Estimated BAC
How BAC is estimated
This calculator uses the Widmark formula, the most widely known method for estimating blood alcohol content. It works by calculating the total grams of pure alcohol consumed, dividing by the volume of water in your body (estimated from your weight and a body-water ratio), then subtracting the alcohol your liver has metabolised over time at a rate of roughly 0.015 BAC per hour.
The body-water ratio — called r in the formula — differs between bodies. On average it is around 0.68 for men and 0.55 for women, because body composition affects how alcohol distributes. This is why the same drinks affect two people of identical weight differently. But these are population averages: your individual ratio could be quite different.
This is the critical limitation: the Widmark formula was never designed to determine whether someone is fit to drive. Real BAC depends on food in the stomach, how fast you drank, medications, liver health, hydration, genetics, hormones, and more — none of which any calculator can measure. Studies show calculator estimates can differ from actual measured BAC by a wide margin in either direction.
The single most important fact about alcohol metabolism: only time reduces BAC, at a fixed rate that nothing can accelerate. Coffee, cold showers, exercise, eating, and drinking water do not lower your blood alcohol — they may make you feel more alert while leaving you just as impaired, which is more dangerous, not less. If you have consumed any alcohol, the safe choice is always to not drive.