BAC Calculator

Blood alcohol content (BAC) is the concentration of alcohol in a person’s bloodstream, expressed as a percentage — a BAC of 0.08%, for example, means roughly 0.08 grams of alcohol per 100 millilitres of blood. It is the standard, widely recognised way of describing how much alcohol is currently circulating in someone’s body, rather than how much they drank, because the same amount of alcohol produces very different concentrations depending on body size, sex, and how much time has passed.

A true BAC can only be established directly, with a calibrated breath, blood or urine test administered under controlled conditions. Everything a formula like the one behind this calculator produces is a population-average estimate, built from body chemistry that varies meaningfully from person to person — a distinction worth stating plainly, because it materially changes how the resulting number should be read.

This calculator uses Widmark’s formula, developed by the Swedish physician and forensic scientist Erik Widmark and published in 1932, which remains the conceptual basis for most BAC estimation used today, from consumer calculators to forensic reconstruction. It treats the body as if alcohol distributes evenly through a single fluid compartment: grams of alcohol consumed, divided by body weight scaled by a distribution ratio that approximates how much of that weight behaves like the water alcohol actually dissolves into.

Widmark’s own published analysis of the formula’s accuracy put individual variation at roughly ±20%, even when every input — weight, sex, drinks and time — is entered correctly. That uncertainty exists because the formula only has room for a handful of variables, while a real BAC is also shaped by factors it cannot see at all: how much food was in the stomach, how quickly the drinks were consumed, real differences in liver enzyme activity, medication interactions, and individual body composition that a single sex-based ratio only roughly approximates.

The formula also assumes all the alcohol consumed has already finished absorbing into the bloodstream, when in reality that process typically takes somewhere around thirty to ninety minutes after a drink. In the window right after drinking, a real BAC can still be rising even while a formula that assumes absorption is complete would suggest it is already falling.

Outside of casual curiosity, BAC estimation of this kind is used in alcohol education and as a teaching tool for understanding how weight, sex, drink count and time interact — not as a substitute for a real reading in any situation where the actual number matters. Forensic toxicologists who apply Widmark-style reasoning professionally do so with far more case-specific data, tighter error bounds, and an explicit acknowledgement of the same underlying uncertainty this calculator states plainly.

The single number this or any BAC formula produces is best understood as a rough, educational midpoint of a real range that only a calibrated instrument can narrow down — never as a precise reading of what is actually happening in a specific person’s bloodstream at a specific moment.