Last updated: 2026-05-13

Blood Alcohol Content (BAC)

Blood alcohol content (BAC) is the concentration of alcohol in the bloodstream, measured in grams of alcohol per 100 mL of blood and expressed as a percentage. A BAC of 0.05% means there are 0.05 grams of alcohol in every 100 mL of blood. Most countries set driving limits between 0.00% and 0.08%, and even small amounts of alcohol begin to affect coordination, reaction time, and judgment before any subjective sense of intoxication.

BAC cannot usually be measured directly without a breathalyzer or blood test, so it is commonly estimated from the amount of alcohol consumed, body size, sex, and elapsed time. This calculator uses the Widmark formula, the most widely used model for that estimation.

Widmark formula

The formula, developed by Swedish pharmacologist Erik Widmark in the 1930s, models BAC as a function of alcohol consumed, body mass, and a sex-dependent distribution factor. Alcohol distributes through the body's water, so a larger body — and a higher body-water fraction — dilutes the same dose to a lower concentration. The liver then clears alcohol at a roughly constant rate over time, which the final term subtracts.

Where:

• A = total volume of pure alcohol consumed (mL), calculated as drinks × volume per drink (mL) × ABV
• W = body weight in kg
• r = Widmark factor: 0.73 for men, 0.66 for women (reflects average body water content)
• H = hours elapsed since drinking started
• 0.789 = density of ethanol (g/mL)
• 0.015 = average BAC eliminated per hour by the liver

The result is bounded at zero — BAC cannot go negative.

Worked example

A man weighing 75 kg165 lb (75 kg) drinks 2 cans of beer (355 mL12 fl oz, 355 mL each, 5% ABV) over 1 hour. Because the Widmark constants are defined in metric units, the calculation uses 355 mL and 75 kg:

• Alcohol consumed: 2 × 355 × 0.05 × 0.789 = 28.1 g
• BAC before elimination: 28.1 / (75 × 10 × 0.73) = 0.0514%
• After 1 hour of elimination: 0.0514 − 0.015 = 0.036%

At 0.036%, the result is below the 0.05% limit common in Europe but above the 0.03% limit in Japan and Korea.

Classification bands

Legal limits vary widely: 0.00% (zero tolerance) in Brazil and many Arab countries; 0.03% in Japan, Taiwan, and South Korea; 0.05% in most of Europe and Australia; 0.08% in North America.

Factors affecting accuracy

The Widmark formula is an estimate. Several factors can shift the real BAC 20–30% above or below the calculated number:

Food slows absorption. Eating a full meal before drinking can reduce peak BAC by 30–40% and delay it by an hour or more. An empty stomach lets alcohol absorb faster and reach a higher peak.

Elimination rates vary individually. The 0.015%/h figure is an average. Regular drinkers tend toward the high end (0.020%/h) and infrequent drinkers toward the low end (0.010%/h), which affects estimates of when BAC returns to zero.

Body composition matters. Fat tissue holds less water than muscle. Two people of the same weight but different body-fat percentages reach different BAC peaks, because alcohol distributes into body water rather than fat.

Medications can amplify alcohol effects. Antihistamines, benzodiazepines, some antibiotics, and many other drugs interact with alcohol, sometimes producing cognitive impairment beyond what BAC alone would predict.

Application and limits

A calculated BAC is an approximation, not a measurement. Only a breathalyzer or blood test reports an actual value: consumer-grade breathalyzers give a directionally correct reading, while evidentiary breathalyzers and blood tests used by police are highly accurate and legally admissible.

Because the estimate carries a wide margin of error, it should not be used to decide whether to drive or operate machinery. The safest course after consuming any alcohol is not to drive.