How BAC is estimated and what it means
Updated: October 25, 2025
Blood Alcohol Concentration (BAC) is the percentage of alcohol in a person's bloodstream. For example, a BAC of 0.08% (commonly used legal driving limit in several countries) means there are 0.08 grams of alcohol per 100 milliliters of blood. Estimating BAC from drinks uses simplified pharmacokinetic formulas that approximate alcohol absorption and elimination. These calculators give an estimate — not a precise measurement — and should not be used to determine fitness to drive or operate machinery.
Widmark formula — the common approach
Most simple calculators use a version of the Widmark formula: BAC = (Alcohol mass in grams / (body weight in grams × r)) − (β × hours), where r is a distribution ratio (≈0.68 for men, ≈0.55 for women) and β is the elimination rate (typical average ≈0.015% BAC per hour). “Alcohol mass” is calculated from the number of standard drinks and their alcohol by volume (ABV).
What a “standard drink” is
A standard drink varies by country. A common definition is about 10–14 grams of pure alcohol per standard drink (e.g., 14 g in the USA). For user-friendliness this calculator uses a default standard-drink size and allows you to enter drink ABV and volume so the alcohol grams calculation is explicit. If you count drinks instead of volumes, be consistent with your country's standard drink definition.
Factors that affect BAC (and why calculators are imperfect)
- Body composition: Muscle vs. fat changes the alcohol distribution ratio — two people of the same weight but different muscle mass can have different BACs.
- Sex/hormones: Biological sex affects distribution (women usually have a lower water fraction making BAC higher for same alcohol mass).
- Food and absorption: A full stomach slows absorption and reduces peak BAC; drinking on an empty stomach raises peak BAC faster.
- Drinking pattern: Rapid drinking raises peak BAC more than the same amount spread over several hours.
- Medications, health and genetics: Certain drugs, liver disease, and genetic differences in alcohol-metabolizing enzymes change elimination rates.
- Temperature, fatigue and dehydration: These can modify effects and subjective intoxication without necessarily changing BAC proportionally.
Legal limits and impairment
Legal BAC limits for driving differ around the world (commonly 0.05% or 0.08%). Any alcohol can impair judgment, reaction time, and coordination; impairment increases with higher BAC. Never rely solely on a calculator to decide whether to drive — only a certified breathalyzer or blood test can measure BAC with legal standing.
Practical safety advice
Plan transportation before drinking, alternate alcoholic drinks with non-alcoholic ones, eat food, and give your body time to metabolize alcohol. If you suspect someone has alcohol poisoning (loss of consciousness, slow or irregular breathing, hypothermia, or seizures), call emergency services immediately.
Limitations & medical considerations
Estimations are general-purpose and for information only. They cannot account for all individual differences. If you are on medication, pregnant, breastfeeding, or have liver disease, consult a healthcare professional about alcohol risks.
FAQs
- How long does alcohol stay in the system?
- On average the body eliminates alcohol at about 0.010–0.020% BAC per hour. Complete elimination depends on peak BAC; for example, if your peak was 0.10%, it may take ~5–10 hours to return to zero depending on elimination rate.
- Can drinking coffee or cold showers sober you up?
- No. Caffeine and cold showers may reduce sleepiness but they do not reduce BAC or the impairment caused by alcohol. Only time allows your body to metabolize alcohol.
- Is it safe to rely on a BAC calculator before driving?
- No. Calculators are rough estimates. Never drive if you feel impaired or uncertain. Use designated drivers, taxis, or public transport.
Sources: medical literature on alcohol pharmacokinetics, public health guidance, and legal BAC standards. This page provides educational information and is not a substitute for professional medical or legal advice.