BAC Terms Glossary: Widmark Formula, Standard Drink & More

July 15, 2026

This glossary is educational only. No BAC estimate, no matter how precise, means it’s safe to drive. If you’ve had any alcohol, use a rideshare, a taxi, or a sober driver.

Here’s what the terms behind the blood alcohol level calculator actually mean.

BAC (Blood Alcohol Content/Concentration)

The percentage of alcohol in a person’s blood, expressed as grams of alcohol per 100 mL of blood (written as a percentage, like 0.08%). BAC can only be measured directly with a breathalyzer or blood test — anything calculated from drinks, weight, and time (including this calculator) is an estimate, not a measurement.

Widmark Formula

The standard equation used since the 1930s to estimate BAC from alcohol consumed, body weight, sex, and time elapsed: BAC(%) = [(alcohol grams × 100) ÷ (body weight in grams × r)] − (β × hours elapsed). It’s built from population averages, which is why it produces an estimate rather than an exact figure for any specific person. See common misconceptions about blood alcohol estimates for how far real BAC can diverge from this formula.

Distribution Ratio (r)

A constant approximating how alcohol spreads through the water in a person’s body. Because men have, on average, a higher proportion of body water relative to body fat than women of the same weight, the commonly used consumer values are r ≈ 0.68 for men and r ≈ 0.55 for women — population averages, not a measurement of any individual’s actual body composition.

Elimination Rate (β)

The rate at which the body metabolizes and removes alcohol from the bloodstream over time, commonly modeled at about 0.015% per hour. Published research shows this rate actually ranges from roughly 0.010% to 0.030% per hour depending on genetics, liver function, and drinking history — one more reason a Widmark estimate is a rough figure, not a precise one.

Standard Drink

A unit defined by the NIAAA as 14 grams of pure alcohol — roughly a 12 oz beer (~5% ABV), a 5 oz glass of table wine (~12% ABV), or a 1.5 oz shot of 80-proof spirits (~40% ABV). Despite very different serving sizes, all three contain about the same amount of pure alcohol, which is why “drinks” can be compared as a unit at all — but a stronger craft beer, a larger wine pour, or a mixed cocktail with more than one shot can easily contain 1.5-2x a standard drink’s worth of alcohol without looking like “more than one drink.”

Absorption

The process of alcohol moving from the stomach and small intestine into the bloodstream, which is what makes BAC rise after drinking. Absorption speed varies significantly based on whether there’s food in the stomach, how quickly the drinks were consumed, and individual physiology — see how long alcohol stays in your system for how this timeline actually works.

Legal BAC Limit

The BAC threshold at which driving becomes a per se offense in a given jurisdiction — commonly 0.08% for standard drivers in the US, 0.04% for commercial (CDL) drivers, and 0.00-0.02% (zero-tolerance) for drivers under 21 in most states. See BAC calculator and legal limits by context for the full breakdown — and remember that impairment to reaction time and judgment can begin well below any of these legal thresholds.

Use the blood alcohol level calculator to understand how these terms combine into a rough estimate — never to decide whether it’s okay to drive.