Units of Alcohol and How Long They Take to Clear

Finds the pure alcohol content as volume × strength ÷ 100 × 0.8. Japanese health guidance puts moderate drinking at around 20 g of pure alcohol a day.

Explanation

The strength of a drink is not the whole story. What your body actually has to deal with is the pure alcohol, which depends on volume times strength.

\text{pure alcohol (g)} = \text{volume (mL)} \times \dfrac{\text{ABV (\%)}}{100} \times 0.8

The 0.8 is the density of ethanol: one millilitre weighs 0.8 grams.

Example

A 500 mL beer at 5% ABV.

500 \times 0.05 \times 0.8 = 20\ \text{g}

Twenty grams — which happens to be exactly the Japanese health ministry's figure for moderate daily drinking.

Drinks that all come to about 20 g

Strikingly, all of these land in the same place.

Beer, 500 mL at 5% — 20 g
Sake, 180 mL at 15% — 21.6 g
Shochu, 100 mL at 25% — 20 g
Wine, two glasses of 200 mL at 12% — 19.2 g
Whisky, a 60 mL double at 43% — 20.6 g

The size of "a drink" shrinks as the drink gets stronger, which is precisely why one serving of anything carries roughly the same alcohol.

How long it takes to clear

The body clears roughly 0.1 g of alcohol per hour for each kilogram of body weight. A 65 kg person manages about 6.5 g an hour.

20 \div 6.5 = 3.1\ \text{hours}

One beer takes three hours to leave. Three beers before bed leaves nine hours of work still to do, which is why the morning after is a real thing.

Watch out

Never use this figure to decide whether you are fit to drive.

Clearance rates vary by a factor of three or more between individuals, depending on enzymes you were born with. This formula is a population average and carries no promise whatsoever about you.

The levels associated with long-term health risk are around 40 g a day for men and 20 g for women.