Purpose and safety boundary
The BAC Calculator is an educational tool. It turns a reported drink size, ABV, body weight, sex used in the formula, and elapsed time into a Widmark-style estimate. It is designed to show the calculation and its uncertainty—not to determine a person's actual BAC, impairment, sober time, or legal driving status.
Step 1: calculate pure alcohol
For each drink, the calculator uses the serving volume and alcohol by volume (ABV):
pure alcohol (g) = volume (ml) × ABV as a decimal × 0.789
The constant 0.789 is the approximate density of ethanol in grams per millilitre. The calculator adds the pure alcohol from all entered drinks. It then translates that amount into a local reference unit only for context. For example, NIAAA defines one U.S. standard drink as 14 g of pure alcohol. Country definitions vary; see alcohol units by country for the displayed references and their public-health sources.
Step 2: apply the Widmark-style estimate
The central estimate displayed by the main calculator follows this implementation:
estimated BAC (%) = max(0, ((A ÷ (W × r × 1000)) × 100) − (β × t))
- A is total reported pure alcohol in grams.
- W is body weight in kilograms. Pounds are converted to kilograms before calculation.
- r is the distribution factor selected by the form: 0.68 for the male setting and 0.55 for the female setting.
- β is 0.015 BAC percentage points per hour in this educational implementation.
- t is the number of hours since the first drink, as reported by the user.
The formula starts with the alcohol amount, body weight, and formula setting, then subtracts a fixed hourly amount. It is intentionally simple and transparent. It does not model every drink's timestamp or a person's changing absorption curve.
Why the tool shows a range
The site presents the central estimate with a visible plus-or-minus 0.015 percentage-point band. This is a communication safeguard, not a clinical confidence interval and not proof that an actual BAC falls within that range. Research comparing reconstructed estimates with breath measurements has found that self-report-based equations leave substantial individual variation unexplained.
Actual BAC can differ because drink sizes and ABV may be misremembered, drinks may not have been absorbed yet, and food, body composition, health, medication, metabolism, and drinking pattern can all matter. Learn more in What is BAC? and Alcohol metabolism and BAC.
What the time estimate can and cannot show
The BAC estimate over time repeats the same fixed hourly assumption over future rows. It can help explain the model's arithmetic. It cannot predict a personal sober time, test whether someone is below a legal limit, or establish that a person is safe to drive.
Implementation and review
Calculations run in the visitor's browser. The site does not send calculator entries to a server. Formula changes, source changes, or material wording corrections are reviewed and dated. See sources and review process for the recurring source checks, or read the editorial policy to report an error.
Sources
- National Institute on Alcohol Abuse and Alcoholism: What Is a Standard Drink?
- National Highway Traffic Safety Administration: Drunk Driving — alcohol and BAC
- Hustad JT, Carey KB. Using calculations to estimate blood alcohol concentrations for naturally occurring drinking episodes: a validity study. Journal of Studies on Alcohol and Drugs. 2005;66(1):130–138. doi:10.15288/jsa.2005.66.130.