drinks.
• Cider 0.5 l, 4.5%: about 17.8 g, nearly 1.8 drinks.
• Long drink 0.33 l, 5.5%: about 14.3 g, nearly 1.4 drinks.
• Wine 150 ml, 12%: about 14.2 g, nearly 1.4 drinks.
• Spirits 40 ml, 40%: about 12.6 g, nearly 1.3 drinks.

Examples: same amount, different person

Three drinks and two hours after starting give very different results in the calculator. An 80 kg man has an estimated 0.25 per mille, while a 60 kg woman has 0.61 per mille, more than twice as much.

The gap comes from two factors: lower body weight and a lower distribution ratio (0.55 for women, 0.68 for men), because a woman's body contains on average less water for the alcohol to spread into. For example, two drinks taken at once by a 55 kg woman give 0.66 per mille by the formula, more than three drinks give an 80 kg man after two hours.

Sober time is counted from when you started drinking

The "Sober in" row does not count from now but from the start of drinking. The calculator works out the peak level as if all alcohol were absorbed at once and divides it by 0.15 per mille per hour.

Example: an 80 kg man has 8 drinks over an evening. By the formula his peak is about 1.47 per mille and he is sober after 9.8 hours. If he started at 22:00, the calculator still shows 0.27 per mille at 06:00, 8 hours later, with almost 2 more hours to go. The morning after is exactly when people most often underestimate their condition behind the wheel.

What the formula leaves out

The Widmark formula is a simplification, so your real level can be higher or lower.

• Absorption: on an empty stomach the peak comes sooner; with food it comes later and lower. The formula assumes instant absorption.
• Timing of drinks: the formula subtracts elimination for the whole period, including for drinks taken right at the end. A 70 kg man who had 4 drinks over 6 hours gets 0.00, even though the last drink he just finished is still being absorbed and his real level is above zero.
• Elimination rate: 0.15 per mille per hour is an average; in practice it ranges from roughly 0.1 to 0.2 per mille per hour between people.
• Body composition: at the same weight, a person with a higher body fat percentage has less water and a higher level.
• Health and medication: liver disease and some medicines change both how alcohol is broken down and how it affects you.
• Coffee, a cold shower, a sauna or sleep do not speed up alcohol elimination. They may change how you feel, but not your blood alcohol level.

Why this number does not tell you whether you can drive

The result is an estimate for an average person, not a measurement. Reaction time and judgement deteriorate even at small amounts, and 0.00 in the calculator does not mean there is really no alcohol in your blood. The only safe choice is not to drive after drinking and to plan your way home by taxi, public transport or with a sober driver.

A breathalyser reading is not the same as the calculator's per mille either: it measures alcohol in exhaled air and uses a different unit.

If someone has drunk heavily and cannot be woken, breathes slowly or irregularly, vomits while semi-conscious, or has cold, bluish skin, call 112 immediately. Until help arrives, turn them onto their side and do not leave them alone.