The BAC Calculator

METHODOLOGY

BAC calculation formula.
Clear assumptions. Clear limits.

Our BAC Calculator uses a Widmark-style educational estimate. It explains the arithmetic, but it cannot measure a personal BAC or tell someone whether they can drive.

How standard drinks become pure alcohol

First, the calculator estimates pure alcohol in grams:

volume in ml × ABV as a decimal × 0.789 = grams of pure alcohol

It then uses the country-specific reference only to describe that total as standard drinks: 14 g for a U.S. standard drink, 8 g for a UK alcohol unit, 10 g for Australia, New Zealand, and Ireland, and 13.45 g for Canada. The Standard Drinks Calculator shows this step for a serving or container.

The BAC calculation formula

The calculator applies a Widmark-style model:

estimated BAC = max(0, ((A ÷ (W × r × 1000)) × 100) − (β × t))

  • A = total pure alcohol in grams
  • W = body weight in kilograms
  • r = 0.68 for male and 0.55 for female in this formula
  • β = 0.015 percentage points per hour in this educational model
  • t = hours since drinking began

The formula is a simplified estimate, not a physiological measurement. It does not know the exact timing of each drink, absorption pattern, health conditions, medication, body composition, food, or individual metabolism.

Why the calculator shows a range

After calculating a central estimate, we show a range of plus or minus 0.015 percentage points. This does not make the result exact; it is a visible reminder that actual BAC can differ substantially from a form-based estimate.

Do not use an online BAC estimate, a BAC formula, or a time-based result to decide whether to drive. If alcohol is involved, arrange a sober ride and avoid operating vehicles or machinery.

What “BAC calculator over time” can and cannot show

The BAC Calculator Over Time repeats the same fixed hourly assumption across future rows. It can illustrate how a mathematical model changes with time; it cannot predict a personal sober time or legal driving status. Read Alcohol Metabolism and BAC for the limits behind that assumption.

BAC conversion

BAC may be expressed as a percentage, g/dL, mg/dL, g/L, or mg per 100 mL. These are different notations for the same concentration. Use the BAC Conversion Calculator to see the mathematical equivalents, not to interpret legal limits.

Sources

  1. National Institute on Alcohol Abuse and Alcoholism, What is a standard drink?
  2. National Highway Traffic Safety Administration, Drunk driving: alcohol and BAC
  3. Hustad JT, Carey KB. Using calculations to estimate blood alcohol concentrations for naturally occurring drinking episodes. Journal of Studies on Alcohol and Drugs, 2005.

Last reviewed: July 20, 2026 · Read our editorial policy or send corrections to hello@thebaccalculator.com.