Blood Alcohol Content (BAC) Calculator
This professional-grade BAC calculator estimates your Blood Alcohol Content using the Widmark formula. It is designed for adults who want an educational estimate of impairment risk based on their drinks, body factors, and time. Results are estimates only—not medical or legal advice. Never drink and drive.
Interactive Calculator
Drinks
Add your drinks. Use presets or enter a custom drink volume and ABV.
Tip: A U.S. standard drink contains about 14 grams of pure alcohol.
Results
Assuming constant β.
≈ 0.0 U.S. standard drinks
Safety reminder
Any amount of alcohol can impair judgment and reaction time. Laws and limits vary by country and state. This tool is for education only and does not replace professional or legal guidance.
Data Source and Methodology
- Widmark, E. M. P. (1932). Alkoholmetrie (Alcoholometry). Berlin: Urban & Schwarzenberg. Core equation and body-water constant approach.
- National Institute on Alcohol Abuse and Alcoholism (NIAAA). Rethinking Drinking — What’s a Standard Drink? (accessed 2025). Direct link
All calculations are strictly based on the formulas and data provided by these sources.
The Formula Explained
US customary formulation
BAC = \frac{A_{\text{oz}} \times 5.14}{W_{\text{lb}} \times r} - \beta \times H
- A_oz = total fluid ounces of pure ethanol consumed (fl oz)
- W_lb = body weight (lb)
- r = body water constant (~0.73 male, ~0.66 female)
- β = elimination rate (typically 0.015 %BAC/hour)
- H = hours since first drink
Metric formulation
A_g = \sum_i \big(V_{i,\text{mL}} \times \text{ABV}_i \times \rho_{\text{EtOH}}\big),\quad \rho_{\text{EtOH}} \approx 0.789\ \text{g/mL}
BAC\% = \frac{A_g}{W_{\text{kg}} \times r \times 10} - \beta \times H
Glossary of Variables
- Unit system: Choose US (lb, fl oz) or Metric (kg, mL).
- Sex: Biological sex used to set r (body water constant).
- Weight: Your body weight in the selected unit.
- Body water constant r: Proportion of body mass as water; typical averages are 0.73 (male) and 0.66 (female).
- Hours since first drink (H): Time elapsed since drinking began.
- Metabolism rate β: Average reduction in %BAC per hour (default 0.015).
- Volume: Beverage amount (fl oz or mL).
- ABV (%): Alcohol by volume percentage of the beverage.
- Estimated BAC: Resulting percentage of alcohol in the blood (g/dL), e.g., 0.08%.
- Time to 0.00%: Estimated hours to metabolize to zero based on β.
- Standard drinks: U.S. standard drinks (14 g pure alcohol each) for context.
How It Works: A Step-by-Step Example
- Add two 12 fl oz beers at 5% ABV (US units). Pure alcohol A_oz = 2 × 12 × 0.05 = 1.2 fl oz.
- Enter Sex = Male → r = 0.73; Weight = 180 lb; Hours H = 1.5; β = 0.015.
- Compute:
\text{Initial BAC} = \frac{1.2 \times 5.14}{180 \times 0.73} \approx 0.0472\text{Final BAC} = 0.0472 - 0.015 \times 1.5 \approx 0.0247 \ (\approx 0.025\%) - Interpretation: About 0.025% BAC. Estimated time to 0.00% = 0.025 / 0.015 ≈ 1.7 hours.
Frequently Asked Questions
Is the Widmark formula valid for everyone?
It is a well-established population model, but real individuals vary due to genetics, health, medications, hydration, age, and food. Treat results as estimates only.
Why do sex and weight matter?
Alcohol distributes mainly in body water. On average, males have a higher proportion of water than females at the same weight, which affects concentration.
Can I customize the elimination rate β?
Yes. The default 0.015 is typical; some people eliminate between 0.010 and 0.025. If you are unsure, keep the default to avoid underestimation.
What if I drank over many hours?
Enter the time since your first drink. The formula subtracts β × hours, approximating ongoing metabolism. It does not model complex absorption kinetics.
Is being below 0.08% always safe to drive?
No. Impairment can occur below common legal limits. Never drive after drinking alcohol. Local legal thresholds vary.
How are standard drinks calculated?
By dividing total pure alcohol (grams) by 14 g per U.S. standard drink. This gives a relatable count, not a legal measure.