About BAC
Blood alcohol concentration — the percentage of alcohol in your bloodstream — is what breathalyzers estimate, laws regulate, and impairment tracks. 0.08% is the driving limit in most US states (0.05% in Utah and much of the world), but impairment measurably begins around 0.02–0.05%, well under every legal line.
Enter your sex, weight, standard drinks, and hours since the first one for the textbook estimate with its working. One critical definition: a “standard drink” is 14 g of alcohol — a 12-oz regular beer, 5-oz wine, or 1.5-oz spirits — and generous pours, craft beers, and cocktails routinely count as 1.5–2 drinks each.
Alcohol also carries ~7 kcal per gram — see where it fits your day with the Calorie Calculator.
The Widmark Formula
The century-old standard, still what most estimators run:
BAC% = (alcohol grams ÷ (weight grams × r)) × 100 − 0.015 × hours r (body-water constant): ~0.68 men · ~0.55 women Standard drink = 14 g alcohol (NIAAA)
Worked example: a 180-lb man, four standard drinks, two hours in: 56 g ÷ (81,600 × 0.68) × 100 − 0.03 = ~0.071% — under the 0.08 line yet measurably impaired, which is precisely the zone where people make bad driving decisions.
Drinks vs Estimated BAC
Widmark estimates two hours after the first drink (elimination included) — illustrative bodies, not yours:
| Standard drinks | Man, 180 lb | Woman, 140 lb |
|---|---|---|
| 1 | 0.00% | 0.01% |
| 2 | 0.02% | 0.05% |
| 3 | 0.05% | 0.09% |
| 4 | 0.07% | 0.13% |
| 5 | 0.10% | 0.17% |
| 6 | 0.12% | 0.21% |
Read the columns' gap: the same drinks put a smaller, lower-body-water drinker dramatically higher — one of several reasons drink-for-drink matching is a bad idea.
What Moves Real BAC Off the Estimate
Upward surprises: drinking fast (absorption outpaces the hourly elimination the formula assumes), empty stomach (food slows absorption substantially), carbonated mixers, higher-ABV pours than the “standard” assumes, and medications that impair alcohol metabolism. Downward: none you can rely on — coffee, cold showers, and “feeling fine” change alertness perception, not blood chemistry. Only time eliminates alcohol.
The legal fine print worth knowing: 0.08% is the per-se limit for regular US drivers, but commercial drivers face 0.04%, under-21 drivers face zero-tolerance thresholds (0.00–0.02%), and impairment charges below 0.08 exist everywhere. Breathalyzer and blood results — not calculators — are what courts use; nothing here has evidentiary value in either direction.