Blood Alcohol Calculator
Add the drinks you had and when, and this calculator estimates your blood alcohol concentration (BAC) over time with the Widmark formula: peak BAC, and roughly when it drops below 0.08%, 0.05% and zero. It is an educational estimate only and must never be used to decide whether you can drive.
Estimate only. Never use this to decide whether you can drive. Your real BAC can be much higher than any calculator shows. If you have been drinking, don't drive.
Drinks
Educational estimate based on population averages. Impairment begins with the first drink. Legal limits vary (0.08% in most US states, 0.05% in Utah and much of Europe, lower or zero for young and commercial drivers).
How to use the Blood Alcohol Calculator
- Enter your sex and body weight, and optionally switch to the Watson method and add your age and height.
- Add each drink: pick a preset or set the volume and ABV, and enter the time you started it. Press Add drink for more rows.
- Choose your stomach contents and the elimination rate (0.015% per hour is average).
- Press Calculate BAC to see your estimated peak, the BAC curve and when it should fall below 0.08%, 0.05% and zero.
- Do not drive after drinking, whatever the estimate says. Plan a taxi, public transport or a sober driver.
The Widmark formula
Swedish chemist Erik Widmark published the classic formula in 1932, and it is still the basis of forensic BAC estimates:
BAC (%) = A ÷ (r × W) × 100 − β × t
- A = grams of alcohol consumed = volume (ml) × ABV × 0.789 g/ml (the density of ethanol).
- W = body weight in grams.
- r = Widmark distribution factor: about 0.68 for men and 0.55 for women (it reflects body water). The Watson option estimates total body water (TBW) from age, height and weight (Watson et al., 1980) and uses r = TBW ÷ (0.8 × weight), since blood is about 80% water.
- β = elimination rate, on average about 0.015% per hour (typical range 0.010–0.020%/h; heavy drinkers can be faster).
- t = hours since drinking began.
The calculator runs the formula minute by minute: each drink is absorbed evenly over 30, 60 or 90 minutes depending on stomach contents, and elimination is subtracted whenever BAC is above zero.
Worked example and standard drinks
A 180 lb (81.6 kg) man drinks one 12 oz 5% beer: 355 × 0.05 × 0.789 = 14.0 g of alcohol. Without any elimination that would give 14.0 ÷ (0.68 × 81,647) × 100 = 0.025%. Three such beers at 8:00, 8:45 and 9:30 pm with a light meal give an estimated peak of about 0.04% around 10:30 pm, and BAC returns to zero about 5 hours after the first drink, because roughly 0.015% is removed every hour while the beers are still being absorbed.
| Drink | Volume | ABV | Alcohol | US standard drinks |
|---|---|---|---|---|
| Regular beer | 12 fl oz (355 ml) | 5% | 14.0 g | 1.0 |
| Craft IPA pint | 16 fl oz (473 ml) | 7% | 26.1 g | 1.9 |
| Wine | 5 fl oz (148 ml) | 12% | 14.0 g | 1.0 |
| Large wine | 250 ml | 13% | 25.6 g | 1.8 |
| Spirits shot | 1.5 fl oz (44 ml) | 40% | 13.9 g | 1.0 |
The US standard drink of 14 g is defined by the National Institute on Alcohol Abuse and Alcoholism (NIAAA). Cocktails often contain two or more standard drinks, and home pours are usually larger than standard.
Why this is only an estimate
Studies comparing Widmark estimates with measured blood alcohol find individual errors of 20% or more. Drinking speed, food, carbonation, medication, liver function, genetics and the actual strength of drinks all change the result. Effects start long before legal limits: at 0.02–0.05% judgement and reaction time are already affected, and crash risk rises steeply above 0.05%. Many countries use a 0.05% limit and some 0.02% or zero. The only safe level for driving is zero: if you have been drinking, use a taxi, public transport or a designated driver, and remember you may still be over the limit the next morning.
If you are worried about your drinking, the SAMHSA National Helpline (1-800-662-4357, US) is free and confidential.
Frequently asked questions
How is BAC calculated?
With the Widmark formula: BAC (%) = alcohol in grams ÷ (body weight in grams × r) × 100 − β × hours, where r is the body water distribution factor (about 0.68 for men and 0.55 for women) and β is the elimination rate (about 0.015% per hour). The calculator applies it minute by minute, with each drink absorbed gradually.
Can I use this to know if I am safe to drive?
No. Real BAC can differ a lot from any formula because of food, drinking speed, medication, liver health, genetics and how drinks were poured. Impairment starts well below legal limits, and in some places the limit is 0.05% or zero for new drivers. If you have been drinking, do not drive.
How long does it take to sober up?
The liver removes alcohol at a roughly constant rate, on average about 0.015% BAC per hour (roughly one US standard drink per hour for many adults). Coffee, cold showers, exercise or sleep do not speed it up; only time does.
What is a standard drink?
In the United States a standard drink contains 14 grams of pure alcohol: 12 fl oz of 5% beer, 5 fl oz of 12% wine or 1.5 fl oz of 40% spirits. Other countries use different sizes, such as 10 g in Australia and 8 g (one unit) in the UK.
Why do women reach a higher BAC from the same drinks?
On average women have less body water per kilogram than men, so the same amount of alcohol is more concentrated. That is why the Widmark factor r is about 0.55 for women versus 0.68 for men.
What does food do?
Eating before or while drinking slows absorption from the stomach, so the peak is lower and later. The calculator models this only roughly by spreading absorption over 30, 60 or 90 minutes.