Widmark's formula, grams per glass and the time back down to 0,2 — with a worked example.
Working out alcohol comes down to three numbers. How many grams you have drunk. What that becomes in per mille in your body. How long it takes to leave. None of them is exact. They give you an order of magnitude, and that is enough to show how wrong a gut feeling usually is.
Grams of alcohol = volume in cl × strength in per cent × 0.079 A large strong beer, 50 cl at 5.2 per cent, gives 50 × 5.2 × 0.079 ≈ 21 grams. A standard drink counts as 12 grams of alcohol. So a large strong beer is nearly two standard drinks, not one.
Per mille = grams of alcohol ÷ (r × body weight in kg) r is the share of the body the alcohol spreads through: about 0.7 for men and 0.6 for women. The difference is body composition, not tolerance. The same beer therefore gives a higher reading in a lighter body. Two people who drank the same amount are not equally affected.
A man of 80 kg drinks two large strong beers, 50 cl at 5.2 per cent. Grams: 2 × 50 × 5.2 × 0.079 ≈ 41 grams. Per mille: 41 ÷ (0.7 × 80) means 41 ÷ 56 ≈ 0,73 promille. He is well over 0,2. He is also nearly halfway to the aggravated limit, after two beers.
The liver breaks down about 0,15 promille per hour. The rate is essentially constant and cannot be hurried. Hours remaining = (your per mille − 0,2) ÷ 0,15 The man in the example: (0,73 − 0,2) ÷ 0,15 ≈ 3.5 hours before he is under the limit. Almost 5 hours before he is back to zero.
You stopped drinking at 01.00. You work out that you were at roughly 1,0 promille then. 1. How far down do you need to get? To 0,2. 2. How much has to go? 1,0 − 0,2 = 0,8. 3. How long does that take? 0,8 ÷ 0,15 ≈ 5.3 hours. 4. When are you under? Around 06.20 at the earliest. Sleep until 07.00 and you are probably under 0,2, but only probably. Guess the wrong way and it is drink-driving.
Nothing speeds up the breakdown. Coffee makes you awake and impaired at the same time, which is more dangerous than simply being tired. Eating before drinking lowers the peak, because the alcohol is absorbed more slowly. Eating afterwards changes nothing. Only time does.
The formula gives an estimate, not a verdict. Sex, age, stomach contents, medication and how you happen to feel move the result both ways. Use it to understand how little it takes, not to run yourself as close to 0,2 as possible. Under Trafikbrottslagen 4 §, it is the per mille in your blood that decides, not your calculation.