Enter your weight, sex, how many standard drinks you've had, and hours since your first drink. This calculator estimates your blood alcohol content (BAC) using the Widmark formula. It is an estimate only — never use it to decide whether you're safe to drive.
How the BAC calculator works
This calculator uses the Widmark formula, developed by Swedish physician Erik Widmark in the 1930s and still the standard tool in forensic toxicology. The idea: alcohol distributes through your body water, so your peak BAC depends on how much alcohol you drank relative to your body weight, adjusted by a distribution ratio (r) that reflects average body water — about 0.68 for men and 0.55 for women. Your body then clears roughly 0.015% BAC per hour, so the formula subtracts that for every hour since your first drink.
The formula
BAC% = ( alcohol (g) ÷ ( body weight (g) × r ) ) × 100 − 0.015 × hours
Alcohol grams = standard drinks × 14 g. Body weight is converted to grams (1 lb = 453.59 g). r is the Widmark distribution ratio: 0.68 for men, 0.55 for women. Hours is time elapsed since your first drink, and the result never goes below zero.
Worked example
A 160 lb man has 3 standard drinks over 2 hours:
Alcohol = 3 × 14 = 42 g. Body weight = 160 × 453.59 = 72,575 g.
BAC = (42 ÷ (72,575 × 0.68)) × 100 − 0.015 × 2 = 0.085 − 0.030 = 0.055%
Below the 0.08% limit — but measurably impaired, and well above the limit for a commercial or under-21 driver.
Why this is only an estimate — and never a green light to drive
The Widmark formula assumes an average body, an empty stomach, and instant absorption. Reality is messier: food slows absorption and lowers peak BAC, some medications amplify alcohol's effects, body composition shifts the distribution ratio, and elimination rates vary from roughly 0.010 to 0.025% per hour between individuals. Two people can drink identically and blow very different numbers. That's why no calculator — this one included — should ever be used to decide whether you're okay to drive. The legal limit is 0.08% in most US states (0.05% in Utah, lower in much of the world, and far lower for commercial and under-21 drivers), but impairment of judgment and reaction time begins around 0.02–0.05%. If you've been drinking, the correct BAC for driving is a number, and the number is zero.
Frequently asked questions
What counts as one standard drink?
In the US, one standard drink contains 14 grams of pure alcohol: a 12 oz beer at 5% ABV, a 5 oz glass of wine at 12%, or a 1.5 oz shot of 80-proof spirits. Craft beers, large pours, and cocktails often contain 1.5 to 2+ standard drinks each, which is the most common reason people underestimate their BAC.
What is the legal BAC limit for driving?
In most US states the limit is 0.08% for drivers 21 and over, and Utah uses 0.05%. Commercial drivers are typically held to 0.04%, and drivers under 21 face zero-tolerance limits of 0.00–0.02%. Many other countries use 0.05% or lower. But impairment starts well below any legal limit — the only safe BAC for driving is zero.
How accurate is a BAC calculator?
Not accurate enough to make safety decisions with. The Widmark formula uses population-average body water values, but food in your stomach, medications, fatigue, hydration, and individual metabolism can swing your true BAC substantially in either direction. Only a calibrated breathalyzer or blood test measures actual BAC.
How long does it take for BAC to return to zero?
Your body eliminates alcohol at a roughly fixed rate of about 0.015% BAC per hour, and nothing speeds it up — not coffee, cold showers, or exercise. From a BAC of 0.08%, that's a bit over 5 hours to reach zero. From a heavy night at 0.15%, it's about 10 hours, which is why morning-after driving can still be drunk driving.
Why does the same number of drinks affect people differently?
Alcohol distributes through body water, so total body water drives BAC. Smaller people and, on average, women have less body water per pound, so the same drinks produce a higher concentration. The Widmark formula captures this with a distribution ratio — about 0.68 for men and 0.55 for women — but individual variation is still large.