Blood Alcohol Content (BAC) Calculator
Estimate blood alcohol content (BAC %) using the Widmark formula. Enter drinks consumed, body weight, sex, and time elapsed.
How to use this tool
- Enter standard drinks consumed, body weight, sex and hours since first drink in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your estimated bac and the full breakdown beneath it.
Educational estimate — not medical advice. Consult a clinician.
Never drive if you have consumed alcohol. This calculator uses the Widmark formula to estimate BAC but individual elimination rates vary. Food intake, medications, and tolerance all affect actual impairment. Only a certified breathalyser or blood test gives an accurate BAC.
Formula
BAC (%) = (drinks × 14) ÷ (weightkg × r × 10) − 0.015 × hours
Where r = 0.73 for males, 0.66 for females (Widmark distribution factor).
How it works
This calculator uses the Widmark formula, the most widely cited pharmacokinetic model for estimating blood alcohol concentration. It converts standard drinks to grams of pure alcohol (14 g per US standard drink), distributes that alcohol through total body water via the sex-based Widmark r factor, and subtracts the average metabolic elimination rate of 0.015 % BAC per hour. Results are estimates only; individual metabolism, food consumption, and hydration all affect actual BAC.
Worked example
Worked example
- Inputs: 2 standard drinks, 70 kg male, 1 hour elapsed.
- Alcohol absorbed: 2 × 14 = 28 g.
- Widmark r for male = 0.73; distribution volume = 70 × 0.73 × 10 = 511.
- Raw BAC: 28 ÷ 511 ≈ 0.0548 %.
- After elimination: 0.0548 − 0.015 × 1 ≈ 0.04 %.
Estimated BAC: 0.04 % — Mild impairment.
Key terms
- BAC (Blood Alcohol Concentration)
- The mass of ethanol per volume of blood, expressed as a percentage (g per 100 mL).
- Widmark formula
- A pharmacokinetic equation developed by Erik Widmark in the 1930s that estimates BAC from alcohol consumed, body weight, sex, and time.
- Standard drink
- A defined unit of alcohol containing 14 g of pure ethanol in the United States.
- Elimination rate
- The average rate at which the liver metabolises alcohol, approximately 0.015 % BAC per hour for most adults.
- Widmark r factor
- A sex-based constant (0.73 male, 0.66 female) representing the fraction of body weight that is water, used to estimate alcohol distribution.
Frequently asked questions
- What counts as one standard drink?
- In the US, one standard drink contains 14 g of pure alcohol: 12 oz regular beer (5%), 5 oz wine (12%), or 1.5 oz spirits (40%). UK units are different (8 g/unit).
- What is the legal driving limit?
- Limits vary by country. In the US most states use 0.08%; in the UK it is 0.08% in England/Wales and 0.05% in Scotland; in many EU countries 0.05%. There is no safe amount of alcohol to drive.
- How does body weight affect BAC?
- Higher body weight means more body water to dilute alcohol, so a heavier person will typically have a lower BAC for the same number of drinks. The Widmark formula captures this via the weight term.