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BAC Calculator

Estimate your Blood Alcohol Content with the Widmark formula from drinks, weight, sex, and time.

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How to use

  1. 1.Pick your sex with the Male / Female toggle so the correct Widmark ratio is used.
  2. 2.Enter the number of US standard drinks (14 g alcohol each), your body weight in kilograms, and the hours since your first drink.
  3. 3.Read the estimated Blood Alcohol Content, which updates instantly — and never use it to decide whether to drive.

About BAC Calculator

This free BAC calculator gives a rough estimate of your Blood Alcohol Content — the percentage of alcohol in your bloodstream — using the classic Widmark formula. It was named after the Swedish chemist Erik M. P. Widmark, who worked out the relationship between the amount of alcohol consumed, body size, and blood alcohol concentration in the 1920s and 1930s. Nearly a century later, a version of his equation is still the starting point for back-of-the-envelope BAC estimates.

The calculation is deliberately simple, and it runs entirely in your browser — nothing you type is ever uploaded. First, the number of standard drinks is converted to grams of pure alcohol. In the United States one standard drink contains about 14 grams of pure alcohol, roughly a 12 oz beer at 5%, a 5 oz glass of wine at 12%, or a 1.5 oz shot of 40% spirits. That grams-of-alcohol figure is then divided by the volume of body water available to dilute it, which is approximated as your body weight multiplied by the Widmark distribution ratio r. Because men and women carry different proportions of body water, this tool uses r = 0.68 for men and r = 0.55 for women. Finally, the body is assumed to eliminate alcohol at an average rate of about 0.015 percentage points per hour, so 0.015 is multiplied by the hours since your first drink and subtracted from the peak figure. The result is clamped at zero, because your BAC cannot fall below nothing.

A worked example makes it concrete: three standard drinks for an 80 kg man one hour after starting works out to roughly 0.062%. Two drinks for a 60 kg woman two hours in comes to about 0.055%. Adjusting any single input moves the answer in the direction you would expect — more drinks push it up, a longer wait pulls it down as your body metabolizes the alcohol, a heavier body dilutes the same alcohol across more body water, and switching from the male to the female ratio raises the estimate because women on average carry a smaller proportion of body water.

The crucial thing to understand is how approximate this really is. The Widmark model uses fixed average ratios and a single average elimination rate, but real Blood Alcohol Content depends on food in the stomach, drinking speed, hydration, medications, liver health, genetics, and how accurately you counted your drinks. Two people with identical inputs can end up with very different actual readings, and the only device that measures true BAC is a calibrated breathalyzer or a blood test. For all these reasons this calculator is an educational estimate only. It must never be used to judge whether you are fit to drive or below any legal limit. If you have been drinking, the safe answer is simply not to drive, and to arrange a taxi, rideshare, or sober driver instead. This tool is not medical or legal advice.

Methodology & sources

This tool estimates Blood Alcohol Content with the Widmark formula. Grams of pure alcohol = standard drinks × 14 (one US standard drink contains 14 g of pure alcohol). BAC% = (gramsAlcohol / (r × weightKg × 1000)) × 100 − 0.015 × hours, where r is the Widmark distribution ratio (0.68 for men, 0.55 for women) and 0.015 is the average elimination rate in percentage points per hour; the result is clamped at 0. Assumptions: US 14 g standard drink, fixed sex-based r values, a single average metabolic elimination rate, and accurate counting of drinks and time. Real BAC also depends on food, drinking pace, hydration, medications, liver health, and individual physiology, so this is a rough educational estimate only. It must NEVER be used to decide whether it is safe or legal to drive — if you have been drinking, do not drive. This is not medical or legal advice.

Frequently asked questions

How does this BAC calculator work?
It uses the Widmark formula. Your standard drinks are converted to grams of alcohol (14 g per US standard drink), divided by your body weight times a distribution ratio r (0.68 for men, 0.55 for women) and 1000, multiplied by 100 to get a percentage, then reduced by 0.015 for every hour since your first drink to account for elimination. The result is clamped at zero.
Can I use this to decide whether it is safe to drive?
No. Absolutely not. This is a rough estimate based on average values and cannot account for the many factors that change real BAC. It must never be used to judge whether you are under a legal limit or fit to drive. If you have been drinking, do not drive — arrange a taxi, rideshare, or sober driver. This is not medical or legal advice.
Why might my real BAC differ from this estimate?
The Widmark model uses fixed average ratios and a single average elimination rate, but actual Blood Alcohol Content depends on food, drinking speed, hydration, medications, liver health, sex differences beyond the ratio, and how accurately you counted drinks. Only a calibrated breathalyzer or a blood test measures true BAC.

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