Blood Alcohol Content (BAC)

How BAC Is Estimated With the Widmark Formula

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.

Key Factors to Consider

• Food in your stomach slows alcohol absorption but doesn’t reduce the total amount absorbed. Eating before or while drinking spreads out how quickly BAC rises, which can make the peak lower and later, but the formula here doesn’t model food intake at all — it estimates as if the alcohol is absorbed on the timeline the formula assumes.
• The 0.015%/hour metabolism rate is a population average, not a fixed constant for everyone. Individual metabolism varies by a meaningful margin due to genetics, liver function, body composition, and other factors — the true rate for any specific person could be somewhat faster or slower than this average.
• Impairment is not a simple on/off switch tied to a single BAC number. Judgment, reaction time, and coordination can all be measurably affected well below any legal driving limit, and the exact effect at a given BAC varies by individual and by how quickly the alcohol was consumed — this calculator’s output is never a “safe to drive” threshold at any level.
• This calculator cannot account for interactions with medications or other substances. Alcohol combined with many common medications (and other substances) can produce effects well beyond what BAC alone predicts — this formula has no way to reflect that.

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: $0.094 - 0.030 \approx 0.064\%$.

Interpreting Your Results

A number from this calculator is a rough, population-average estimate — not a reading from a breathalyzer or blood test, and not a threshold for any decision about driving or drinking more. Two people with identical inputs can have meaningfully different real BAC due to individual metabolism, so treat the output as a general awareness tool rather than a precise personal measurement. If the estimate is trending upward as more drinks are logged, remember that BAC continues to rise for a while even after the last drink, since absorption isn’t instant.

Useful to Know

In the United States, the “illegal per se” BAC limit for driving is 0.08% for drivers 21 and over, 0.04% for commercial drivers operating a commercial vehicle, and effectively 0.00%-0.02% (“zero tolerance”) for drivers under 21, depending on the state. These are legal thresholds for being charged with driving under the influence — they are not a statement that driving below them is safe, since measurable impairment to judgment and reaction time can begin at BAC levels well under 0.08%.