Blood Alcohol Content (BAC) Calculator

The Widmark Formula The Widmark formula, developed by Swedish physician Erik Widmark in the 1920s, is the gold standard for estimating blood alcohol content. It accounts for body weight, gender, alcohol consumed, and time elapsed.

Formula: BAC = (A × 5.14) / (W × r) − (0.015 × H)

Where:

• A = fluid ounces of pure ethanol consumed
• W = body weight in pounds
• r = Widmark factor (0.68 for males, 0.55 for females)
• H = hours since drinking began
• 5.14 = conversion constant
• 0.015 = average alcohol metabolism rate (BAC% per hour)

Where 5.14 comes from, and what A actually means

A is a volume, not a mass. One fluid ounce of ethanol is 29.57 mL and weighs 29.57 × 0.789 = 23.33 g. A pound is 453.6 g. So converting to a percentage:

(A × 23.33) ÷ (W × 453.6 × r) × 100 = A × 5.14 ÷ (W × r)

This matters because a US standard drink contains 0.6 fl oz of pure ethanol, which is 14 g. A 12 oz beer at 5% ABV gives 12 × 0.05 = 0.6 fl oz of ethanol. There is no density factor to apply at that step, since 5.14 already carries the density. Multiplying the 0.6 by 0.789 or 0.816 as well double-counts it and undershoots the real BAC by about 20%.

Gender Difference The Widmark factor (r) differs by sex because females generally have a higher proportion of body fat and lower total body water, so alcohol is distributed into a smaller volume and reaches a higher concentration at the same dose.

On the r values, since calculators disagree about them

Widmark’s original figures are r = 0.68 for men and 0.55 for women, and those are what the pharmacology literature uses. Many online calculators instead use 0.73 and 0.66, which come from later regression work on a different population. The gap is not small: at the same dose a woman is told a BAC roughly 20% apart depending on which pair the tool picked, which is a large part of why two BAC calculators rarely give you the same number.

This site uses 0.68 and 0.55 on every BAC page, so the answers here at least agree with each other.

Standard Drinks A standard drink in the United States contains 14 grams (0.6 oz) of pure alcohol. Common examples:

• Beer (5% ABV): 12 oz can/bottle
• Wine (12% ABV): 5 oz glass
• Spirits (40% ABV): 1.5 oz shot

Important Limitations This formula provides an estimate only. Actual BAC varies with food intake, individual metabolism, medications, liver health, and drinking speed. Never rely solely on this calculator to decide whether to drive.

Metabolism Rate The body metabolizes alcohol at roughly 0.015% BAC per hour, which is about one standard drink per hour for an average adult. That rate does not speed up with coffee, food, a cold shower or exercise once drinking has begun. It also varies between people, from about 0.010 to 0.020 per hour, so treat any single time-to-sober figure as the middle of a range rather than a promise.

Counting the grams. To find the pure alcohol in a specific drink, multiply volume (mL) × (ABV ÷ 100) × 0.789, where 0.789 g/mL is the density of ethanol. A 355 mL (12 oz) beer at 5% comes out to about 14 g, which is exactly one US standard drink. The same 14 g shows up in a 5 oz glass of 12% wine or a 1.5 oz shot of 40% spirits, which is why those three pours are treated as equivalent.