This calculation estimates what the Blood Alcohol Concentration (BAC) will be in mg/dL based upon a single ingestion of alcohol (at one time) and the time since the ethanol was consumed.

The patient's weight must be entered as this affects the BAC, the volume of the alcoholic beverage and the strength of that beverage all factor into this calculation; as does the time since consumption.

The strength of the beverage can be entered in either % ethanol or "proof."  The Proof of a beverage is 2 x the % ethanol.  For example, vodka, is often 80 proof (or 40% ethanol) and beer and wine are more commonly reported in %, for example 12% (24 proof).

This calculator is a modification of the Widmark equation, which was published by a Swedish physician EMP Widmark in 1932.  One modification of this equation is that we will use metabolism of ethanol to occur at 20 mg/dL/hr (his equation had it as 15 mg/dL/hr).

There are separate equations for male and female as the volume of distribution differs by gender (due to differences in volume of distribution).

Based upon % Ethanol

Male BAC Estimate = 1.22 x mL x % Ethanol/kg weight

Female BAC Estimate = 1.51 x mL x % Ethanol/kg weight

Based upon Proof

Male BAC Estimate = 0.610 x mL x Proof/kg weight

Female BAC Estimate = 0.754 x mL x Proof/kg weight

Time factor

Subtracted from this is 20 mg/dL/hr x hrs since consumed ethanol

Notes

• Multiple modifications of this formula have occurred over time; each with their advantages and disadvantages and inaccuracies
• In chronic alcoholics; the metabolism rate of ethanol may occur at up to 30 mg/dL/hr and thus, the predicted BAC used here (due to only 20 mg/dL/hr being subtracted) may be higher than the patient's actual BAC.

Reference:

Forrest ARW. The estimation of Widmark's factor, J. For Sci Soc. 1986:26:249-52.

Watson PE, Watson ID, Batt RD. Prediction of blood alcohol in human subjects. Updating the Widmark equation. Journal of Studies on Alcohol 1981; 42:547-56.

Widmark EMP. Principles and Applications of Medicolegal Alcohol Determination. Davis, CA. Biomedical Publications. 1981, pp. 107-8.