How much have I had? The Widmark formula gives an answer
After an evening with a few drinks, almost everyone eventually asks the same question: how much alcohol is actually in my blood? Only a breath or blood test gives an exact answer. For a rough estimate, people have relied for nearly a century on the Widmark formula — the same core formula used by the <a href="/en/blood-alcohol-calculator">blood alcohol calculator</a>. In this article we explain step by step how it works, which quantities go into it, and why the result can only ever be an estimate.
Who was Widmark?
The formula goes back to the Swedish chemist and forensic scientist Erik Matteo Prochet Widmark, who developed it in the 1920s and 1930s. Widmark was looking for a method to derive the expected blood alcohol concentration from a known amount of alcohol consumed. His work became the foundation of forensic alcohol assessment and remains part of expert medico-legal reports to this day.
The formula at a glance
At its heart the Widmark formula is remarkably simple. Blood alcohol concentration (BAC) in per mille is calculated as: BAC = A ÷ (r × m). Here A is the amount of alcohol absorbed in grams, r is the reduction factor and m is body weight in kilograms. Let us look at each of the three quantities in turn.
Step 1: The amount of alcohol A
First we need to know how much pure alcohol we actually drank. The amount in grams is the volume of liquid in millilitres, multiplied by the percentage by volume divided by 100, multiplied by the density of ethanol, which is around 0.8 grams per millilitre.
An example: a large beer of 0.5 litres at 5 percent contains 500 × 0.05 × 0.8 = 20 grams of pure alcohol. A glass of wine of 0.2 litres at 11 percent comes to 200 × 0.11 × 0.8 = 17.6 grams. A shot of spirits of 4 centilitres at 40 percent gives 40 × 0.40 × 0.8 = 12.8 grams. If you have several drinks, you simply add up the individual amounts. The <a href="/en/blood-alcohol-calculator">blood alcohol calculator</a> forms exactly this sum automatically through its drink builder.
Step 2: The reduction factor r
Alcohol does not distribute evenly throughout the body but mainly into body water. The reduction factor r describes what share of body weight serves as the distribution space. Because men on average have a higher proportion of body water than women, different values apply: around 0.7 for men and 0.6 for women.
These numbers are statistical averages. An individual's factor depends on the ratio of muscle to fat tissue — muscle holds more water than fat. A well-trained person tends to have a higher factor than someone with a high body fat percentage. In practice the standard values are used anyway, because exact body composition is rarely known.
Step 3: Body weight m
Body weight enters the formula directly: the heavier a person, the larger the distribution volume and the lower the concentration for the same amount of alcohol. This is why a light person reaches a higher level after the same drinks than a heavy one.
A full worked example
Take a man weighing 80 kilograms who has drunk two large beers. The amount of alcohol is 2 × 20 = 40 grams. The reduction factor is 0.7 and the weight 80 kilograms. This gives a peak value of 40 ÷ (0.7 × 80) = 40 ÷ 56 ≈ 0.71 per mille. If we also account for the absorption deficit, the value drops to around 0.64 per mille. This is the peak straight after full absorption — before the body has made any meaningful progress on elimination.
The absorption deficit
Not all the alcohol you drink ends up in your blood. Some is broken down in the stomach and gut, or never absorbed at all. This absorption deficit is usually set at about 10 percent, so only around 90 percent of the alcohol takes effect. The <a href="/en/blood-alcohol-calculator">blood alcohol calculator</a> applies this deficit by default, but can be switched to the less favourable value without the deduction. In court back-calculations the deficit is often omitted in the defendant's favour, to stay on the safe side.
Alcohol elimination over time
The Widmark formula first delivers only the peak value. After that the body eliminates alcohol continuously — mostly in the liver. As a rule of thumb, BAC falls by about 0.1 to 0.2 per mille per hour, on average around 0.15 per mille. This elimination is largely constant and cannot be sped up by anything: coffee, cold showers, exercise or a hearty meal do not help. Only time lowers the value.
To get from the peak to the current BAC, you subtract the product of the elimination rate and the hours elapsed since the first drink. In our example, after four hours at an elimination rate of 0.15 per mille, around 0.6 per mille would have been eliminated — the man would then be at about 0.04 per mille. The calculator plots this course as a curve and marks the relevant limits of 0.5 and 0.3 per mille.
Why the result is only an estimate
As elegant as the formula is, it works with average values and ignores many individual factors. Actual BAC depends, among other things, on how full the stomach was, how quickly the drinking happened, which medications are involved and what a person's body composition looks like. Two people of the same weight and the same amount of drink can therefore end up with markedly different values.
So keep in mind: the <a href="/en/blood-alcohol-calculator">blood alcohol calculator</a> provides rough guidance, not a reliable measurement. It makes no statement about whether you are fit to drive and does not replace a breath or blood test. Legally, only the measured value counts. When in doubt, the simplest and safest rule applies: do not drive.
Conclusion
The Widmark formula combines the amount of alcohol, the reduction factor and body weight into a quick estimate of blood alcohol concentration. Once you understand the three quantities, you can see why drink strength, sex and weight have such a large effect. Use the <a href="/en/blood-alcohol-calculator">blood alcohol calculator</a> to run your own example — and always treat the result as what it is: a non-binding approximation.