How we estimate blood alcohol content — the formula, constants, assumptions, and limitations.
The Widmark Equation
Our calculator is built on the Widmark equation, a pharmacokinetic model introduced by Swedish scientist Erik M. P. Widmark in 1932 and still the de-facto standard in forensic toxicology for estimating blood alcohol concentration from a known alcohol dose:
BAC (%) = (A / (r × W)) × 100 − (β × t)
A = mass of pure ethanol consumed (grams)
r = Widmark distribution factor (dimensionless)
W = body weight (grams)
β = ethanol elimination rate (% BAC / hour)
t = time elapsed since drinking began (hours)
For each individual drink, we compute its contribution to BAC and apply time-based elimination from the moment the drink was consumed. The live chart sums these per-drink contributions at one-minute resolution.
Constants We Use
| Constant | Value | Justification |
|---|
| Ethanol density | 0.789 g/mL | Standard density of ethanol at 20 °C used in pharmacology and beverage-alcohol calculations. |
| Widmark factor — men | r = 0.68 | Widely cited adult-male mean from Widmark (1932) and replicated in later forensic literature. Reflects that ~68% of body mass is water in a reference adult male. |
| Widmark factor — women | r = 0.55 | Widely cited adult-female mean. Reflects lower body-water fraction and reduced gastric ADH activity (see Frezza et al., 1990, NEJM). |
| Elimination rate (β) | 0.015 %/h | Mean of the range (0.010–0.025 %/h) established in Jones (2010) across thousands of controlled subjects, assuming zero-order kinetics at moderate BAC. |
Assumptions the Model Makes
- Zero-order elimination. The liver metabolises ethanol at a roughly constant rate across the moderate-to-high BAC range. This is the standard assumption of the Widmark model.
- Instant absorption approximation. The basic Widmark formula treats each drink as if it enters the bloodstream at the moment it is consumed. Real absorption takes 15–90 minutes depending on food and drink concentration, so the model can slightly over-estimate BAC in the first hour and under-estimate the true peak if drinks are spaced.
- Average body composition. A single r value per sex ignores variation in body fat percentage, age, and ethnic background that influence body-water distribution.
- Healthy liver function. Elimination rates are altered by liver disease, chronic heavy drinking, fasting state, and certain medications.
Known Limitations
Because of the assumptions above, the calculator should be read as a population-average estimate, not a personal measurement. Evidence shows that:
- Individual BAC can differ from the Widmark estimate by ±30% or more.
- Food substantially reduces peak BAC; the basic formula does not model this.
- Carbonated mixers, drink temperature, and drinking pace all shift absorption rates.
- Chronic heavy drinkers develop metabolic tolerance that may raise β.
The only way to measure BAC accurately is with a calibrated evidential breath-testing device or a venous blood draw analysed in a certified laboratory.
Primary References
- Widmark, E. M. P. (1932). Die theoretischen Grundlagen und die praktische Verwendbarkeit der gerichtlich-medizinischen Alkoholbestimmung. Urban & Schwarzenberg, Berlin.
- Jones, A. W. (2010). Evidence-based survey of the elimination rates of ethanol from blood with applications in forensic casework. Forensic Science International, 200(1–3), 1–20. doi.org/10.1016/j.forsciint.2010.02.021
- Frezza, M., di Padova, C., Pozzato, G., Terpin, M., Baraona, E., & Lieber, C. S. (1990). High blood alcohol levels in women: the role of decreased gastric alcohol dehydrogenase activity and first-pass metabolism. New England Journal of Medicine, 322(2), 95–99. doi.org/10.1056/NEJM199001113220205
- Cederbaum, A. I. (2012). Alcohol metabolism. Clinics in Liver Disease, 16(4), 667–685. doi.org/10.1016/j.cld.2012.08.002
Additional peer-reviewed sources are catalogued on our Research page.