How to work out how long alcohol takes to clear (and the catch)
"How many hours for a 500ml beer?" "What about two cans?" These are common searches. Estimating a rough answer yourself actually takes two steps. Here they are, in order, followed by an important catch.
The conclusion first: the number you get is an average-case estimate only, and it can never be used to decide whether the alcohol has cleared or whether you can drive. The reasons come later.
Step 1: work out the pure alcohol (grams)
A drink's "size" is best measured not in millilitres but in grams of pure alcohol. The formula is simple.
pure alcohol (g) = volume (ml) × ABV (%) ÷ 100 × 0.8 (the density of alcohol)
For a 5% beer, 500ml: 500 × 0.05 × 0.8 = 20g. Common drinks come out roughly like this.
| Drink (example) | Pure alcohol, approx. |
|---|---|
| Beer (5%) 350ml | ~14g |
| Beer (5%) 500ml | ~20g |
| Beer (5%) 500ml × 2 cans | ~40g |
| Highball (7%) 350ml | ~20g |
| Lemon sour (7%) 350ml | ~20g |
| Sake (15%) 1 gō (180ml) | ~22g |
| Wine (12%) one glass (120ml) | ~12g |
| Shōchū (25%) on the rocks, 60ml | ~12g |
Had a few? Just add up the pure alcohol from each. There is also a calculator that does it for you once you enter the volume and ABV.
Step 2: estimate how long it takes to break down
There is a widely used rule of thumb for the speed of breakdown: roughly body weight (kg) × 0.1g of pure alcohol per hour.[1] For a 60kg person that is about 6g an hour. Two cans of beer (about 40g of pure alcohol) would therefore take at least about 7 hours.
"At least" is deliberate. This formula is an average, and in reality it more often takes longer than this. Estimating on the short side would err in the dangerous direction.
The catch: don't trust the number too much
Charts and calculators are handy, but what they give you is a number for an average person. In reality it varies a lot, for reasons like these.
- Breakdown speed varies from person to person with enzyme genetics, body size, and the day's condition and food.[1]
- You cannot pin down the exact moment it "clears." All you get is a lower bound: it takes at least this long.
- Water, coffee, a sauna, or a nap do not make the breakdown itself faster. Only "no more drinks" and "time" do.
For why there is no single number that fits everyone, see this article.
Never a basis for driving
To be clear: no calculation can be used to decide whether you can drive. Japan's Road Traffic Act sets a very strict limit (0.15mg of alcohol per litre of breath). Charts and calculators are estimates from your inputs, not real measurements. "It should have cleared by now" is not a reason to take the wheel. If you have been drinking, don't drive. That is the only answer.
How Nomicho shows it
Nomicho reflects this "no single number" fact directly in its design. It estimates for you (from your sex, weight, and flush response), tracks your concentration in real time as you drink, and shows a cautious guide to roughly when it clears (leaning to the later side, never the earlier). It is a way to watch the part a single calculation cannot show: what comes next.
References
- [1] Ministry of Health, Labour and Welfare, e-Healthnet, "Absorption and breakdown of alcohol" (Japanese).
The exact equations and primary sources Nomicho uses are published in the methodology.
This article is general information, not medical or legal advice. The tables and figures are approximations for explanation and do not represent any specific person's values.