BAC guide
BAC Formula: How Online Estimates Are Calculated
Understand the variables and assumptions used by this site’s Widmark, Watson, Seidl and NHTSA-style BAC estimation options.
Use the BAC CalculatorWhat the calculator estimates
The calculator estimates alcohol grams from serving count, volume and ABV, applies the selected body-distribution method, then subtracts a fixed amount for elapsed time.
This guide documents the four existing options. It does not add another formula or calculation engine.
Inputs used by the four methods
| Method | Inputs used | Main modeling choice |
|---|---|---|
| Widmark | Alcohol grams, weight, sex, time | Sex-based distribution factor |
| Watson | Alcohol grams, weight, sex, height, age, time | Estimates total body water |
| Seidl | Alcohol grams, weight, sex, height, age, time | Uses the calculator’s body-water pathway |
| NHTSA-style | Alcohol grams, weight, sex, time | Standard-drink and weight assumptions |
From drink details to alcohol grams
Each entry combines servings, volume and ABV. Alcohol density converts that volume to estimated ethanol grams.
- ABV is a percentage of liquid volume.
- A mixed drink may contain more than one standard serving.
- Metric and imperial weights are converted before calculation.
Why formula outputs differ
Formulas simplify body composition, absorption and elimination, so the same drinking history can produce different estimates. More inputs do not turn an estimate into a measurement.
For time, the calculator uses its existing fixed 0.015 percentage-points-per-hour assumption. Real elimination varies.