Blood Alcohol Content (BAC) Calculator
Estimate blood alcohol content from your drinks, weight, sex, and time using the Widmark formula, with a BAC-over-time chart and time until sober
By Waqar Mushtaq · Last updated:
Blood alcohol content (BAC) is the concentration of alcohol in your blood, written as grams per 100 mL (g/dL) -- so a BAC of 0.08% means 0.08 g of alcohol in every 100 mL of blood, the legal driving limit across the US. This calculator estimates your BAC with the Widmark formula, the same dilution model used in forensic toxicology, from what you drank, your body weight, your sex, and how long ago you started. Beyond the single number it shows how long until you're below 0.08% and back to zero, a BAC-over-time chart with the US and Utah limits marked, and what effects are typical at your estimated level.
This is an estimate, not a breathalyzer. Food, drinking speed, medications, and individual metabolism move real BAC by 20% or more in either direction. Never use a calculator to decide whether you're fit to drive.
How BAC Is Estimated (the Widmark Formula)
O(n) in drinksA = grams of alcohol · W = body weight in grams · r = Widmark factor (0.68 men, 0.55 women) · β = elimination rate (0.015 g/dL per hour) · t = hours since the first drink
0.789 g/mL is the density of ethanol
The formula treats alcohol as dissolving evenly through your body water. r is the fraction of body weight the alcohol effectively spreads into: on average about 0.68 for men and 0.55 for women, because women typically carry more body fat (which holds almost no water) per kilogram. Dividing the alcohol by that diluting mass gives the peak concentration; your liver then clears it at a roughly constant rate of about 0.015% per hour.
Example -- a 180 lb (81.6 kg) man drinks four 12 oz beers at 5% ABV and it's been 2 hours:
- Alcohol: 4 × 354.9 mL × 0.05 × 0.789 = 56.0 g (four US standard drinks)
- Peak: 56.0 / (81,647 g × 0.68) × 100 = 0.101%
- After 2 hours: 0.101 − 0.015 × 2 = 0.071%
- Back to zero: 0.101 / 0.015 ≈ 6.7 hours after the first drink
The same four beers give a 130 lb woman a peak of about 0.17% -- more than two-thirds higher -- because both the lower body weight and the lower r value concentrate the same alcohol into less body water.
Like most online BAC calculators, this assumes the alcohol is absorbed all at once. In reality absorption takes 30-90 minutes (longer with food), so BAC typically keeps rising for a while after the last drink -- the real peak is somewhat lower and later than the estimate, but the time to get back to zero is about the same.
Use Cases
Understanding How Much Is Too Much
See how quickly a few drinks add up for your weight and sex -- and why the same number of drinks affects two people so differently. The effects table puts the estimate in context.
Planning the Morning After
The time-to-zero estimate shows why you can still be over the limit the next morning after a late night of heavy drinking. Sleep doesn't speed elimination; only time does.
Alcohol Education and Training
Health educators, server-training programs, and driving courses use Widmark estimates to show the relationship between standard drinks, body weight, and BAC -- the chart makes the linear decline easy to explain.
Comparing Drinks by Strength
A 16 oz pint of 8% craft IPA is more than two standard drinks, not one. Enter a drink's real size and ABV to see how it compares with a standard 5% beer.
Frequently Asked Questions
How accurate is a BAC calculator?
Only roughly. Forensic studies put the uncertainty of a Widmark estimate at around ±20% even with good inputs, because the r factor and elimination rate vary from person to person (elimination ranges from about 0.010% to over 0.030% per hour). Eating, drinking speed, medications, and health conditions all shift the result. Treat the number as a ballpark, and never as permission to drive.
What is a standard drink?
In the US, a standard drink contains about 14 grams (0.6 fl oz) 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. Many real drinks are bigger: a large restaurant wine pour or a strong craft beer can be two standard drinks or more. Other countries define it differently (a UK "unit" is 8 g).
How long does it take to get back to 0.00%?
On average BAC falls by about 0.015% per hour, so divide your peak BAC by 0.015. A peak of 0.12% takes about 8 hours to clear. Coffee, water, cold showers, and exercise do not speed this up -- your liver works at a roughly fixed rate.
What is the legal BAC limit for driving?
It's illegal to drive with a BAC of 0.08% or higher in every US state, and Utah's limit is 0.05%. Commercial drivers are held to 0.04%, and drivers under 21 face zero-tolerance laws (typically 0.00-0.02%). Many countries use 0.05% or lower. Impairment begins well below any legal limit.
Why does sex change the result?
On average, women have a higher proportion of body fat and less body water than men of the same weight, so the same amount of alcohol ends up more concentrated in the blood. The Widmark factor captures this: 0.55 for women versus 0.68 for men. It's an average -- a very lean woman or a man with a high body-fat percentage will differ from it.
Is this the same as a breathalyzer reading?
No. A breathalyzer measures alcohol in your breath and converts it to an estimated BAC; this calculator predicts BAC from what you drank. Both are estimates of blood alcohol, and only a blood test measures it directly.
Methodology
The formula above is implemented as a standalone module covered by unit tests, so its worked examples are re-checked on every change to the site. It is verified against the authoritative sources listed below.
Not medical advice. This calculator is a general informational tool and cannot account for your individual circumstances. See the terms of use.
Maintained by Waqar Mushtaq. Spotted a result that looks wrong? Report it and it becomes a failing test.