Blood Alcohol Content Guide: The Science of How Your Body Processes Alcohol
A 0.08% BAC — the US legal limit — means 0.08 grams of alcohol per 100 milliliters of blood. At this level, reaction time is impaired by 40%, lane tracking is degraded, and risk of a fatal crash is 7× higher than sober driving. Yet most people have no accurate sense of what their BAC actually is based on what they've drunk.
Key Takeaways
- The liver metabolizes ~1 standard drink/hour — coffee, food, cold showers do not speed this up
- Women reach higher BAC per drink because of lower body water percentage and less alcohol dehydrogenase enzyme
- 0.08% BAC (US limit) impairs reaction time by 40%; crash risk is 7× that of sober driving
- Breath vs blood alcohol: breathalyzers measure vapor in alveolar air and apply a 2100:1 partition ratio
- Time is the only cure — BAC drops at roughly 0.015%/hour regardless of actions taken
How BAC Is Calculated: The Widmark Formula
The Widmark formula (developed by Swedish chemist Erik Widmark in the 1920s) estimates BAC from alcohol consumed, body weight, sex, and time elapsed. The r factor — called the Widmark factor — accounts for the different ratios of body water to total body weight between men (r ≈ 0.73) and women (r ≈ 0.66). Women have a lower r because they have a higher percentage of body fat relative to water, so alcohol distributes into a smaller volume.
BAC is expressed as grams per 100mL of blood (g/dL), also written as a percentage. A BAC of 0.08% means 0.08g of alcohol per 100mL of blood — or about 0.08 mL of pure ethanol per 100 mL blood. The elimination rate of approximately 0.015–0.017% per hour is relatively constant for moderate drinkers.
BAC Effects by Level
Mild relaxation, slight mood elevation. Most cognitive functions relatively intact. Legal in all countries for driving.
Reduced coordination, impaired judgment. Tracking moving objects degraded. Legal limit in many European countries (0.05%). Many people feel overconfident at this range.
Clearly impaired motor control. Reaction time reduced by 40%. Slurred speech may appear. Illegal to drive in the US, Canada, UK, Australia. Crash risk 7× sober.
Major loss of motor control, vomiting likely, significant memory impairment (blackouts). Crash risk 25× sober. Medical attention may be required.
Life-threatening. Loss of consciousness, respiratory depression, risk of death. BAC above 0.45% is generally considered fatal.
Legal BAC Limits by Country
0.08% Limit
- 🇺🇸 United States
- 🇨🇦 Canada
- 🇳🇿 New Zealand
- 🇲🇽 Mexico
0.05% Limit
- 🇩🇪 Germany
- 🇫🇷 France
- 🇦🇺 Australia
- 🇯🇵 Japan (0.03%)
Zero Tolerance
- 🇸🇦 Saudi Arabia
- 🇦🇪 UAE
- 🇷🇴 Romania (0.0%)
- 🇭🇺 Hungary (0.0%)
How Breathalyzers Work
Breathalyzers measure alcohol in deep lung air (alveolar air) and apply a partition ratio of 2100:1 — the ratio of alcohol in blood to alcohol in breath vapor at 34°C (lung temperature). This ratio varies between individuals by as much as ±15–20%, which is why breathalyzer readings have legal margins of error built into them.
Modern evidential breathalyzers (Intoxilyzer, Drager) use infrared spectroscopy to identify ethanol by its molecular absorption spectrum, preventing interference from acetone (which ketogenic dieters exhale in higher quantities). Consumer devices use electrochemical fuel cells that are less selective and more prone to false positives.
Calculate Your BAC
BAC Calculator
Estimate your blood alcohol content based on drinks consumed, body weight, sex, and time. For informational purposes — never drive impaired regardless of estimated BAC.
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