How Is BAC Calculated? The Widmark Formula Explained
Disclaimer: This article is for educational purposes only. Never use a BAC estimate to decide whether it is safe to drive. The only safe amount of alcohol before driving is zero, and individual responses to alcohol vary significantly.
Estimate your BAC based on weight, drinks, and time with the BAC Calculator, or read on to understand the formula behind the number.
Blood alcohol content — the percentage of alcohol in your bloodstream by volume — follows predictable chemistry. A Swedish physician named Erik Widmark worked out the core formula in the 1930s, and it is still the basis for every BAC calculator and breathalyzer calibration in use today.
The Widmark Formula
The formula for calculating BAC is:
BAC = (Alcohol consumed in grams / (Body weight in grams × r)) × 100
Where r is the Widmark distribution factor:
- r = 0.73 for biological males
- r = 0.66 for biological females
The difference exists because women typically have a higher proportion of body fat relative to lean tissue, and alcohol does not distribute into fat the way it distributes into muscle and water-rich tissue. Two people who weigh exactly the same but have different body compositions will reach different BAC levels after consuming identical amounts of alcohol.
What Counts as One Standard Drink?
To use the formula, you need to know how many grams of pure alcohol you consumed. In the United States, one standard drink contains 14 grams of pure alcohol. That equals:
- 12 oz of regular beer (5% ABV)
- 5 oz of wine (12% ABV)
- 1.5 oz of distilled spirits (40% ABV)
A 16 oz pint of craft beer at 7% ABV is not one standard drink — it is closer to 1.9 standard drinks. Knowing this matters enormously for accurate calculations.
How Your Body Clears Alcohol
The liver metabolizes alcohol at a fairly consistent rate regardless of how much you drink, how much water you consume, or whether you eat a large meal. The standard metabolism rate is approximately 0.015% BAC per hour.
This means that if your BAC peaks at 0.09%, it will take roughly 6 hours for your system to clear the alcohol entirely — regardless of coffee, food, exercise, or cold showers. Those remedies affect how alert you feel, not your actual BAC.
A Worked Example
A 180-pound male (approximately 81,650 grams) drinks 3 standard beers over the course of an evening. He wants to know his BAC after 2 hours.
Step 1 — Calculate alcohol consumed: 3 drinks × 14 grams = 42 grams of alcohol
Step 2 — Apply the Widmark formula: BAC (peak) = (42 / (81,650 × 0.73)) × 100 BAC (peak) = (42 / 59,604) × 100 BAC (peak) ≈ 0.070%
Step 3 — Subtract metabolism over 2 hours: 2 hours × 0.015% = 0.030% BAC after 2 hours = 0.070% − 0.030% = 0.040%
At 0.04%, this person is below the US legal limit of 0.08%, but is still measurably impaired. Reaction time and judgment are affected at BAC levels well below the legal threshold.
Legal Limits Around the World
The legal BAC limit for driving varies by country:
| Region | Limit | |---|---| | United States | 0.08% | | United Kingdom | 0.08% (0.05% in Scotland) | | Most of the EU | 0.05% | | Sweden, Norway | 0.02% | | Czech Republic, Hungary | 0.00% |
Many countries have moved toward or below 0.05%, which reflects research showing meaningful cognitive and motor impairment begins at that level — not at 0.08%.
Factors That Affect BAC Beyond the Formula
The Widmark formula gives a reasonable estimate, but several variables push results higher or lower in practice:
- Food in the stomach slows alcohol absorption, lowering peak BAC but not the total amount metabolized
- Carbonated mixers like tonic water or soda speed up absorption
- Medications — many common drugs interact with alcohol and amplify its effects at lower BAC levels
- Fatigue and stress increase perceived impairment at a given BAC
- Tolerance affects how impaired you feel, but does not change your actual BAC or driving ability
The BAC Calculator runs the Widmark formula for you — enter your weight, drinks, and time elapsed to see an estimated BAC.