How Is Blood Alcohol Level Calculated? Understanding BAC and the Widmark Formula

What does BAC mean?

BAC (blood alcohol concentration) is the share of alcohol in your blood. A BAC of 0.05% means 0.05 g of alcohol per 100 ml of blood — the same as 0.5‰ (per mille), the unit used in much of Europe. When people say "blood alcohol level", they mean the same thing.

How is alcohol concentration estimated?

Without a breath or blood test, BAC can only be estimated. A calculator needs three things:

• How much pure alcohol you drank (volume × alcohol strength)
• How much of your body that alcohol spreads through (weight and body composition)
• How much time has passed, because your liver keeps breaking alcohol down

The Widmark formula

In the 1930s, Swedish scientist Erik Widmark described a simple model that most BAC calculators still use:

BAC = (grams of alcohol ÷ (body weight in grams × r)) × 100 − (β × hours)

• r is the distribution factor — roughly 0.68 for men and 0.55 for women on average
• β is the elimination rate — about 0.015% per hour for an average adult

A quick example

A 500 ml beer at 5% contains about 20 g of alcohol. For an 80 kg man: 20 ÷ (80,000 × 0.68) × 100 ≈ 0.037% at peak, before any time has passed.

How body weight affects BAC

The same drink spreads through more body water in a heavier person, so the concentration is lower. A lighter person drinking the same amount will usually reach a higher BAC.

How biological sex and body composition matter

Alcohol dissolves in water, not fat. On average, women have a higher share of body fat and less body water than men of the same weight, which is why the formula uses a smaller r value. Muscular people of either sex tend to land lower than the average estimate.

How the amount of alcohol matters

BAC rises roughly in proportion to the alcohol you drink. Two strong cocktails can easily contain as much alcohol as four beers — strength and serving size matter as much as the number of drinks.

Why time matters

Your body removes alcohol at a fairly steady rate — about one standard drink per hour for many adults. Water, coffee, sleep or a cold shower do not speed this up. Only time lowers BAC.

Why it is only an estimate

Real BAC depends on food, drinking speed, health, medication, genetics and more. The Widmark formula uses averages, so your real value can be noticeably higher or lower. Read more in why two people get different BAC.

Never use a calculator as proof you can drive

A calculator cannot confirm that you are safe or legally allowed to drive. The only reliable measurement is a proper breath or blood test. If there is any doubt, don't drive. Check the legal limits by country for local rules.