Blood Alcohol Content (BAC) Calculator
Estimates your blood alcohol content (BAC) as a percentage from drinks consumed, body weight, sex and time since the first drink, using the Widmark formula. Use it to understand roughly where you are relative to the 0.08% legal limit; it is an estimate, not a test.
Last updated: September 2026
Formula below · 3 sources (CDC, NIH, Wikipedia) · Updated Sep 2026
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About this calculator
The Widmark formula estimates BAC using the equation: BAC (%) = (drinks × 14) / (weight × 1,000 × r) × 100 − (0.015 × hoursElapsed), where 14 grams is the mass of alcohol in one US standard drink, weight is converted from kilograms to grams (× 1,000) so that grams of alcohol per gram of body weight × 100 gives grams per 100 mL, the unit of BAC %, and r is the Widmark distribution factor — approximately 0.68 for males and 0.55 for females, reflecting differences in body water percentage. The subtracted term (0.015 × hours) represents the liver's average elimination rate of 0.015% BAC per hour. The result is floored at zero because BAC cannot be negative. A BAC of 0.08% is the legal driving limit in most US states and many countries. The formula provides an estimate; actual BAC varies with food intake, hydration, and individual metabolism, and it assumes all the alcohol has been absorbed (BAC can still be rising for 30–90 minutes after the last drink). To estimate when you will be back to zero, divide the result by 0.015 to get the hours still needed. Never use it to decide whether you are fit to drive.
How to use
A 70 kg male has consumed 3 standard drinks and 2 hours have passed since his first drink. Step 1 — alcohol consumed: 3 × 14 = 42 g. Step 2 — distribution volume: 70 kg × 1,000 × 0.68 = 47,600 g of body water equivalent. Step 3 — peak BAC: 42 / 47,600 × 100 = 0.0882%. Step 4 — subtract elimination: 0.0882 − (0.015 × 2) = 0.0882 − 0.030 = 0.0582%. The calculator shows about 0.06% — below the 0.08% legal limit but already in the range where reaction time and judgment are impaired, and it would take about 0.0582 / 0.015 ≈ 4 more hours to reach zero. A 60 kg woman with the same drinks and time would be at 42 / (60,000 × 0.55) × 100 − 0.03 ≈ 0.097%, over the limit.
Frequently asked questions
How long does it take for blood alcohol content to reach zero after drinking?
The liver metabolises alcohol at a roughly constant rate of about 0.015% BAC per hour for most adults, regardless of coffee, food, or exercise after drinking. Dividing your peak BAC by 0.015 gives the approximate hours to sobriety; for example, a BAC of 0.09% takes about 6 hours to clear. Factors like liver health, medication, and genetic variation can slow this rate significantly. There is no safe shortcut — only time reduces BAC reliably.
Why does gender affect blood alcohol content calculations?
Females generally have a lower proportion of total body water than males of the same weight, meaning alcohol is distributed into a smaller volume and produces a higher concentration. The Widmark r factor is 0.68 for males and 0.55 for females to reflect this difference. Hormonal fluctuations can also influence alcohol absorption rate, and females tend to have lower levels of alcohol dehydrogenase, the enzyme that begins breaking down alcohol in the stomach. These biological differences mean women typically reach higher BAC levels from the same number of drinks.
What counts as one standard drink for blood alcohol content purposes?
In the United States, one standard drink contains 14 grams of pure ethanol — equivalent to a 355 ml (12 oz) regular beer at 5% ABV, a 148 ml (5 oz) glass of wine at 12% ABV, or a 44 ml (1.5 oz) shot of 40% spirit. Craft beers, cocktails, and large wine pours often contain 1.5 to 2.5 standard drinks, which is why BAC can rise faster than expected. To use this calculator accurately, convert what you actually drank into equivalent standard drink units before entering the number.