How BAC is estimated
The calculator uses the Widmark formula, the standard method in toxicology. It divides the alcohol you drank by the water in your body, which depends on weight and sex, then subtracts what your liver has processed since you started.
Example: a 180 lb man who has 4 standard drinks has a peak BAC of 56 g ÷ (81,647 g × 0.68) × 100 = 0.101%. Two hours after his first drink, about 0.030% has been processed, leaving about 0.071%, which takes roughly 4¾ more hours to clear.
BAC and its effects
| BAC | Typical effects |
|---|---|
| 0.02% | Some loss of judgment, relaxation, trouble doing two things at once |
| 0.05% | Lowered alertness, reduced coordination, slower response to emergencies |
| 0.08% | Legal limit for driving in most states; poor muscle coordination and reasoning |
| 0.10% | Clear slowing of reaction time and difficulty staying in lane |
| 0.15% | Far less muscle control, major loss of balance |
Only time lowers BAC. Coffee, water, food after drinking or a cold shower don't speed it up. If someone is hard to wake, vomiting while confused, or has slow or irregular breathing, call 911: these can be signs of alcohol poisoning.
Frequently asked questions
How long does it take to get alcohol out of your system?
BAC falls by about 0.015% an hour, so roughly one standard drink an hour for many adults. A BAC of 0.08% takes about 5 to 6 hours to reach zero.
How many drinks gets you to 0.08?
It varies widely. A 180 lb man might reach it after about 4 drinks in an hour; a 130 lb woman after about 2 to 3. Food, health and drinking speed change it, which is why no calculator can tell you you're safe to drive.
Is this accurate enough to know if I can drive?
No. It's an estimate for education only. Impairment begins below the legal limit, and real BAC can be much higher than the estimate. If you've been drinking, don't drive.
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