Blood Alcohol Content (BAC) Calculator

Estimate blood alcohol content using the Widmark formula. For education only — never a substitute for not driving after drinking.

⏱ Updated: 17 Sep 2026

Calculator

Free BAC calculator: estimate blood alcohol content using the Widmark formula. For education only — never a substitute for not driving after drinking.

What counts as a "standard drink" varies by country — roughly 14g of pure alcohol in the US, 8g in the UK, 10g in Australia. Adjust the grams-per-drink field to match your country, or a specific drink's volume × ABV × 0.789.

Estimated BAC
%
This is a rough estimate, not a measurement. Actual BAC depends on metabolism, food intake, medication, and many other factors this formula can't see. Never use this number to decide whether it's safe to drive — if you've been drinking, don't drive.

Enter your weight, sex, drinks consumed, and hours elapsed to estimate blood alcohol content using the Widmark formula.

How to Use the Blood Alcohol Content (BAC) Calculator

Set units and sex first, then work through weight, drink count, and hours elapsed — the estimate recalculates with every field, so it's easy to see how another hour or another drink shifts the number.

Blood alcohol content estimation goes back to Erik Widmark's original 1930s research, and the formula built from it is still the basis for most BAC calculators in use today — including breathalyzer-adjacent estimates used for rough, non-evidentiary purposes.

BAC% = (Alcohol Grams ÷ (r × Weight in grams)) × 100 − (0.015 × Hours)

r is the Widmark factor — a distribution constant reflecting body water percentage, commonly cited as 0.68 for men and 0.55 for women, since alcohol distributes through body water and the two sexes differ in average body-water proportion at the same weight. The 0.015 metabolism term is the commonly cited average rate the body clears alcohol per hour — real elimination rates vary by individual, ranging roughly 0.010-0.020 in published studies.

"Standard drink" size varies by country — about 14 grams of pure alcohol in the US, 8 in the UK, 10 in Australia — so this calculator asks for grams of alcohol directly rather than assuming one definition. A drink's grams work out to its volume in milliliters times its ABV as a decimal times 0.789 (ethanol's density).

This is a rough estimate, not a measurement, and it is never a basis for deciding whether to drive. Individual metabolism, food intake, medication, and other factors this formula can't see all shift real BAC away from what any calculator predicts. If you've been drinking, don't drive — call a ride instead.