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Blood Alcohol Content (BAC) Calculator

What is my blood alcohol level?

How it is calculated

The tool uses a Widmark-style estimate: BAC (‰) = (drinks × 14) / (weight × r) − 0.15 × hours, floored at zero. Each drink is a 14 g standard drink of pure alcohol (the US size; a UK unit is 8 g and many European standard drinks are 10 g, so count grams rather than glasses), weight is in kilograms, and hours is the time since you started drinking. The distribution factor r is 0.68 for male and 0.55 for female, and 0.15‰ per hour is assumed to be burned off. The result is in per mille (‰, grams per litre) with the percent figure in brackets: 1‰ = 0.1%, so 0.80‰ is 0.080%. The tool gives no legal verdict: limits differ by country and driver type (0.5‰ in much of Europe, 0.8‰ or 0.08% in the US and England, lower for new or professional drivers), and no number makes driving safe. It is only an estimate.

Worked example

A 75 kg man has 4 drinks and the last 2 hours have passed. Alcohol: 4 × 14 = 56 g. Body factor: 75 × 0.68 = 51. Peak level: 56 / 51 ≈ 1.10‰. Elimination: 0.15 × 2 = 0.30‰. Estimate: 1.10 − 0.30 = 0.80‰ (0.080%). That is an estimate, not a verdict on whether anyone may, or can safely, drive.

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Common mistakes

  • Counting a drink as one glass whatever its size. The tool assumes 14 g of alcohol per drink, so a large strong pour counts as more than one.
  • Entering the time since the last drink instead of since the first. Elimination is counted from when drinking began.
  • Treating a result under your country's limit as safe to drive. Food, medication and individual metabolism move the real value, and impairment starts below any legal limit.
  • Mixing up ‰ and %. 0.5‰ is 0.05%, not 0.5%.
  • Entering weight in pounds. The field expects kilograms.
  • Leaving the sex setting at its default. The female factor (0.55) gives a noticeably higher result for the same weight and drinks.

Sources

About this calculator

This calculator estimates blood alcohol content in per mille using the Widmark formula, which combines total alcohol consumed with body weight, a sex-specific body water distribution constant, and the time elapsed since drinking began (since the liver metabolizes alcohol at a roughly constant rate over time). Because men and women distribute alcohol differently through body water, the same weight and drink count produce different estimated BAC for each.

People use it to get a rough sense of impairment level after drinking — for context against legal driving limits, which vary by country but are commonly around 0.5 per mille — or to estimate how long it will take for the level to drop to a safer point. It is explicitly an estimate: individual metabolism, food intake, medication and other factors all shift the real number, so it should never be treated as a precise measurement or used to decide whether it's actually safe to drive.

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Comments (1)

  • Editorial team

    The estimate counts standard drinks of 14 g of pure alcohol, so a large wine pour, a strong craft beer or a double spirit can be two drinks or more. Convert what you actually drank before entering it. The result assumes steady elimination at 0.15 per mille per hour, but food, medication and the individual all shift this, so it is an illustration and never a green light for driving. Which local serving sizes confuse you most?

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