The science behind the estimate
The Widmark formula is the most widely used method for estimating blood alcohol concentration. It accounts for your gender, body weight, the amount of alcohol consumed, and time elapsed since drinking began. This calculator applies that formula to give you a reasonable estimate—but it's important to remember that actual BAC varies based on food intake, individual metabolism, liver function, and other health factors. This is an estimate for educational purposes, not professional medical or legal advice.
The formula
BAC = (alcohol grams ÷ (body weight grams × gender factor)) × 100 − (metabolism rate × hours)
Where:
- Alcohol grams = number of standard drinks × 14 g
- Gender factor = 0.58 for men, 0.49 for women (distribution ratio)
- Metabolism rate ≈ 0.015% per hour (one drink per hour)
Worked example
Let's say you're a 75 kg (165 lb) male who drank 3 standard drinks over the past 2 hours.
Step 1: Calculate total alcohol consumed
- 3 drinks × 14 g/drink = 42 grams of alcohol
Step 2: Apply the Widmark distribution
- Body weight in grams: 75 kg = 75,000 g
- Distribution factor for males: 0.58
- Peak BAC (at time of last drink) = 42 ÷ (75,000 × 0.58) × 100 = 0.097%
Step 3: Account for metabolism over time
- Elimination rate: 0.015% per hour
- Time elapsed: 2 hours
- Alcohol metabolized: 0.015 × 2 = 0.030%
Step 4: Calculate current BAC
- Current BAC = 0.097% − 0.030% = 0.067%
At this level, you'd likely notice mild impairment: slower reaction time, reduced judgment, and slight loss of coordination. In most places, 0.08% is the legal driving limit, so this person is approaching that threshold.
Common mistakes to avoid
Confusing drink size with standard drinks: A large beer or wine pour may contain 1.5–2 standard drinks, not just one. Check the alcohol by volume (ABV) and pour size to count accurately.
Assuming food doesn't matter: Food slows alcohol absorption and can delay peak BAC by 30–60 minutes, but this calculator assumes the drinks are already in your system. If you drank on an empty stomach, your peak BAC was likely higher than the estimate.
Forgetting that metabolism isn't instant: The body processes alcohol at roughly one drink per hour, but this rate varies. Liver disease, certain medications, and fatigue can slow it down. Never assume you're sober based on time alone.
Ignoring individual variation: The Widmark formula uses population averages. Your actual BAC may differ by ±0.01–0.02% depending on genetics, body composition, hydration, and other factors. A breathalyzer is the only way to know for certain.