How Long Does Alcohol Stay in Your System? The Widmark Math

“How long until I’m sober?” is one of the most-searched questions about alcohol, and it has a frustrating answer: longer than you think, and only your liver sets the pace. If you want a rough estimate for planning around sleep, training, or simply understanding what a night of drinking does to your blood, the Alcohol Metabolism / Sober-By Calculator applies the same math this article explains. This piece is about how that estimate is built — and, just as importantly, how much you should trust it.

What “in your system” actually means

Once you drink, alcohol is absorbed from the stomach and small intestine into the bloodstream, where it’s measured as blood alcohol concentration (BAC) — grams of ethanol per 100 mL of blood, expressed as a percentage. A BAC of 0.08% means 0.08 grams of ethanol per 100 mL.

The important detail is that most of the alcohol you drink is broken down by the liver, largely by the enzyme alcohol dehydrogenase, at a nearly constant rate regardless of how much is present. This is what makes alcohol unusual: unlike most substances, which clear on a half-life curve, alcohol elimination is roughly linear. Your BAC falls by about 0.015% per hour for most people, with a realistic range of about 0.012% to 0.020% per hour depending on the individual.

Linear clearance has a blunt consequence: nothing speeds it up. Coffee, a cold shower, water, food after the fact, or “sweating it out” don’t change the liver’s rate. They may make you feel more alert, but a more alert person with a high BAC is still impaired. Time is the only variable that matters.

The standard drink, so the numbers mean something

To estimate anything, you first need to know how much ethanol you actually consumed. In the US, one standard drink = 14 grams of pure ethanol. That works out to roughly:

Beverage	Typical serving	Approx. alcohol
Regular beer (~5%)	12 oz	1 standard drink
Malt liquor (~7%)	8–9 oz	1 standard drink
Table wine (~12%)	5 oz	1 standard drink
Distilled spirits (~40%)	1.5 oz	1 standard drink

The catch is that real-world pours rarely match these. A generous restaurant glass of wine can be closer to two standard drinks, and a mixed cocktail may contain two or three. If you underestimate the grams going in, every downstream estimate is wrong from the start.

The Widmark formula, explained

The most widely used way to estimate peak BAC is the Widmark formula, developed by Swedish scientist Erik Widmark in the 1920s and still the backbone of most blood alcohol calculators. Conceptually, it says peak BAC depends on how much ethanol you drank relative to how much body water you have to dilute it in:

Peak BAC ≈ (grams of alcohol) ÷ (body weight in grams × r)

Here r is the “distribution ratio” — the fraction of your body that behaves like water, since alcohol distributes into body water rather than fat. Because men on average carry a higher proportion of body water than women, r is typically taken as about 0.68 for men and 0.55 for women. These are population averages, not personal measurements.

That formula gives the peak BAC — roughly where you’d land if all the alcohol were absorbed at once. To estimate BAC at a later time, you subtract the linear elimination that has happened since:

Estimated BAC ≈ Peak BAC − (0.015 × hours elapsed)

A worked example

Suppose a 175 lb (about 79,400 g) man drinks 4 standard drinks over the evening. That’s 4 × 14 = 56 g of ethanol.

• Peak BAC ≈ 56 ÷ (79,400 × 0.68) ≈ 56 ÷ 54,000 ≈ 0.104%
• After 3 hours of elimination: 0.104 − (0.015 × 3) = 0.104 − 0.045 ≈ 0.059%
• Hours to reach ~0.00%: 0.104 ÷ 0.015 ≈ ~7 hours from peak.

So even after those drinks stop, this person is looking at most of the night before alcohol has cleared. A woman of the same weight, using r ≈ 0.55, would peak higher — around 0.128% — and take correspondingly longer, because the same ethanol is diluted in less body water. That sex difference is one reason a single “hours until sober” rule of thumb doesn’t fit everyone.

Why your real BAC won’t match the estimate

The Widmark method is a useful teaching tool and a reasonable average, but it is an estimate built on averages, and several things move real BAC away from the number:

• Food. Drinking on an empty stomach speeds absorption and raises peak BAC; a full meal slows it and blunts the peak. Widmark’s basic formula doesn’t capture this.
• Sex and body composition. The fixed r values (0.68 / 0.55) are averages. Two people of identical weight but different muscle-to-fat ratios have different body-water fractions and therefore different true r.
• Genetics and enzymes. Activity of alcohol- and aldehyde-dehydrogenase varies widely between individuals and populations, changing how fast alcohol is cleared — the 0.015%/hour figure is a middle-of-the-road average, not a personal constant.
• Medications and health. Liver function, certain medications, and hydration status all shift the picture, sometimes substantially.
• Drinking rate. The formula assumes a tidy peak; sipping over hours versus slamming drinks changes the actual curve, and elimination is happening the whole time you’re still drinking.

Because of all this, any calculated BAC should be read as a rough, educational picture of the general timeline — not a precise measurement of your blood at a given minute. That’s also why the estimate pairs naturally with understanding other slow-clearing substances, like the way caffeine’s half-life keeps it circulating long after your last cup.

Worth noting too: “cleared from your blood” and “recovered” aren’t the same milestone. Even after BAC returns to zero, the night’s sleep and overnight recovery can still be blunted — a separate cost we cover in Alcohol and Recovery: What One Drink Costs.

The safety line that isn’t negotiable

This is the part that matters more than the math: an estimate must never be used to decide whether it’s safe to drive. The formula’s individual variation is wide enough that a calculator can tell you you’re “under the limit” while your actual BAC — and your actual impairment — is higher. Impairment of reaction time, judgment, and coordination begins well below any legal threshold, and legal limits are ceilings, not safety guarantees.

Say it plainly: the only safe amount of alcohol before driving is none. If you have been drinking, do not drive. Plan a ride, stay put, or wait far longer than any number suggests. A blood alcohol calculator is for curiosity and planning around rest and recovery — it is not a breathalyzer, and it is never permission to get behind the wheel.

The takeaway

Alcohol leaves your system on a nearly straight line — about 0.015% BAC per hour — and only time moves that line. The Widmark formula gives you a reasonable, educational estimate of where your BAC peaked and roughly when it should return to zero, but food, body composition, genetics, medications, and how fast you drank all bend the real curve. Use the Alcohol Metabolism / Sober-By Calculator to see your own rough timeline, treat the result as a ballpark rather than a verdict, and remember the one rule the math can’t soften: if you’ve been drinking, you don’t drive.

Sources

• National Institute on Alcohol Abuse and Alcoholism (NIAAA) — Alcohol Metabolism and What Is a Standard Drink? (niaaa.nih.gov)
• Widmark, E.M.P. — Principles and Applications of Medicolegal Alcohol Determination (the original basis for the Widmark formula).