An estimate of blood alcohol using the Widmark equation, from what you drank, your weight and the time since. It is a population estimate and does not replace a breathalyser.
Estimated blood alcohol now
—
Estimated blood alcohol now—
Peak after drinking—
As a percentage—
Standard drinks—
Pure alcohol consumed—
Against the limit—
How this was worked out
Indicative result. This figure is a statistical estimate with a very wide margin and cannot be used to decide whether to drive. If you have been drinking, do not drive.
Widmark's equation spreads the alcohol consumed through body water and subtracts what the liver has already cleared. The grams of alcohol come from volume times strength times ethanol's density, 0.789. The constant r is the fraction of the body it distributes into: 0.68 in men and 0.55 in women on average, and that difference is why the same drink at the same weight gives a higher reading in a woman.
How to work it out by hand
Work out the grams of alcohol: millilitres × strength ÷ 100 × 0.789
Divide by your weight in kilos times the r constant for your sex
That is the peak blood alcohol in grams per litre
Subtract 0.15 for every hour since you drank
What is worth knowing
All three numbers in the formula are population averages and individual variation is enormous: the r constant depends on body composition, clearance runs from 0.10 to 0.20 grams per litre per hour depending on the person, and drinking on an empty stomach or during a meal changes the curve entirely. So this result is for understanding the order of magnitude and never for deciding whether to drive. Only a breathalyser is reliable, and only not driving is safe.
Frequently asked questions
Can I rely on this number to drive?
No. It is a population estimate with an enormous margin. If you have been drinking, do not drive: that is the only safe answer.
Why does sex matter?
Because alcohol distributes into body water, and its proportion differs on average. At the same weight and the same drink, the reading comes out higher in a woman.
Can clearance be sped up?
No. Not coffee, not a cold shower, not eating afterwards. The liver works at its own rate and only time passes.
What is a standard drink?
Ten grams of pure alcohol. A small beer, a glass of wine or half a spirit measure are each around one unit.
Does drinking on an empty stomach count the same?
No. On an empty stomach the peak comes sooner and higher. The formula does not capture that, and it is one of its limits.
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