How to work out how long alcohol takes to clear (and the catch)
"How many hours for a 12 oz beer?" "What about a couple of pints?" 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 standard drinks
A drink's "size" is best measured not in fluid ounces but in how much pure alcohol it contains. In the US that unit is the standard drink: 14 grams of pure alcohol.
pure alcohol (g) = volume (fl oz) × 29.57 × ABV (%) ÷ 100 × 0.8 (the density of alcohol)
For a 5% beer, 12 fl oz: 12 × 29.57 × 0.05 × 0.8 ≈ 14 g — one standard drink. Common pours come out roughly like this.
| Drink (example) | Standard drinks | Pure alcohol, approx. |
|---|---|---|
| Beer (5%) 12 fl oz | 1 | ~14 g |
| Light beer (4.2%) 12 fl oz | 0.9 | ~12 g |
| Beer (5%) 12 fl oz × 2 | 2 | ~28 g |
| IPA (6.8%) 16 fl oz | 1.8 | ~26 g |
| Hard seltzer (5%) 12 fl oz | 1 | ~14 g |
| Wine (13.5%) 5 fl oz | 1.1 | ~16 g |
| Vodka soda (7.5%) 8 fl oz | 1 | ~14 g |
| Shot of spirits (40%) 1.5 fl oz | 1 | ~14 g |
Had a few? Just add them up. 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 0.1 g of pure alcohol per hour for every kilogram of body weight — about 0.045 g per hour per pound.[1] For a 150 lb person that is about 7 g an hour. Four 12 fl oz beers (four standard drinks, about 56 g of pure alcohol) would therefore take at least about 8.5 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. Every US state sets a per se limit of 0.08% BAC (0.05% in Utah), drivers under 21 are subject to zero-tolerance laws, and you can be charged with impaired driving below any of those numbers. 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.