BAC Calculator

Estimated blood alcohol content from drinks and time

This estimator applies the Widmark formula to sex, body weight, the number of standard drinks, and the hours since the first drink. It returns an estimated BAC, the hours until that estimate reaches zero, and the common 0.08% legal threshold for comparison. It is an educational estimate, not a breath test.

How the Widmark formula works

Alcohol is distributed across the water in the body rather than its full weight, so the calculation uses a sex-specific ratio: 0.68 for men and 0.55 for women. Each standard drink is treated as 14 g of pure alcohol. Weight is converted to kilograms, and the alcohol is spread across body water, then reduced by a fixed metabolic rate of about 0.015% BAC per hour.

BAC % = (drinks × 14 g) ÷ (weight(kg) × r × 1,000 g) × 100 − (0.015 × hours)

The result is floored at zero, and hours to zero is the remaining BAC divided by 0.015.

Worked example with the default inputs

The tool opens as male, 180 lb, 4 standard drinks, 2 hours since the first drink.

1. weight = 180 × 0.453592 = 81.65 kg
2. Body water = 81.65 × 0.68 = 55.52 kg, or 55,520 g
3. Alcohol = 4 × 14 = 56 g
4. Peak BAC = (56 ÷ 55,520) × 100 = 0.1009%
5. Subtract metabolism = 0.1009 − (0.015 × 2) = 0.071%
6. Hours to zero = 0.071 ÷ 0.015 = 4.7 hours

At 0.071% the estimate sits below the 0.08% threshold shown. Remove the two hours of metabolism and the same four drinks would read above it, which illustrates why pace matters as much as quantity.

Reference table

Because the r value is lower for women, the same weight and drink count produce a higher BAC. That is a body-composition difference, not a tolerance difference.

Common mistakes

• Using it to decide whether to drive. The estimate carries too much individual error to be used that way; if in doubt, do not drive.
• Under-counting drinks. A standard drink is smaller than a typical pour; craft beer, large wine glasses, and doubles all exceed it.
• Believing coffee or a cold shower helps. Nothing speeds metabolism; only time lowers BAC.
• Measuring hours from the last drink. The formula subtracts from the first drink, so use the full time alcohol has been in the body.

How accurate is a BAC calculator?

The Widmark formula is the standard forensic estimate, but real BAC varies with food in the stomach, body composition, medication, and drinking pace. Results can be off by roughly 0.02% either way, which is far more than the margin that matters near a legal limit. Only a calibrated breath or blood test is evidential.

Does eating slow alcohol absorption?

Food slows the rate at which alcohol enters the bloodstream, which lowers the peak for a given number of drinks, but it does not change the total alcohol or the speed at which it is eventually metabolised. A meal spreads the rise over a longer period rather than removing it.

How long until my BAC reaches zero?

Divide the estimated BAC by 0.015 to get the hours. The default example reaches zero in about 4.7 hours. Sleep does not accelerate clearance; it only passes the time.

How many drinks does the estimate count as one?

One US standard drink is 14 g of pure alcohol: a 12 oz beer at 5%, a 5 oz glass of wine at 12%, or a 1.5 oz shot of 40% spirits. A pint of craft beer at 7% is closer to 1.5 standard drinks, and a generous restaurant pour of wine can be nearly two.

Does body fat percentage affect BAC?

Indirectly. Alcohol mixes into body water, and fat holds little water, so a person with more body fat has less water to dilute a drink. The fixed ratios of 0.68 and 0.55 are population averages that partly capture this, but individual body composition still adds error.

This is an educational estimate, not legal or medical advice, and it is never a safe-to-drive determination. The same body-weight distribution logic appears in the body fat calculator, and alcohol’s effect on hydration is covered by the water intake calculator.