BAC Calculator
Estimate your blood alcohol concentration with the Widmark formula based on drinks, weight, sex and time elapsed.
BAC Calculator — input your values
What does this health calculator do?
This BAC calculator estimates your blood alcohol concentration using the Widmark formula, which combines the grams of alcohol you consumed, your body weight, your sex (which affects body water distribution) and the hours that have passed since your first drink.
It shows your estimated BAC as a percentage, the grams of pure alcohol in what you drank, and a curve of how your BAC declines over the following hours. A table explains typical impairment at each BAC level, and the result is compared with the 0.08% US legal driving limit.
IMPORTANT: This is an educational estimate, not a measurement, and it is never a basis for deciding whether it is safe to drive. BAC varies with food intake, hydration, medications and individual metabolism, and breathalyzer results can differ. The only safe choice is to not drive after drinking.
Formula and variables
The 0.015 per hour term represents average metabolism; actual rates range from about 0.01 to 0.02 per hour.
| Symbol | Meaning |
|---|---|
| alcohol grams | Ounces × ABV × 29.5735 × 0.789 |
| weight | Body weight in grams |
| r | Widmark ratio: 0.68 (men), 0.55 (women) |
| hours | Hours since the first drink |
Worked example
A 75 kg man drinks 3 beers (12 oz, 5% ABV) over about 1 hour.
- Alcohol grams = 3 × 12 × 0.05 × 29.5735 × 0.789 = 42.0 g.
- Widmark numerator = 42.0 × 100 ÷ (75,000 × 0.68) = 0.0824.
- Metabolism = 0.015 × 1 hr = 0.015.
- BAC = 0.0824 − 0.015 = 0.0674, about 0.067%.
Answer: Estimated BAC ≈ 0.067% — below the 0.08% limit, but still impaired.
Tips and common mistakes
Tips
- Eat before and during drinking — food slows alcohol absorption, though it does not speed elimination.
- Drink water between alcoholic drinks and pace yourself to about one standard drink per hour.
- Only time lowers BAC: coffee, cold showers and exercise do not speed up metabolism.
- If you plan to drink, arrange a designated driver, ride-share or public transport before you start.
Common mistakes to avoid
- Treating the estimate as a real breathalyzer reading — individual variation can shift results by 0.01–0.02% or more.
- Ignoring that mixed drinks and craft beers often contain more alcohol than a standard drink.
- Believing the BAC has dropped because you feel more alert; impairment can persist after the feeling fades.
BAC Calculator — frequently asked questions
What is BAC?
Blood alcohol concentration is the percentage of alcohol in your blood, expressed as grams per deciliter. A BAC of 0.08% means 0.08 g of alcohol per 100 ml of blood.
How accurate is the Widmark formula?
It's a well-established estimate, but real BAC depends on food, hydration, medications, body composition and genetics. Only a breath or blood test measures it accurately.
How long does it take to sober up?
On average the body eliminates about 0.015% BAC per hour. At 0.08%, reaching zero typically takes about five hours — and only time accomplishes it.
Is 0.08% the legal limit everywhere?
It is the standard limit for adult drivers in the US. Many other countries use 0.05%, and some have zero-tolerance policies for new or commercial drivers.
Why does sex matter in the calculation?
Men typically have a higher water-to-fat ratio, diluting alcohol more. The Widmark ratio uses 0.68 for men and 0.55 for women to reflect this.
Can one drink put me over the limit?
For a small or average woman, one standard drink can approach or exceed 0.08% within the first hour. Weight, sex and time all affect the result.