Health
Blood Alcohol Content (BAC) Calculator
Get a rough Widmark-formula estimate of blood alcohol concentration from body weight, sex, drinks and time. This is not a test of whether you are safe or legal to drive.
Calculation inputs
One standard drink holds 0.6 fl oz (about 14 g) of pure alcohol — see the table below.
Results
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Complete the fields and select Calculate to show results here.
IMPORTANT: This is a rough estimate only. It cannot tell you whether you are sober, unimpaired or legally fit to drive, and it must never be used to make that decision. Alcohol affects judgment and reaction time even at low levels. If you have been drinking, do not drive — use a sober driver, a taxi, rideshare or public transport.
About this calculator
This tool gives a rough estimate of blood alcohol concentration using the Widmark formula. It is an educational guide to how alcohol builds up and wears off, and it cannot tell you whether you are safe or legal to drive. If you have been drinking, do not drive.
How the calculation works
- Alcohol (g) = drinks × 14
- BAC % = alcohol (g) ÷ (weight in g × r) × 100 − 0.015 × hours
- r = 0.68 for males, 0.55 for females
Notes and assumptions
Food, medication, health, drink strength and how fast you drank all change real BAC, so this formula can be far off for any one person.
In every U.S. state the legal limit for most adult drivers is 0.08% (0.05% in Utah), and lower for commercial and under-21 drivers.
How it works in plain English
The Widmark formula, developed in the 1930s, estimates how concentrated alcohol becomes in the body. It starts with the total amount of pure alcohol consumed. In the United States one standard drink holds 0.6 fluid ounces, or about 14 grams, of pure alcohol, so the tool multiplies your number of standard drinks by 14 grams.
That alcohol spreads through the water in your body rather than through fat. The distribution ratio, often written r, adjusts body weight for this. The commonly used averages are 0.68 for men and 0.55 for women, reflecting typical differences in body water. Alcohol in grams is divided by body weight in grams times r, and multiplied by 100 to give a percentage.
The liver then removes alcohol at a fairly steady pace. A standard average is about 0.015 percentage points per hour, so that amount is subtracted for every hour since the first drink. The result is the estimated BAC now, and dividing it by 0.015 gives a rough time until it reaches zero.
Real BAC depends on food, medications, health, genetics, drink strength and drinking speed, and can be much higher than this estimate. Impairment begins well below the legal limit. This number must never be used to decide whether you can drive.
The formula
- Alcohol (g) = standard drinks × 14
- BAC (%) = alcohol (g) ÷ (body weight (g) × r) × 100 − 0.015 × hours
- r = 0.68 (male), 0.55 (female)
Worked example
Consider a man weighing 170 pounds who has had three standard drinks, with two hours passing since his first one.
Three drinks contain about 42 grams of pure alcohol. His weight in grams is 170 × 453.592, or about 77,111 grams, and multiplying by the male ratio of 0.68 gives 52,435. Dividing 42 by 52,435 and multiplying by 100 gives a starting estimate of about 0.080 percent.
Over two hours his body would remove roughly 2 × 0.015, or 0.030, leaving an estimated BAC of about 0.050 percent. That is already above Utah's 0.05 limit and is a level at which reaction time and judgment are measurably affected. Even though the figure sits below 0.08, he should not drive.
U.S. standard drink sizes (each ≈ 0.6 fl oz / 14 g pure alcohol)
| Drink | Typical alcohol by volume | One standard drink |
|---|---|---|
| Regular beer | About 5% | 12 fl oz |
| Malt liquor | About 7% | 8–9 fl oz |
| Table wine | About 12% | 5 fl oz |
| Fortified wine | About 17% | 3–4 fl oz |
| Distilled spirits (80 proof) | About 40% | 1.5 fl oz |
Source: National Institute on Alcohol Abuse and Alcoholism (NIAAA), "What Is a Standard Drink?"; legal limits from NHTSA.
Frequently asked questions
Can I use this to decide whether I can drive?
No. This is a rough population-average estimate, and your real blood alcohol level could be far higher. Driving skills begin to decline after even one drink, long before the legal limit. Police and courts rely on breath or blood tests, not formulas. If you have had any alcohol, the only safe choice is to let a sober person drive or use a taxi, rideshare or public transport.
Why are the ratios different for men and women?
Alcohol dissolves in body water, not fat. On average, women's bodies contain a lower share of water than men's of the same weight, so the same amount of alcohol produces a higher concentration. The values 0.68 and 0.55 are the standard averages used with the Widmark formula, but individual body composition varies widely.
What is the legal BAC limit for driving?
For most adult drivers in the United States the limit is 0.08 percent, except Utah, where it is 0.05 percent. Commercial drivers face a 0.04 percent limit, and drivers under 21 are held to zero or near-zero tolerance. You can still be charged with impaired driving below these limits if your driving is affected.
Does coffee, water or food sober you up faster?
No. Only time lowers blood alcohol, because the liver processes it at a roughly fixed pace. Coffee may make you feel more alert, but it does not reduce impairment. Eating before or while drinking slows how quickly alcohol is absorbed, which can lower the peak level, but it does not speed up its removal.
Why might my real BAC be higher than this estimate?
Drinks are often larger or stronger than a standard drink, especially cocktails, craft beers and generous wine pours. Drinking quickly, an empty stomach, some medications, illness and individual metabolism can all raise the level. The formula also assumes all alcohol is absorbed and processed at an average rate, which is rarely exactly true.