Blood Alcohol
Concentration Calculator

BAC guide

How blood alcohol concentration is calculated.

Blood Alcohol Concentration (BAC) measures the percentage of alcohol (ethanol) in your bloodstream by volume. A BAC of 0.08% means 0.08 grams of alcohol per 100 mL of blood — the legal driving limit in the United States, Canada, and the United Kingdom (England and Wales). Understanding how BAC is calculated helps explain why alcohol affects people so differently and why age-based rules like "one drink per hour" are dangerously oversimplified.

The Widmark formula

The standard method for estimating BAC is the Widmark formula, developed by Swedish physician Erik Widmark in 1932 and refined extensively since. It remains the foundation of forensic toxicology, DUI law, and public health research worldwide:

• A: fluid ounces of pure (absolute) alcohol consumed
• W: body weight in pounds
• r: Widmark distribution ratio (0.73 male, 0.66 female)
• H: hours elapsed since drinking began
• 5.14: unit conversion constant (≈ 23.34 g/oz ÷ 453.6 g/lb × 100)
• 0.015: average metabolic elimination rate (%/hour)

What is a "standard drink"?

In the United States, one standard drink contains exactly 14 grams (0.6 fluid ounces) of pure alcohol. This is the amount found in:

A pint of 7% craft beer contains nearly twice the alcohol of a standard light beer. Many people significantly underestimate their consumption because they count containers rather than standard drinks.

The Widmark r-factor: why sex matters

The Widmark factor r represents the ratio of alcohol in blood to alcohol in the total body. It differs by sex because men typically have a higher percentage of total body water than women of the same weight. Since alcohol distributes into body water (not fat), a 150 lb man and a 150 lb woman drinking the same amount will have different BAC levels, even after controlling for weight; the woman will generally have a higher BAC because r = 0.66 vs r = 0.73.

This also explains why overweight individuals aren't as protected as total body weight might suggest — adipose (fat) tissue has a very low water content, so excess body fat doesn't increase alcohol distribution significantly.

Metabolism: the 0.015% rule

Your liver metabolises alcohol at a rate of approximately 0.015% BAC per hour on average, regardless of how much you've drunk, how much water you drink, or whether you've eaten. This means:

• A BAC of 0.08% will take roughly 0.08 / 0.015 = 5.3 hours to reach zero — not one hour per drink.
• Individual metabolic rates range from 0.010 to 0.020%/hr. A fast metaboliser might clear 0.08% in 4 hours; a slow one might take 8 hours.
• Food, coffee, cold showers, and sleep do not speed up metabolism; they can mask impairment symptoms while BAC remains elevated.

BAC and impairment: what the research shows

Impairment begins well below the legal limit. The National Highway Traffic Safety Administration (NHTSA) has documented measurable driving-relevant impairment starting at 0.02% BAC:

• 0.02%: Decline in visual function, difficulty tracking moving targets.
• 0.04%: Reduced vigilance, impaired lane-tracking.
• 0.05%: Legal limit in the EU, Australia, and Scotland. Reduced coordination and slowed reaction time.
• 0.08%: US/UK/Canada legal limit. Significant impairment of concentration, short-term memory, speed control, and information processing.
• 0.15%+: Severe impairment. Balance and reflexes dramatically reduced. High risk of vomiting.
• 0.30%+: Loss of consciousness; potentially fatal without medical intervention.

Research consistently shows that crash risk roughly doubles at 0.05% BAC and increases 7-fold at 0.08% compared to sober driving. This is why many countries set limits below 0.08%.

Factors that affect true BAC

The Widmark formula produces an estimate. Real BAC depends on many variables the formula does not capture:

• Food consumption: Eating before or during drinking slows alcohol absorption significantly — peak BAC may be 30–50% lower on a full stomach, and delayed by 1–2 hours.
• Carbonated mixers: Champagne, sparkling wine, and spirits mixed with carbonated drinks absorb faster than still beverages.
• Body composition: The Widmark r-factor is a population average. An individual's true value varies with their actual lean body mass.
• Medications: Certain drugs (antihistamines, sedatives, antidepressants) amplify alcohol's effects. Some (e.g., aspirin) can inhibit gastric alcohol dehydrogenase and raise BAC.
• Tolerance: Regular heavy drinkers may feel less impaired at the same BAC due to central nervous system adaptation, but their actual driving impairment is not reduced — tolerance masks symptoms, not capability.
• Absorption lag: Alcohol takes 30–90 minutes to be fully absorbed from the digestive tract. BAC typically peaks 45–75 minutes after drinking stops.

Legal limits worldwide

There is no universal BAC threshold for legal driving. Limits vary significantly by country, and many jurisdictions impose stricter limits for commercial drivers, young drivers, and repeat offenders:

• 0.08%: United States, Canada, United Kingdom (England and Wales), Ireland, New Zealand
• 0.05%: Most of the EU, Australia, Japan, South Africa, Scotland
• 0.02%: Sweden, Norway, Poland
• 0.00%: Czech Republic, Hungary, Slovakia, Brazil (zero tolerance)

In the US, all states impose zero tolerance (0.01–0.02% limits) for drivers under 21. Commercial drivers face a 0.04% federal limit.

Important disclaimer

This calculator is for educational and informational purposes only. It is not a reliable method for determining whether it is safe or legal to drive. Individual BAC can vary substantially from formula estimates due to body composition, food intake, medications, alcohol tolerance, and absorption timing. If you have consumed any alcohol, the safest choice is not to drive — call a rideshare service, arrange a designated driver, or wait until you are confident you have fully metabolised the alcohol. No estimate, however accurate, substitutes for this principle.