How BAC is Calculated

The science and sources behind drinkYs' blood alcohol estimates.

drinkYs estimates blood alcohol concentration (BAC) using a minute-by-minute pharmacokinetic simulation built on peer-reviewed research. This page explains the model, its inputs, and its limitations.

The Widmark Formula

The foundation of the calculation is the Widmark formula, developed by Swedish physician Erik Widmark in 1932 and still used today in clinical and forensic toxicology:

BAC = (Alcohol consumed in grams) ÷ (Body weight kg × r) Where r is the Widmark factor — a measure of body water distribution.
Typical values: r = 0.68 for males, r = 0.55 for females (Watson et al., 1981).

This gives the peak BAC if all alcohol were absorbed instantly. drinkYs adds a 45-minute linear absorption ramp on top of this, so each drink's contribution builds gradually — reflecting how alcohol actually enters the bloodstream after ingestion.

Elimination Rate

Once absorbed, alcohol is eliminated by the liver at a near-constant rate (zero-order kinetics). Rather than using a fixed number, drinkYs derives your personal elimination rate from three factors:

Biological sex — females generally have a slower elimination rate than males due to differences in enzyme activity and body composition
Body weight — heavier individuals typically have higher liver mass and clearance capacity
Drinking experience — regular or heavy drinkers show upregulation of the CYP2E1 enzyme, resulting in faster elimination (~0.020 %/hr vs ~0.012 %/hr for light drinkers)

This approach is based on the variability data documented by Norberg et al. (2003) and Jones (2010).

The Simulation

Rather than a single static calculation, drinkYs runs a step-by-step simulation in one-minute increments from the time of the first drink. At each step it adds the newly absorbed alcohol and subtracts the elimination. This correctly models scenarios like multiple drinks over several hours, where BAC rises and falls in a complex curve — something a simple formula cannot capture.

Important Disclaimer

These calculations are estimates based on population averages. Every person is different. Factors including food intake, fatigue, medication, health conditions, and genetics can significantly affect how your body processes alcohol.

The BAC values shown in drinkYs should never be used to determine whether it is safe to drive, operate machinery, or perform any safety-critical activity. Always err on the side of caution.

If you have specific questions or concerns about alcohol and your health, please consult your family doctor or a qualified healthcare professional.

Scientific References

[1] Widmark, E.M.P. (1932). Die theoretischen Grundlagen und die praktische Verwendbarkeit der gerichtlich-medizinischen Alkoholbestimmung. Urban & Schwarzenberg, Berlin. — Original description of the Widmark formula for BAC estimation.

[2] Watson, P.E., Watson, I.D., Batt, R.D. (1981). Prediction of blood alcohol concentrations in human subjects. Journal of Studies on Alcohol, 42(7), 547–556. — Source for sex-specific Widmark r factors based on body water distribution.

[3] Norberg, Å., Jones, A.W., Hahn, R.G., Gabrielsson, J.L. (2003). Role of variability in explaining ethanol pharmacokinetics. Clinical Pharmacokinetics, 42(1), 1–31. — Comprehensive review of inter-individual variability in alcohol elimination rates.

[4] 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. — Population data on elimination rates and the effect of drinking experience and CYP2E1 induction.