BAC Calculator

How the Widmark Formula Estimates BAC

The Widmark formula, developed by Swedish scientist Erik Widmark and published in 1932, estimates blood alcohol content from the amount of alcohol consumed, body weight, a gender-based distribution constant, and time elapsed since drinking began:

$$\text{BAC} = \left(\frac{\text{grams of alcohol}}{\text{weight (g)} \times r}\right) \times 100 - (\text{hours} \times 0.015)$$

Why the Distribution Constant Differs by Gender

Alcohol distributes through the water content of the body, not through fat tissue. On average, men's bodies are about 68% water by weight, while women's bodies are about 55% water by weight — women on average carry a higher percentage of body fat, which doesn't absorb alcohol the way water-containing tissue does. The same amount of alcohol, in a body with a lower water percentage, becomes more concentrated — which is why the formula uses a smaller r constant for women, producing a higher estimated BAC for the same drink count and body weight, all else equal.

Worked Example

A 170 lb man has 3 standard drinks (3 × 14 = 42 grams of alcohol) over 2 hours. Body weight in grams: 170 × 453.6 ≈ 77,112 g. BAC = (42 / (77,112 × 0.68)) × 100 − (2 × 0.015) = (42 / 52,436) × 100 − 0.03 ≈ 0.080 − 0.030 = 0.050%. The result is floored at zero if the subtraction would otherwise go negative — BAC cannot be a negative number.

Why This Is Only a Rough Estimate

The Widmark formula assumes even alcohol absorption and a fixed elimination rate, neither of which holds precisely in real life. Food in the stomach slows alcohol absorption significantly, so the same drinks on an empty stomach versus after a full meal can produce very different actual BAC curves. Individual metabolism, certain medications, liver function, and even the pace of drinking all shift the real elimination rate away from the 0.015 average used here. Because of this variability, the number this calculator produces should be treated as a rough, general-education estimate — not a measurement, and never a basis for deciding whether someone is fit to perform any safety-critical task, including driving.

Common BAC Calculator Mistakes

Using this or any BAC calculator to judge fitness to drive is the most serious mistake possible with this tool — see the warning above the result. Miscounting standard drinks is another common error: a large glass of wine, a strong cocktail with multiple shots, or a high-ABV craft beer can easily contain more than one standard drink's worth of alcohol, even though it's served as a single drink. Forgetting that BAC keeps rising for a while after the last drink — as alcohol already swallowed continues to absorb — is a third: entering "hours since first drink" doesn't capture that absorption is still in progress shortly after drinking stops, which this formula, like all Widmark-based estimates, does not model in detail.

BAC Terms You Should Know

Standard Drink — in the US, defined as about 14 grams of pure alcohol, roughly one 12 oz regular beer, one 5 oz glass of wine, or one 1.5 oz shot of distilled spirits.

Widmark Constant (r) — the gender-based body-water distribution factor in the Widmark formula, about 0.68 for men and 0.55 for women.

Elimination Rate — the rate at which the body metabolizes and removes alcohol from the bloodstream, commonly cited as an average of about 0.015% BAC per hour, though it varies by individual.

Legal BAC Limit — the maximum BAC allowed for driving under the law in a given jurisdiction (commonly 0.08% for most drivers in the US) — noting that impairment itself can begin well below this threshold.