BAC calculator for a quick blood alcohol estimate

The calculator uses the Widmark formula, a method published by the Swedish scientist Erik Widmark in the 1930s and still taught in forensic toxicology courses. It starts with the grams of pure alcohol you drank and spreads them through the water in your body.

Alcohol in grams comes from each drink row: volume in ounces, times 29.57 to get milliliters, times the ABV, times 0.789, which is the density of ethanol. A 12 oz beer at 5% ABV holds about 14 grams, which is the US standard drink.

That amount is divided by your body weight in grams multiplied by a distribution factor called r. The factor reflects how much of your body is water. The calculator uses an average of 0.68 for men and 0.55 for women, then widens the result to cover people above and below that average.

Finally it subtracts what your liver has likely cleared since your first drink. The textbook average elimination rate is about 0.015 BAC per hour, and healthy adults commonly fall somewhere between 0.010 and 0.020. The low end of your range assumes faster clearance and a higher r, the high end assumes slower clearance and a lower r.

• Grams of alcohol, from volume and ABV of each drink
• Divided by body weight times the distribution factor r
• Minus about 0.015 for each hour since the first drink
• Shown as a range, because both r and elimination vary from person to person

A formula assumes all the alcohol is already absorbed. In real life absorption takes roughly 30 to 90 minutes and depends heavily on food. A full stomach slows it down and flattens the peak, while drinking fast on an empty stomach pushes the peak higher and earlier.

Body composition matters more than weight alone. Two people who both weigh 180 lb can carry very different amounts of body water, so the same drinks produce different levels.

Pours are rarely standard. A restaurant glass of wine is often 6 to 9 oz rather than 5, craft beers can run 7 to 10% ABV, and mixed drinks often contain more than one shot. Typing the real volume and strength makes the biggest single difference to the estimate.

Medication, fatigue, illness, liver health and hormonal changes can all shift how alcohol behaves. None of them can be captured by a few input fields.

• Absorption is not instant, so a recent drink may still be rising
• Body water varies even at the same weight
• Real pours are larger and stronger than the standard
• Health and medication change how you process alcohol

If you want to know what is in your breath rather than what a formula predicts, a breath tester is the tool for that. Fuel cell testers react specifically to alcohol and hold their calibration better than cheaper semiconductor sensors.

Before choosing one, compare sensor type, published accuracy and how the maker handles recalibration in our best breathalyzer ranking, or go straight to the fuel cell breathalyzers we compare from published specifications.

Choosing Female in the calculator changes the distribution factor from 0.68 to 0.55. On average women carry a higher share of body fat and a lower share of water than men of the same weight, so alcohol is less diluted and the level climbs higher.

Research also points to lower activity of the stomach enzyme that breaks down a small part of alcohol before it reaches the blood, and to hormonal phases that may change absorption. The formula cannot follow those individual differences, which is why the result is a band and the upper edge deserves the most attention.

If your body composition sits far from the average, for example a very athletic build or a high body fat percentage, treat the estimate as even rougher.