How the Widmark estimate works
The Widmark equation converts the alcohol you have drunk into an estimated blood alcohol concentration. It divides the grams of pure ethanol consumed by your body weight adjusted for the fraction of your body that is water, since alcohol distributes through body water rather than fat.
Men average a distribution ratio of about 0.68 and women about 0.55, because body-water percentage differs on average between the sexes. The body then clears alcohol at a fairly steady rate of roughly 0.015 percent per hour, which this tool subtracts based on the time since your first drink.
What counts as a standard drink
A standard drink in this calculator is about 14 grams of pure alcohol, the figure used in the United States. The same volume of different beverages can contain very different amounts of alcohol, so estimating drinks accurately is the hardest part of getting a sensible result.
- A 350 ml (12 oz) regular beer at about 5% alcohol
- A 150 ml (5 oz) glass of wine at about 12% alcohol
- A 45 ml (1.5 oz) shot of spirits at about 40% alcohol
- Large pours, craft beers and cocktails often count as more than one standard drink
Why the real number varies so much
Actual BAC depends on factors the formula cannot capture: whether you ate, how fast you drank, your liver function, medications, hydration and individual metabolism. Two people of the same weight can reach noticeably different concentrations from the same drinks.
Because alcohol absorption is still ongoing in the first hour or so after drinking, an estimate taken early can read lower than your true peak. The result is a rough guide, never a precise measurement.
Safety and health disclaimer
This is an educational estimate only and must never be used to decide whether it is safe to drive or operate machinery. The only reliable way to know your BAC is a breath or blood test, and the only safe amount of alcohol before driving is none. If you are concerned about alcohol use, speak with a healthcare professional.
Formula
BAC% = alcohol(g) / (weight(g) × r) × 100 − 0.015 × hours; r = 0.68 (male), 0.55 (female)Frequently asked questions
- Can I rely on this to decide if I can drive?
- No. Individual factors such as metabolism, food, medication and the exact drink size make real BAC vary widely. The only safe amount of alcohol before driving is none.