Read this first: this calculator gives a rough estimate from an average-person formula. Your real blood alcohol can be much higher than shown. Never use the result to decide whether to drive, ride, operate machinery, swim or look after children — the only safe blood alcohol level for driving is zero. If you have been drinking, use a taxi, public transport or a sober driver.
How to use the BAC calculator
- Choose US or metric units, your sex and your body weight.
- Enter the hours since your first drink — not since the last one. The formula counts the time your body has been breaking alcohol down.
- Count your drinks by type, pick the size and check the strength (ABV, alcohol by volume) on the label. Craft beers are often 6–9% rather than 5%, and wines range from about 11% to 15%. Use Other drinks for cocktails, cider, hard seltzer or anything else, with its own size and strength.
- Optionally add your age and height: the estimate then uses your own body water (the Watson formula) instead of the average for your sex.
The result shows the estimated BAC now as a percentage (grams of alcohol per 100 mL of blood, the US unit) and in per mille (‰, grams per liter, used in much of Europe), the grams of pure alcohol you drank, and how long the estimate takes to fall to zero.
The formula
The calculator uses the equation published by the Swedish chemist Erik Widmark in 1932, still the basis of forensic alcohol calculations:
BAC = A ÷ (r × W) × 100 − β × t
- A — grams of pure alcohol: volume in mL × ABV × 0.789 (the density of ethanol, in g/mL).
- W — body weight in grams.
- r — the Widmark factor, the share of the body that alcohol spreads into (mostly body water). Widmark’s averages are 0.68 for men and 0.55 for women; real people range widely around them.
- β — the elimination rate. A 2010 review of forensic studies by A. W. Jones found about 0.010–0.015% per hour on an empty stomach, 0.015–0.020% after a meal and up to 0.025–0.035% in people with alcohol dependence, with 0.015% a good average for moderate drinkers. The calculator uses 0.015% and also shows the time to zero at 0.020% and 0.010%.
- t — hours since the first drink.
With age and height, r is replaced by your estimated total body water, from the equations of Watson, Watson and Batt (1980), divided by the water content of blood (about 80%) and your weight. Lean and tall people have more body water and reach a lower BAC from the same drinks.
The model assumes all the alcohol is absorbed at once. In reality alcohol takes time to pass from the stomach and intestine into the blood, longer after a meal, so shortly after the last drink your BAC may still be climbing.
Worked example
A 160-lb (72.6 kg) man drinks three 12-oz beers at 5% over two hours.
- Alcohol: 3 × 355 mL × 0.05 × 0.789 = 42.0 g, exactly three US standard drinks.
- Peak if absorbed at once: 42.0 ÷ (0.68 × 72,575 g) × 100 = 0.085%.
- After two hours: 0.085 − 0.015 × 2 = 0.055% (0.55 ‰).
- Time to reach zero: 0.055 ÷ 0.015 ≈ 3 hours 41 minutes more — or anywhere from 2¾ hours (at 0.020% per hour) to 5½ hours (at 0.010%).
The same three beers give a 130-lb (59 kg) woman an estimate of about 0.10% after two hours, because she has less body weight and less body water for the alcohol to spread into.
Standard drink sizes
A “standard drink” is a fixed amount of pure alcohol, and countries define it differently — so “two drinks” means different amounts in different places:
| Country | One standard drink | Example |
|---|---|---|
| United States (NIAAA) | 14 g | 12 fl oz of 5% beer, 5 fl oz of 12% wine or 1.5 fl oz of 40% (80-proof) spirits |
| Canada (Health Canada) | 13.45 g | 341 mL of 5% beer, 142 mL of 12% wine or 43 mL of 40% spirits |
| Australia (NHMRC) | 10 g | 12.7 mL of pure alcohol, about 255 mL of 5% beer |
| United Kingdom (NHS) | 1 unit = 8 g (10 mL) | units = ABV × mL ÷ 1,000; a pint of 5% beer is 2.8 units |
Legal driving limits around the world
Limits are written in different units: 0.05% (grams per 100 mL, the US way) is the same as 0.5 ‰, 0.5 g/L or 50 mg per 100 mL. The World Health Organization recommends a limit of no more than 0.05% for drivers in general and 0.02% for young and novice drivers. Selected countries, from official sources (checked October 2026; laws change, so confirm locally):
| Country | General limit | Stricter limits | Source |
|---|---|---|---|
| United States | 0.08%; Utah 0.05% (since December 30, 2018) | — | NHTSA, Utah Highway Safety Office |
| Canada | 0.08% (80 mg per 100 mL, Criminal Code) | Ontario: roadside suspension from 0.05%; zero for drivers 21 and under | Criminal Code s. 320.14, Ontario |
| England, Wales, Northern Ireland | 0.08% (80 mg per 100 mL) | — | GOV.UK |
| Scotland | 0.05% (50 mg per 100 mL) | — | GOV.UK |
| Ireland | 0.05% | 0.02% for learners, the first 2 years of a first license, truck, bus and taxi drivers | Road Traffic Act 2010 |
| Germany | 0.05% (0.5 ‰) | zero during the probation period and under 21 | StVG § 24a, § 24c |
