Blood Alcohol Content (BAC) Calculator

How This Calculator Works

Blood alcohol content (BAC) is the percentage of alcohol in a person’s bloodstream, and this calculator gives a rough estimate of it using the widely-published Widmark formula. Enter the number of standard drinks consumed, body weight, gender at birth, and how many hours have passed since the first drink, and it estimates the resulting BAC.

This is an estimate, not a measurement, and it should never be used to decide whether it’s safe to drive. Real BAC is affected by many things this formula can’t account for — food in your stomach, medications, individual metabolism, how quickly the drinks were consumed, and more — and judgment and reaction time can be measurably impaired at BAC levels well below any legal driving limit. Use a taxi, rideshare, public transit, or a sober friend instead.

A “standard drink” (in the U.S.) is about 0.6 fl oz of pure alcohol — roughly one 12 oz beer at 5% ABV, one 5 oz glass of wine at 12% ABV, or one 1.5 oz shot of 80-proof spirits.

The Formula

$$\text{BAC} = \left(\frac{\vA{\text{Standard Drinks}} \times 0.6 \times 5.14}{\vB{\text{Weight (lbs)}} \times \vC{r}}\right) - (0.015 \times \vD{\text{Hours Elapsed}})$$

Where $\vC{r}$ is the Widmark distribution ratio — a widely-cited average reflecting typical differences in body water content, commonly given as 0.73 for men and 0.66 for women. These are population averages, not a fact about any specific individual. The result is clamped at zero, since BAC can’t go negative once fully metabolized.

Worked Example

A 180 lb male who has had 4 standard drinks over 2 hours:

1. Alcohol consumed: $\vA{4} \times 0.6 = 2.4 \text{ fl oz}$.
2. Raw estimate: $\frac{2.4 \times 5.14}{\vB{180} \times \vC{0.73}} \approx 0.094\%$.
3. Metabolized so far: $0.015 \times \vD{2} = 0.030\%$.
4. Estimated BAC: $0.094 - 0.030 \approx 0.064\%$.