How long until alcohol wears off? Why there is no single number
"One beer — how many hours until it's gone?" It's a common search, and there are plenty of calculators and lookup tables for it. A clear number would obviously be handy. But honestly: there is no single clearance time that holds for everyone. Here's why, and how to treat the "rough estimate" sensibly — grounded in public health sources.
One important thing first: this article is not about deciding whether you can drive. That answer comes at the end.
Start with pure-alcohol grams
The "amount" of a drink isn't its milliliters but the grams of pure alcohol it contains:
Pure alcohol (g) = volume (ml) × ABV (%) ÷ 100 × 0.8 (density of alcohol)
A 500 ml beer at 5% is 500 × 0.05 × 0.8 = 20 g — often called "one unit." Once swallowed, roughly 25% is absorbed in the stomach and most of the rest in the small intestine, and nearly all of it enters the blood,[2] so the grams you drink drive how high your blood alcohol concentration climbs.[1]
For the step-by-step way to work out the pure alcohol and a rough time yourself, see the how-to-calculate article and the calculator.
The "0.1 g per kg per hour" rule, and its limits
There's a common rule of thumb for clearance speed: you break down roughly body weight (kg) × 0.1 g of pure alcohol per hour. By that math, 20 g (one unit) takes a 60 kg person about 3–4 hours; two units, 6–7.
But here's the crux: that figure is only a population average. Most metabolism happens in the liver, and its speed varies widely with enzyme genotype, body size, liver size, and drinking habits.[1][5] And it isn't just the downward slope that varies — how high you peak (the starting point) varies too.[6][7]
| What varies | Effect on clearance time |
|---|---|
| Body size / weight | Smaller bodies peak higher and tend to take longer to clear |
| Whether you ate | An empty stomach absorbs faster, with a steeper rise |
| Constitution (metabolizing enzymes) | Inborn differences in the enzymes that break alcohol down |
| The day's condition / sleep | Fatigue or sleep can make it slower than usual |
| Drinking habits | Habitual drinking bears on metabolic speed |
A worked example: two medium beers
Two medium beers (5%, ~435 ml each), 30 minutes apart. The figure below sketches, conceptually, how things look about 60 minutes after the first sip. Most tables and apps report this moment as a single value — "X%" (the grey dot). But in reality it can swing up or down with your body and condition, so the honest read is a range with slack on the high side (the orange band). Same "now," seen as a point or as a range — that's the difference.
Can you speed it up?
Water, coffee, a sauna, a nap — none of these are understood to make the breakdown itself faster. If anything, metabolism is said to slow while you sleep. There's no quick sober-up; the only things that affect how fast your level falls are not drinking more and time.
About driving — "it's probably worn off" is not a reason
Let's be direct: no estimate can tell you whether you're okay to drive. Under Japan's Road Traffic Act, 0.15 mg of alcohol per litre of breath is already a drink-driving offense, and the limits are strict — and impaired driving is an offense regardless of concentration. Tables, calculators, and app estimates are all guesses from inputs, not measurements. "It's probably gone by now" is never a reason to drive. If you drank, don't drive.
"Cleared" and "no hangover" are not the same
Your blood alcohol dropping to near zero doesn't mean an easy next morning. The mechanism of a hangover is, in fact, poorly understood. Acetaldehyde was long blamed, yet it's rarely detected in the blood during a hangover; the leading view is that several factors combine — mild withdrawal, dehydration, low blood sugar, poorer sleep, and congeners (trace compounds in drinks).[3][4] People who flush easily (weaker ALDH2) tend to have it worse. So "time to clear" and "how you feel tomorrow" are separate questions. What drives hangover severity, and how it can be estimated, is the subject of the hangover-forecast article.
What you see in Nomicho
Nomicho builds this "no single number" fact into the design. It shows the outlook as the kind of range above, with slack on the slow-to-clear side. It won't tell you "when you can drive" — it's a way to watch, out of curiosity, how alcohol moves through your own body.
Sources
- [1] MHLW e-healthnet, "Absorption and breakdown of alcohol" (Japanese)
- [2] MHLW e-healthnet, "Blood alcohol concentration" (Japanese)
- [3] MHLW e-healthnet, "Mechanism of hangover" (Japanese)
- [4] NIAAA, "Hangovers (Japanese edition)"
- [5] Jones AW. Evidence-based survey of the elimination rates of ethanol from blood. Forensic Sci Int. 2010 (the mean clearance rate β and its person-to-person spread).
- [6] Watson PE, Watson ID, Batt RD. Total body water volumes for adult males and females from simple anthropometric measurements. Am J Clin Nutr. 1980;33(1):27–39. (body size sets total body water — why the same dose peaks higher or lower depending on your build)
- [7] Forrest ARW. The estimation of Widmark's factor. J Forensic Sci. 1986 (the women's formula, re-derived by Searle 2014).
For exactly which equations and primary sources Nomicho uses to compute this, see the methodology.
This article is general information, not medical or legal advice. The chart is a conceptual illustration, not a specific person's values.