How to Calculate Blood Alcohol Content (BAC)
Blood Alcohol Content (BAC) measures the concentration of alcohol in your bloodstream, expressed as a percentage. At 0.08%, you're legally impaired to drive in the US and most countries. Understanding BAC helps you make safer decisions about drinking and driving.
The Widmark Formula
BAC = (A ร 5.14) / (W ร r) โ 0.015 ร H
Where:
- A = total ounces of pure alcohol consumed
- W = body weight in pounds
- r = gender constant (0.73 for men, 0.66 for women)
- H = hours since drinking started
- 0.015 = alcohol eliminated per hour
Note: A "standard drink" in the US contains 0.6 oz of pure alcohol (12 oz beer at 5%, 5 oz wine at 12%, or 1.5 oz spirits at 40%).
Step-by-Step Example
Male, 170 lbs, drinks 3 beers (3 ร 0.6 = 1.8 oz pure alcohol) over 2 hours:
BAC = (1.8 ร 5.14) / (170 ร 0.73) โ (0.015 ร 2) = 9.252 / 124.1 โ 0.030 = 0.0745 โ 0.030 = 0.0445%
BAC Effects at Different Levels
| BAC | Effects |
|---|---|
| 0.02โ0.04% | Mild relaxation, slight impairment |
| 0.05โ0.07% | Reduced inhibition, slowed reaction time |
| 0.08% | Legal limit in US โ significantly impaired |
| 0.10โ0.15% | Slurred speech, poor coordination |
| 0.20%+ | Vomiting, blackouts, danger of unconsciousness |
| 0.30%+ | Risk of alcohol poisoning, death |
Important Limitations
The Widmark formula is an estimate. Actual BAC varies with food intake (eating slows absorption), carbonation (speeds absorption), tolerance, liver health, and medications. Never rely on a calculation to decide if you're safe to drive.
Use our BAC calculator for any drinking scenario.