Your blood alcohol level can be estimated from how much alcohol you drank, your weight, your sex, and how long ago you started. The standard way to do it is the Widmark formula, developed by the Swedish professor Erik Widmark, and it is what Mypromille and the BAC calculator on this blog use.
If you just want the number, the BAC calculator works it out in your browser. This page explains what happens underneath.
Warning
The intention of this service is to give more awareness of the effects of alcohol on the body, not to encourage drinking. If you are a minor, you should close this page now. Alcohol is meant to be consumed in moderation, because it increases your risk of personal injury.
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The Widmark formula
The formula estimates the peak level first, as if all the alcohol had been absorbed at once, and then subtracts what the body has broken down since:
per mille = grams of alcohol ÷ (body weight in kg × r) − β × hours
| Symbol | What it is | Typical value |
|---|---|---|
| grams of alcohol | the pure alcohol in everything you drank | worked out from each drink’s size and ABV |
| body weight | your weight in kilograms | yours |
| r | the Widmark factor: the share of your body weight that alcohol spreads into | about 0.7 for men, 0.6 for women |
| β | how fast the body breaks alcohol down, in per mille per hour | 0.10–0.15 on average; the calculator on this blog shows the result for the whole range |
| hours | time since you started drinking | counted from your first drink |
The factor r is why sex matters. Alcohol dissolves in body water rather than fat, and women on average carry a lower share of body water, so the same drinks spread into less of their body and give a higher level.
To get the grams of alcohol in a drink, multiply the volume in ml by the ABV and by 0.789, the density of ethanol. A 33 cl beer at 5 % is 330 × 0.05 × 0.789 = 13 g. Understanding standard drinks goes through that calculation in detail.
A worked example
Four standard drinks of 12 g each is 48 g of alcohol, roughly four 33 cl beers at 4.5 %.
A man weighing 80 kg:
Peak: 48 ÷ (80 × 0.7) = 48 ÷ 56 = 0.86 per mille
Three hours later: 0.86 − (0.10 to 0.15 × 3) = 0.41–0.56 per mille
A woman weighing 60 kg:
Peak: 48 ÷ (60 × 0.6) = 48 ÷ 36 = 1.33 per mille
Three hours later: 1.33 − (0.10 to 0.15 × 3) = 0.88–1.03 per mille
Same drinks, same evening: her level is more than 50 % higher than his, and she will need many more hours to reach zero. Both are still well above Sweden’s 0.2 limit three hours later.
Why it is only an estimate
The formula is a good average, but real bodies vary:
- Food slows absorption, so the peak comes later and lower than the formula assumes.
- Drinking fast brings the real level closer to the calculated peak.
- The burn-off rate differs from person to person, which is why it is given as a range rather than one number. Nothing speeds it up, not coffee, water or sleep.
- Body composition matters: the factor r is an average for each sex, not a measurement of you.
That is why Mypromille shows an estimate and can also connect to a Bluetooth breathalyzer for a real measurement. And it is why no calculation, here or anywhere else, can tell you when you are fit to drive: the legal limits are in the drink-driving limits by country, and how long alcohol stays in the blood is covered in driving the day after.
For more background, see Wikipedia on blood alcohol content.
Per mille or percent?
Per mille means parts per thousand. One per mille is a thousandth, just as one per cent is a hundredth. Blood alcohol content is given in per mille across Sweden and much of Europe, while the USA and other English-speaking countries tend to give the same figure as a percentage. It is the same measurement on a different scale: divide per mille by ten to get percent. (Wikipedia)
| Per mille | Percent | For reference |
|---|---|---|
| 0.2 ‰ | 0.02 % | Sweden’s limit |
| 0.5 ‰ | 0.05 % | the limit in most of Europe |
| 0.8 ‰ | 0.08 % | the US limit |
| 1.0 ‰ | 0.10 % | aggravated drink-driving in Sweden |
The app can show your level in either unit. You can change it on the settings page.
Using the formula in Mypromille
Knowing you
To run the formula, Mypromille needs your sex and weight. The app asks for them the first time you open it, and you can change them at any time.
Adding drinks
Now you are ready to add drinks. Mypromille needs the size of each drink (cl, ml or ounces) and its ABV (alcohol by volume). If you don’t know the ABV of a particular beer, you can search a database of millions of them.
Custom drinks
If you can’t find your drink, or you are drinking something different, you can add your own, as long as you know the ABV and the size.
Frequently asked questions
What is the Widmark formula?
What are the Widmark factors for men and women?
How do I convert per mille to percent?
How accurate is a calculated blood alcohol level?
Stop guessing. Be sure.
I hope Mypromille helps you become more aware of your own drinking, for your safety and your health. The app is available for iPhone, Android and straight in the browser.