BAC Calculator

Formula

How to use it

1. Select your sex and enter your weight.
2. Enter how many drinks you have had and pick the drink size; choose “Other” to enter grams of alcohol per drink.
3. Enter the hours since your first drink.
4. Read the estimated BAC and the time until it is likely to reach zero. Treat both as rough.

Worked examples

Never use this to decide whether to drive

Alcohol affects judgment and reaction time well below any legal limit, and you cannot feel or calculate your way to a safe number. Real BAC can be much higher than this estimate for the same drinks: absorption depends on food, how fast you drank, medicines and health conditions, and the rate the body clears alcohol differs from person to person. If you have been drinking, do not drive: use a designated driver, public transport or a taxi.

Real clearance can take hours longer than the time to zero shown, so the estimate cannot be used to judge driving the next morning either.

What the estimate leaves out

The Widmark formula assumes all the alcohol has been absorbed and spread through the body’s water. In reality BAC keeps rising for a while after the last drink, especially on an empty stomach, so early on the estimate can be too low or too high. The r factors are averages for men and women; people with more body fat for their weight tend to have a lower r and therefore a higher BAC. The elimination rate of 0.015 percentage points per hour is a commonly used average; actual rates vary between people.

Coffee, cold showers, exercise or sleep do not speed up the rate at which the liver clears alcohol. Only time lowers BAC.

The result is also shown in grams per liter, which equals per mille (‰), the way most European limits are written: 0.8 g/L is 0.8‰.

No amount of alcohol is known to be safe in pregnancy. Medicines such as sedatives, opioids and some antihistamines increase the effect of alcohol, and the estimate does not apply with liver disease.

When to get help

Call 911 (or your local emergency number) if someone who has been drinking is confused, has a seizure, is vomiting while drowsy, breathes slowly or irregularly, has bluish or pale skin, feels very cold, or cannot be woken. Do not leave them alone, and keep them on their side. Above about 0.20% the result shows these signs directly.

If you are worried about your own drinking, talk to your doctor. In the US you can also call the SAMHSA National Helpline at 1-800-662-4357 (free and confidential).

This is an estimate for general information, not medical advice. Talk to a doctor or other qualified clinician before making health decisions based on it.

Sources

The Widmark formula and the distribution factor r (about 0.68 for men and 0.55 for women): Widmark EMP. Die theoretischen Grundlagen und die praktische Verwendbarkeit der gerichtlich-medizinischen Alkoholbestimmung. Berlin: Urban & Schwarzenberg, 1932.

Elimination rates of alcohol from blood and their variation between people: Jones AW. “Evidence-based survey of the elimination rates of ethanol from blood with applications in forensic casework.” Forensic Science International 2010;200:1–20.

US standard drink of 14 g of pure alcohol (12 oz beer at 5%, 5 oz wine at 12%, 1.5 oz spirits at 40%), and that only time lowers BAC: National Institute on Alcohol Abuse and Alcoholism (NIAAA), “What Is a Standard Drink?” and “Understanding the Dangers of Alcohol Overdose” (niaaa.nih.gov).

UK unit of 8 g (10 mL) of pure alcohol: NHS, “Alcohol units” (nhs.uk). Australian standard drink of 10 g: National Health and Medical Research Council, Australian Guidelines to Reduce Health Risks from Drinking Alcohol (2020).

Signs of alcohol overdose and when to call for help: NIAAA, “Understanding the Dangers of Alcohol Overdose”. Helpline: Substance Abuse and Mental Health Services Administration (SAMHSA), National Helpline.