This calculator gives a rough estimate only. Never use it to decide whether you are safe to drive. Your real blood alcohol can be much higher than any formula predicts. If you have been drinking, don't drive.
How to use the BAC calculator
- Choose your sex and units, then enter your body weight.
- Pick the type of drink: a US standard drink, a common beer, wine or spirit serving, or other to enter the size and alcohol by volume (ABV) yourself.
- Enter the number of drinks and the hours since your first drink.
- Read the estimated BAC, the time until it is likely to reach zero, and the hour-by-hour table.
The Widmark formula
The formula was developed by the Swedish scientist Erik Widmark in the 1930s and is still the basis of most BAC estimates. Alcohol spreads through the water in your body, so the r factor is the share of body weight that alcohol can spread into. It is lower on average for women, who tend to have less body water for the same weight. The body then clears alcohol at a fairly steady rate, about 0.015% per hour on average. Ethanol weighs 0.789 g per mL. In the United States a standard drink contains 14 g of alcohol (0.6 fl oz), according to the National Institute on Alcohol Abuse and Alcoholism (NIAAA). That is about 12 fl oz of 5% beer, 5 fl oz of 12% wine, or 1.5 fl oz of 40% spirits.
Worked example
The calculator's defaults are a man weighing 80 kg who has had 3 standard drinks, starting 2 hours ago:
- Alcohol: 3 × 14 g = 42 g
- Peak if absorbed at once: 42 ÷ (80,000 × 0.68) × 100 = 0.077%
- Minus elimination: 0.015 × 2 hours = 0.030%
- Estimated BAC: 0.077 − 0.030 = 0.047%
- Time until about 0.00%: 0.047 ÷ 0.015 = 3.1 hours, or about 3 hours 9 minutes
A 60 kg woman who drank the same would be at about 0.097%, double the estimate. Two 5 fl oz glasses of 12% wine for the 80 kg man, one hour after the first, give about 0.036%.
Why the real number can be very different
Widmark's formula assumes that all the alcohol is absorbed straight away and cleared at an average rate. Real life is messier:
- Food slows absorption. On an empty stomach, BAC rises faster and peaks higher.
- Drinking speed matters. Several drinks in a short time produce a higher peak than the same drinks spread out.
- Elimination rates vary between people, and with liver health, medicines and drinking habits.
- Drink sizes are often bigger than a standard drink: a large glass of wine can hold two.
- Absorption continues after you stop drinking, so your BAC may still be rising for 30 to 90 minutes after the last drink.
Breath and blood tests can still give results well above this estimate. Only time lowers BAC: coffee, cold showers and exercise don't speed it up.
Legal limits and impairment
In the United States the legal limit for drivers aged 21 and over is 0.08% in every state except Utah, where it is 0.05%. Drivers under 21 face zero-tolerance limits, and commercial drivers a limit of 0.04%. Many other countries use 0.05% or lower. These are legal thresholds, not safe levels. Coordination, judgment and reaction time start to suffer at much lower levels, and the risk of a crash rises with every drink.
If someone is confused, vomiting, having a seizure, breathing slowly or irregularly, or hard to wake, they may have alcohol poisoning. Call emergency services and don't leave them alone.
Frequently asked questions
How accurate is this BAC calculator?
Only roughly. The Widmark formula uses average values, and your true BAC can be noticeably higher or lower. Treat the result as a general guide to how alcohol builds up and clears, never as a measurement.
How long does it take to get back to zero?
On average the body clears about 0.015% per hour, roughly one standard drink an hour for many adults. After a night of heavy drinking you can still be over the limit the next morning, even after sleeping.
Why do women reach a higher BAC from the same drinks?
On average, women have a lower proportion of body water, so the same amount of alcohol is more concentrated in their blood. The formula reflects this with a lower r factor.
Is it safe to drive if the calculator shows 0.00%?
Don't use the calculator to decide. It can't measure your actual blood alcohol, and you may still be impaired. If you have been drinking, use a taxi, public transportation or a sober driver, or wait until the next day.
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Results are estimates for general information and are not medical advice. Talk to a doctor or other qualified professional about your health. See our disclaimer.