What Is Blood Alcohol Content (BAC)?
Blood Alcohol Content (BAC) is a measure of the concentration of alcohol (ethanol) in the bloodstream, expressed as a percentage by volume. A BAC of 0.08% means 0.08 grams of alcohol per 100 milliliters of blood. BAC is the legal standard used in most countries to determine driving impairment.
The Widmark Formula
BAC is estimated using the Widmark formula: BAC (%) = (Alcohol consumed in grams) ÷ (Body weight in grams × Distribution ratio) − (Elimination rate × Hours elapsed). The distribution ratio (Widmark factor r) is approximately 0.68 for men and 0.55 for women, reflecting differences in body water percentage. The body eliminates approximately 0.015% BAC per hour.
Effects at Different BAC Levels
0.02–0.03%: Mild relaxation, slight mood elevation. 0.05–0.06%: Judgment impaired, coordination reduced. 0.08%: Legal driving limit in most US states, Canada, Australia, UK. 0.10–0.15%: Significant impairment, slurred speech, poor balance. 0.20–0.25%: Confusion, nausea, possible blackout. 0.30%+: Risk of unconsciousness, respiratory depression. 0.40%+: Potentially fatal.
Legal BAC Limits by Country
0.00% (zero tolerance): Saudi Arabia, UAE, Russia, Hungary, Czech Republic. 0.02%: Sweden, Norway, Poland, Estonia. 0.05%: Most of Europe (Germany, France, Spain, Italy, Netherlands, Australia). 0.08%: United States (most states), Canada, United Kingdom. Japan uses 0.03% but has a stricter 0.015% operational limit for professional drivers.
Factors That Affect BAC
Body weight and composition, sex (women generally reach higher BAC from the same amount of alcohol), food intake (slows alcohol absorption), drink strength, drinking speed, age, liver health, and medications all affect BAC. Tolerance — feeling less drunk at a given BAC — does not reduce impairment, it just masks the symptoms.