Blood Alcohol Content (BAC) \u2014 The Complete Guide
Blog › Health · 12 min read · Published 2026-06-01
Understand BAC, the Widmark formula, elimination rates, legal limits worldwide, and how drinks, food, sex and weight change your number.
What Blood Alcohol Content Actually Measures
BAC is the percentage of alcohol — ethanol — in your bloodstream by volume. A BAC of 0.08% means 0.08 grams of ethanol per 100 mL of blood, or roughly 80 mg/dL. It's the same measure used by breathalyzers (after applying a 2,100:1 blood-to-breath partition ratio) and by police-station blood draws. Because alcohol distributes through the body's water and not its fat, BAC depends not just on what you drank but on how much water your body holds — which is why the same two beers produce very different numbers in a 60 kg woman and a 100 kg man. Use the BAC Calculator to model your own scenario before you ever drive.
The Widmark Formula — Where The Numbers Come From
Swedish chemist Erik Widmark published the formula in 1932 and it is still the backbone of every modern BAC estimator. In its standard form: BAC% = (A × 5.14 / (W × r)) − 0.015 × H, where A is alcohol in ounces, W is body weight in pounds, r is the gender constant (0.73 male, 0.66 female) reflecting body-water fraction, and H is hours since the first drink. The metric form replaces 5.14 with grams of ethanol and pounds with kilograms. The −0.015 × H term subtracts the body's elimination rate, which we'll cover next. The formula is an estimate: real-world BAC can swing ±0.02 around the prediction because of food, hydration, medications, and genetic differences in alcohol dehydrogenase activity.
Elimination Rates and the 'One Drink Per Hour' Myth
The liver oxidises ethanol at roughly 0.015 BAC per hour for most adults — about one standard drink per hour. But that average hides huge variation: heavy drinkers metabolise up to 0.020/hr through enzyme induction; some East Asian populations clear at 0.010/hr or less because of the ALDH2*2 variant that slows acetaldehyde breakdown. The popular 'one drink an hour keeps you sober' rule only works if you start at 0.00 — once you've stacked four drinks in 90 minutes, your BAC peaks 30–90 minutes later and you'll need 4+ hours to clear it regardless of how slowly the next drinks come.
What Counts as a Standard Drink
A US standard drink contains 14 g of pure alcohol: 12 oz (355 mL) of 5% beer, 5 oz (148 mL) of 12% wine, or 1.5 oz (44 mL) of 40% spirits. UK 'units' are smaller (8 g), and Australian standard drinks are 10 g — which is why translating drink counts between countries is misleading. Craft beer at 7–9% ABV, cocktails poured by hand, and oversized wine glasses routinely deliver 1.5–2 standard drinks each. Always count grams of ethanol, not 'drinks'.
Legal BAC Limits Around The World
Most US states, Canada and the UK (excluding Scotland) set the legal driving limit at 0.08%. Scotland, most of continental Europe, Australia, Brazil and Japan use 0.05%. Sweden enforces 0.02%, and dozens of countries — Hungary, Czech Republic, UAE, Saudi Arabia, India for commercial drivers — operate zero-tolerance 0.00 laws. Commercial drivers face stricter limits everywhere: 0.04% in the US, 0.02% in much of Europe. Under-21 drivers in the US face 0.02% 'zero-tolerance' laws.
How Food, Sex, Medications and Hydration Shift Your BAC
Eating a fat- and protein-rich meal before drinking can reduce peak BAC by 30–50% by slowing gastric emptying. Carbonated drinks (champagne, vodka-soda) accelerate absorption and raise peak BAC. Women generally reach a higher BAC than men for the same drink count because they carry a smaller water fraction (52% vs 61%) and have lower gastric alcohol dehydrogenase. Antihistamines, certain antidepressants, painkillers (acetaminophen plus alcohol is hepatotoxic) and metronidazole all interact badly with alcohol — sometimes producing intense disulfiram-like reactions even at low BAC.
BAC Effects — What the Numbers Feel Like
0.02–0.03: mild euphoria, slight impairment of judgement. 0.05: relaxation, lowered alertness, measurable reaction-time delay. 0.08: muscle coordination clearly impaired, short-term memory problems, the legal limit in most of North America. 0.10: slurred speech, slowed reflexes. 0.15: major balance loss, vomiting likely. 0.25: severe impairment, blackout territory. 0.30: stupor, loss of consciousness. 0.40+: respiratory depression, coma, potentially fatal. The lethal dose in adults is around 0.45–0.50, but tolerance lets chronic drinkers survive higher numbers.
Breath, Blood and Saliva — How BAC Is Measured
Breathalyzers use either a fuel-cell sensor (modern, accurate) or semiconductor sensor (cheap, drift-prone) and apply the 2,100:1 partition ratio to estimate blood concentration from breath. They can over-read by 10–15% in people with GERD, low body temperature or recently used mouthwash. Police blood draws are the gold standard — accuracy within ±0.005. Saliva strips and wearable sensors are improving but still ±0.02 at best.
Sobering Up — What Works, What Doesn't
Nothing speeds up ethanol metabolism except time. Coffee, cold showers, exercise and 'rally' food make you a more awake drunk, not a sober one. Sleep does not eliminate alcohol any faster — your liver runs at 0.015/hr whether you're awake or asleep. The only effective interventions during alcohol poisoning are airway protection and IV fluids; severe cases need hospital monitoring for hypoglycaemia and aspiration risk.
Planning a Safe Night Out
Use the BAC Calculator to plan in advance: enter weight, sex, drink list and time. Aim to finish drinking 1 hour per drink before you need to drive. Better still, build a default 'designated driver / rideshare / overnight stay' rule that removes the temptation to estimate. If you track caffeine intake to fine-tune your evening, our Caffeine Half-Life Calculator pairs nicely — caffeine masks BAC's sedative effects but doesn't lower the number.
The Bottom Line
BAC is a function of grams of ethanol consumed, body water, time, and roughly a dozen secondary factors. The Widmark formula gets you within ±0.02 of reality, which is precise enough for planning but never precise enough to flirt with a 0.08 legal limit. When in doubt, don't drive. When confident, verify with a personal breathalyzer — they've dropped under \$100 for fuel-cell accuracy.