How To Calculate Blood Alcohol Content

Estimated blood alcohol content is the grams of ethanol in the blood divided by the volume of blood, expressed per decilitre. Three standard drinks, at fourteen grams of ethanol each, is forty two grams. In a seventy kilogram man that ethanol is diluted by roughly forty seven litres of body water, giving a peak of about 0.088 grams per decilitre before any of it is metabolised.

The calculation is the Widmark formula, published in the nineteen thirties and still the basis of nearly every drink-driving limit table in use. It is a mass balance: the ethanol consumed is spread through the body water, and the liver removes it at a roughly constant rate.

A standard drink is a defined unit, not a glass size. In the United States it is fourteen grams of ethanol, which is twelve ounces of five percent beer, five ounces of twelve percent wine, or one and a half ounces of forty percent spirits. A generous home pour can easily be two standard drinks while looking like one.

Body water fraction is the main source of individual variation. Men average about 0.68 of body mass as water and women about 0.55, because of differences in average body composition. The same number of drinks therefore produces a higher concentration in a smaller body and in a body with less water.

Metabolism is modelled as a fixed hourly clearance of about 0.015 grams per decilitre. The liver processes ethanol at roughly one standard drink per hour and largely cannot be hurried, so time is the only thing that lowers a blood alcohol content.

The result is an estimate with a wide error band. Food in the stomach slows absorption, medication and liver condition change clearance, and the measured breath or blood figure can differ from the estimate by a meaningful margin. Treat the output as a planning figure, never as a measurement.

Because the estimate is uncertain, the only number that supports driving is zero. Legal limits vary from about 0.02 in some jurisdictions to 0.05 or 0.08 in others, and impairment begins below all of them. The calculation is useful for understanding limits, not for deciding whether a specific drive is safe.

Peak concentration occurs later than the last drink, because absorption continues after drinking stops. A stomach full of food can delay the peak by an hour or more, which is why a figure measured immediately after drinking is lower than the figure an hour later.

Gender is entered through the body water fraction rather than through the formula. The arithmetic is identical for everyone; only the distribution volume differs, and that difference is what the coefficient captures.

Body weight in pounds has to be converted before it is used. A one hundred and ninety pound man is about eighty six kilograms, and using the pound figure directly would overstate the dilution by more than a factor of two.

Finally, the formula assumes ethanol is distributed through total body water rather than through blood alone. Blood is about eighty percent water, so the estimates are calibrated to the figures that instruments actually report, which is why the constants are empirical rather than purely physical.