| France | 0.05% (0.5 g/L) | 0.02% with a probationary license | Service-Public.fr |
| Spain | 0.05% (0.5 g/L) | 0.03% for the first 2 years of a license and for professional drivers | Reglamento General de Circulación, art. 20 |
| Italy | 0.05% (0.5 g/L) | zero under 21, in the first 3 years of a license and for professional drivers | Polizia di Stato |
| Netherlands | 0.05% (0.5 ‰) | 0.02% for novice drivers (first 5 years) | Rijksoverheid |
| Poland | 0.02% (0.2 ‰); above 0.05% it is a crime | — | Sejm |
| Sweden | 0.02% (0.2 ‰) | — | Riksdagen |
| Norway | 0.02% (0.2 ‰) | — | Lovdata |
| Czechia, Hungary, Romania, Slovakia | zero: no alcohol allowed | — | Service-Public.fr |
| Russia | 0.03% (0.3 g/L blood, 0.16 mg/L breath) | — | Supreme Court of Russia |
| Brazil | any alcohol is penalized; 0.06% (6 dg/L) or more is a crime | — | Código de Trânsito, arts. 165, 276, 306 |
| Japan | 0.03% (0.3 mg/mL blood, 0.15 mg/L breath) | — | e-Gov |
| China | 0.02% drink driving; 0.08% drunk driving | — | GB 19522-2024 |
| India | 0.03% (30 mg per 100 mL) | — | Motor Vehicles Act, s. 185 |
| Australia (New South Wales) | 0.05% | zero for learner and provisional drivers; 0.02% for bus, taxi, heavy-vehicle and dangerous-goods drivers | NSW Government |
| New Zealand | 0.05% (250 µg/L breath) | zero under 20 | NZ Police |
| South Africa | 0.05% | 0.02% for professional drivers | SAnews |
Many places also punish driving while impaired at any BAC, and penalties rise steeply at higher levels. A legal limit is not a safe level.
What different BAC levels do
The US National Highway Traffic Safety Administration describes the effects that typically appear at each level (The ABCs of BAC):
| BAC | Typical effects | Effects on driving |
|---|---|---|
| 0.02% | Some loss of judgment, relaxation, slight body warmth, altered mood | Weaker visual functions; harder to do two things at once |
| 0.05% | Exaggerated behavior, impaired judgment, lower alertness, released inhibitions | Reduced coordination, harder to track moving objects and to steer, slower response to emergencies |
| 0.08% | Poor muscle coordination, harder to detect danger; judgment, self-control, reasoning and memory impaired | Trouble concentrating and controlling speed, short-term memory loss, slower information processing, impaired perception |
What changes your real BAC
- Food. A meal slows the absorption of alcohol and lowers the peak; on an empty stomach BAC rises faster and higher.
- Drinking speed. Several drinks in a short time push the peak up, because the liver clears alcohol at a roughly fixed rate.
- Body composition and sex. Alcohol spreads into body water, not fat. For the same weight, women and people with more body fat usually reach a higher BAC.
- Medication and health. Many medicines interact with alcohol, and liver disease slows its removal.
- Tolerance does not lower BAC. Regular drinkers may feel less drunk at the same level, but their reactions are still impaired.
Only time lowers BAC. Coffee, water, food after drinking, a cold shower or sleep for a few hours do not speed up the liver. If someone is confused, vomiting, breathing slowly or cannot be woken, call emergency services — alcohol poisoning is a medical emergency.
This page is for general information and does not replace advice from a doctor. If you are worried about how much you drink, talk to a health professional; in the US, SAMHSA’s free, confidential National Helpline is 1-800-662-4357.
Frequently asked questions
Can I use this calculator to tell whether I can drive?
No. It is an estimate based on population averages, and your real BAC can be considerably higher. Driving skills are affected well below legal limits, and some places allow no alcohol at all. If you have been drinking, don’t drive.
How long does alcohol take to leave the body?
On average the body removes about 0.015% BAC per hour, so a BAC of 0.08% takes over five hours to reach zero, and longer for slow eliminators. Nothing speeds this up: coffee, water and cold showers do not.
What is the difference between BAC in % and in per mille (‰)?
They measure the same thing on different scales. 0.08% means 0.08 grams of alcohol per 100 mL of blood, which is 0.8 grams per liter or 0.8 ‰. European limits are often written as 0.5 ‰ or 0.5 g/L, the same as 0.05%.
Why do women reach a higher BAC from the same drinks?
On average women weigh less and have a lower share of body water, the part of the body that alcohol dissolves in, so the same amount of alcohol ends up more concentrated in the blood.
How accurate is a BAC calculator?
Only roughly. The formula uses averages: the Widmark factor varies from person to person even at the same sex and weight, absorption depends on food and drinking speed, and elimination ranges from about 0.010% to 0.020% per hour in moderate drinkers. Your real BAC can differ a lot from the estimate. Breath and blood tests measure; a calculator only estimates.
Last updated: October 15, 2026