Blood Alcohol (BAC) Calculator
Estimate blood alcohol content. An estimate only, never for driving.
One US standard drink = 14 g of pure alcohol (a 12 oz 5% beer, 5 oz wine, or 1.5 oz spirit).
Time elapsed since you started drinking. Your body clears about 0.015% BAC per hour.
Estimated BAC
- Effect band
- At or over the 0.08% US driving limit
- Pure alcohol consumed
- 56 g
This is an estimate only and must never be used to decide whether it is safe to drive. There is no safe way to calculate that you are under a legal limit. If you have been drinking, do not drive.
Rough estimate from the Widmark equation. Your real BAC depends on food, drink strength, pace, medication, and your own metabolism, and can be much higher than shown.
How the BAC calculator works
This tool estimates blood alcohol concentration (BAC) using the Widmark equation, the standard back-of-envelope model taught in toxicology. BAC is expressed as a percentage by volume, where 0.08% is the common legal driving limit across the United States.
The calculation has two parts. First it finds the peak concentration from the alcohol in your system, then it subtracts what your body has cleared over time.
The grams of pure alcohol come from your drink count: alcoholGrams = drinks * 14, since one US standard drink holds about 14 grams. That alcohol distributes through your body water, which the model approximates as weightKg * 1000 grams scaled by a distribution ratio r (0.68 for men, 0.55 for women). The peak is (alcoholGrams / (weightKg * 1000 * r)) * 100. Finally, because the body eliminates roughly 0.015% per hour, the tool subtracts 0.015 * hours since your first drink. The result is floored at zero, since you can never have a negative amount of alcohol in your blood.
Worked example
A man weighing 80 kg has 4 standard drinks, and it has been 1 hour since his first.
- Alcohol consumed:
4 * 14 = 56 g - Body water term:
80 * 1000 * 0.68 = 54,400 - Peak BAC:
(56 / 54,400) * 100 = 0.103% - Eliminated in 1 hour:
0.015 * 1 = 0.015 - Estimated BAC:
0.103 - 0.015 = 0.088%
At about 0.088% this estimate already sits over the 0.08% driving limit. Wait two more hours with nothing further to drink and the elimination term grows to 0.015 * 3 = 0.045, leaving 0.103 - 0.045 = 0.058%. The alcohol leaves only with time, never faster.
How to use it
- Count your standard drinks honestly. A large glass of wine or a strong cocktail can be two drinks or more, so round up when unsure.
- Enter your body weight. You can switch between kilograms and pounds with the unit toggle; pounds are converted to kilograms before the math runs, so the result is the same either way.
- Choose your sex, which sets the distribution ratio in the formula.
- Enter the hours since your first drink, which drives how much alcohol your body has had time to clear.
Limitations and safety
This calculator is for general education only. It is not a breathalyzer and it is not medical or legal advice. The Widmark equation assumes full absorption and uses average constants, so your real BAC can be considerably higher than the number shown, especially if you drank quickly, on an empty stomach, or had stronger drinks than you counted. Sex, medication, illness, and individual metabolism all move the true figure.
Do not use this result to decide whether to drive, operate machinery, or do anything where impairment is dangerous. There is no BAC reading from a tool like this that makes driving safe. If you have been drinking, arrange another way home. If you are worried about your own or someone else’s drinking, speak to a qualified health professional.
Frequently asked questions
Can I use this to check whether I am safe to drive?
No. This is an estimate, not a measurement, and it can be wrong by a wide margin in either direction. The only honest answer is that if you have been drinking, you should not drive. No calculator can tell you that you are under a legal limit, because the real figure depends on details this tool cannot know. Treat any result above zero as a reason not to get behind the wheel.
What counts as one standard drink?
In the United States a standard drink contains about 14 grams of pure alcohol. That is roughly a 12 ounce beer at 5 percent, a 5 ounce glass of wine at 12 percent, or a 1.5 ounce shot of spirits at 40 percent. Cocktails, craft beers, and large pours often hold more than one standard drink, so count generously rather than optimistically.
Why does the same number of drinks give a woman a higher result?
The Widmark equation uses a distribution ratio that reflects how much of the body is water, since alcohol spreads through body water rather than fat. The conventional average ratio is 0.68 for men and 0.55 for women, so at the same weight and dose the estimate is higher for a woman. These are population averages and do not capture every individual difference.
How fast does the body clear alcohol?
On average the body eliminates roughly 0.015 percent of BAC per hour, and that rate is fairly steady regardless of how much you have had. This tool subtracts 0.015 times the hours since your first drink. Drinking coffee, taking a cold shower, or eating afterwards does not speed this up. Only time lowers your BAC.
Why might my real BAC be higher than the estimate?
The Widmark model assumes alcohol is fully absorbed and uses average constants. Drinking on an empty stomach, drinking quickly, stronger drinks than you counted, medication, illness, and individual metabolism can all push your true concentration above the figure shown. Because of that, a low estimate is never a green light.