Blood Alcohol Content (BAC) Explained

A blood alcohol content (BAC) estimate starts with how much pure ethanol you drank, divides it by the water in your body (which depends on weight and sex), and then subtracts what your liver has already burned off since the first drink. It's an estimate, not a measurement — but understanding the inputs shows why two people drink the same amount and feel very different.

How much alcohol did you actually drink?

A "drink" is not a glass, it's a dose of ethanol. A standard US drink is 14 g of pure alcohol — roughly a 12 oz beer at 5%, a 5 oz glass of wine at 12%, or a 1.5 oz shot of 40% spirit. The grams of alcohol from any drink is a product of its volume, its strength, and ethanol's density:

grams = volume_ml × (ABV / 100) × 0.789

The 0.789 is ethanol's density in g/mL. A 500 ml beer at 5% holds 500 × 0.05 × 0.789 ≈ 19.7 g, or about 1.4 standard drinks. Sum this across every drink to get the total alcohol grams entering the body.

The Widmark formula: spread over body water

Once you know the alcohol grams, the Widmark formula spreads them across the body's water to estimate the concentration in blood:

BAC (%) = grams / (weight_g × r) × 100

The r is the Widmark factor — the fraction of body mass that is water. It's about 0.68 for men and 0.55 for women, because women typically carry a higher proportion of fat (which holds little water) relative to lean mass. Same dose, smaller water volume, higher concentration: that's the single biggest reason a man and a woman of the same weight reach different BACs from the same drinks.

Metabolism: the line that falls over time

Your liver clears alcohol at a fairly steady rate of about 0.015% BAC per hour, regardless of coffee, water, or willpower. The estimate subtracts that burn from the peak:

effective BAC = peak BAC − 0.015 × hours

So the number that matters is not just how much you drank but when. Two drinks consumed in an hour leave a much higher BAC than the same two drinks spread across four hours, because the liver has had time to work. The result is clamped at zero — you can't have negative alcohol in your blood.

Why the estimate is only an estimate

The Widmark factor is an average. Real body-water fraction varies with body composition, age, genetics, and whether you've eaten (food slows absorption, lowering the peak). The 0.015%/h metabolism rate is a population average too — some people clear faster, some slower. A calculator built on these averages can be off by 20% or more for any one person, which is why a BAC estimate is never a legal or medical determination of impairment. Only a breathalyzer or blood test measures BAC directly.

Impairment starts well below the legal limit

In the US, 0.08% BAC is the per-se drunk-driving limit for adults, but reaction time, judgment, and coordination degrade much earlier. A 0.04% BAC — about two drinks for an average man — already measurably impairs divided-attention tasks like driving. The estimate's job is to make the relationship between drinks, weight, sex, and time visible so people make better decisions, not to give anyone a target.

Try it yourself

The BAC Estimator takes sex, body weight, a list of drinks (volume, ABV, count), and hours since the first drink, then applies the Widmark formula with metabolism to estimate peak and effective BAC and a plain-language impairment category. It is an educational estimate, not a legal or medical reading — never use it to decide whether you are safe to drive.