The Widmark formula
BAC estimates use the Widmark equation, developed in the 1930s. It's the same math driving every BAC calculator and breathalyzer-prediction app.
BAC = (oz_alcohol × 5.14 / weight_lb × r) − (0.015 × hours)
Where r is the distribution ratio - about 0.68 for men, 0.55 for women on average, reflecting differences in body water content. The 0.015 per hour is the average elimination rate; your liver clears about one standard drink per hour.
Frequently asked
How accurate is this?
For an average person of average size with average metabolism, it's within about ±0.02 of a measured BAC. Outliers are common. Asians, women, the elderly, anyone on medications, and people with liver issues clear alcohol differently. The only way to know your real BAC is to test it.
Can I drive at 0.05%?
In most US states, the legal limit is 0.08%, but driving impairment starts well before that. Studies show reaction time slows by 15-20% at 0.05%. Several countries set the limit at 0.05% for that reason. Even at 0.03%, you're a statistically worse driver than sober.
Does coffee, food, or a shower sober me up?
No. Nothing speeds alcohol metabolism. Coffee makes you feel more alert without lowering your BAC - which is dangerous, because you think you're fine when you're not. Time is the only thing that sobers you up. About one hour per standard drink.
What about edibles and cannabis?
This calculator only models ethanol. Mixing alcohol with cannabis, opioids, or sedatives compounds impairment in ways the math here doesn't capture. Don't drive on combinations.