Blood alcohol concentration (BAC) is calculated using a formula that accounts for how much alcohol you drank, your body weight, your sex, and how much time has passed. The most widely used method is the Widmark formula, and while online calculators can do the math for you, understanding the formula yourself gives you a much clearer picture of how alcohol actually affects your body.
The Widmark Formula
The core calculation is straightforward: divide the total grams of alcohol you consumed by your body weight in kilograms, then divide again by a distribution factor (called the Widmark r-factor) that reflects how alcohol spreads through your body. The result is your estimated BAC before accounting for metabolism over time.
Written out, it looks like this:
BAC = (grams of alcohol) / (body weight in kg × r-factor)
The r-factor differs by sex and body composition. For a simplified calculation, many sources use a fixed value of about 0.68 for men and 0.55 for women. A more precise version, developed by researcher A.R.W. Forrest, ties the r-factor to BMI:
- Men: r = 1.0181 − (0.01213 × BMI)
- Women: r = 0.9367 − (0.01240 × BMI)
The reason for the sex difference is body water. Alcohol dissolves in water, and male bodies typically carry a higher percentage of water relative to fat. More water means the same amount of alcohol gets diluted into a larger volume, producing a lower BAC. Higher body fat percentages (reflected by a higher BMI) reduce the r-factor for both sexes, meaning the alcohol concentrates in a smaller volume and BAC goes up.
How to Convert Drinks Into Grams of Alcohol
The formula requires grams of pure alcohol, not the volume of your drink. In the United States, one standard drink contains 14 grams of pure alcohol. That standard drink is:
- Beer: 12 ounces at 5% ABV
- Wine: 5 ounces at 12% ABV
- Spirits: 1.5 ounces at 40% ABV (80 proof)
If your drink doesn’t match these sizes or strengths, you can calculate the grams yourself. Multiply the volume of the drink in milliliters by the ABV (as a decimal) by 0.789, which is the density of ethanol in grams per milliliter. A 16-ounce (473 mL) craft IPA at 7% ABV, for example, contains about 473 × 0.07 × 0.789 = 26.1 grams of alcohol, nearly double a standard drink.
A Worked Example
Say a 180-pound man with a BMI of 26 drinks three standard beers over two hours. Here’s how the math works step by step.
First, convert body weight: 180 pounds ÷ 2.205 = 81.6 kg. Next, total alcohol consumed: 3 drinks × 14 grams = 42 grams. Then calculate the r-factor: 1.0181 − (0.01213 × 26) = 0.703.
Plug those into the formula: 42 / (81.6 × 0.703) = 0.073, or about 0.073% BAC before accounting for metabolism. That’s your peak theoretical BAC, assuming all the alcohol hit your bloodstream at once. In reality, your body started breaking down alcohol while you were still drinking, which brings us to the time adjustment.
Adjusting for Time and Metabolism
Your liver processes alcohol at a roughly constant rate regardless of how much you’ve had. The average person eliminates about 7 grams of alcohol per hour, which works out to approximately 0.015% BAC per hour for most body sizes. Some people metabolize slightly faster or slower, with the typical range falling between 0.010% and 0.020% per hour.
To adjust for elapsed time, subtract the hourly elimination rate multiplied by the number of hours since you started drinking:
Adjusted BAC = peak BAC − (0.015 × hours)
Returning to the example above: the man started drinking two hours ago, so his adjusted BAC would be roughly 0.073 − (0.015 × 2) = 0.043%. That puts him below the U.S. legal driving limit of 0.08%, but alcohol still affects coordination and judgment well below that threshold.
Why the Formula Is an Estimate
The Widmark formula gives you a useful ballpark, not a precise measurement. Several real-world factors can push your actual BAC higher or lower than the calculated number.
Food in your stomach slows alcohol absorption significantly. Drinking on an empty stomach lets alcohol pass into your small intestine faster, producing a higher and earlier peak BAC than the formula predicts. Carbonated mixers can also speed absorption.
The formula assumes all the alcohol you drank has been fully absorbed, which takes roughly 30 to 90 minutes after your last sip. If you just finished a drink, your actual BAC may still be rising. Individual variation in liver enzyme activity also matters: regular heavy drinkers often metabolize alcohol faster than occasional drinkers, while certain genetic variants (particularly common in East Asian populations) slow metabolism and produce higher BAC from the same intake.
Hydration, medications, and overall health all introduce additional variability. The Widmark r-factor captures some of this through BMI, but it can’t account for everything. Treat your calculated BAC as an approximation with a meaningful margin of error, not a number you can rely on to judge whether you’re safe to drive.
BAC Levels and Their Effects
The legal driving limit in every U.S. state is 0.08%, but impairment begins well before that. At 0.02% to 0.03%, you may feel relaxed and slightly warm, with minor changes in mood. By 0.05%, reaction time slows noticeably, and your ability to track moving objects and steer accurately starts to decline. At 0.08%, muscle coordination, balance, and judgment are clearly impaired. Above 0.15%, most people experience significant loss of balance and vomiting. BAC levels above 0.30% carry serious risk of loss of consciousness, and levels above 0.40% can be fatal.
These thresholds vary by individual tolerance, but the physiological impairment at each level is real regardless of how sober you feel. People who drink frequently often perceive themselves as less impaired than they actually are, because their brain adapts to the sensation without gaining any real improvement in motor control or reaction time.
Quick Reference for Common Scenarios
If you don’t want to run the full formula, here are rough BAC estimates for a few common situations, assuming drinking on a moderately full stomach and using average r-factors:
- 150-lb woman, 2 glasses of wine over 2 hours: approximately 0.06%
- 150-lb woman, 3 glasses of wine over 2 hours: approximately 0.11%
- 200-lb man, 3 beers over 2 hours: approximately 0.03%
- 200-lb man, 5 beers over 3 hours: approximately 0.06%
- 130-lb woman, 2 cocktails over 1 hour: approximately 0.07%
These are rough estimates. Stronger drinks, faster drinking, lower body weight, and an empty stomach will all push the number higher. When in doubt, wait longer than you think you need to, or don’t drive at all.