How BAC is calculated
This calculator uses the Widmark equation, the standard method for estimating blood alcohol content:
BAC% = (alcohol grams ÷ (body weight g × r)) × 100 − 0.015 × hours
Where r is a body-water constant — about 0.68 for men and 0.55 for women — and 0.015 is the average BAC your body clears per hour. Each standard drink adds about 14 grams of alcohol.
Women tend to reach a higher BAC than men of the same weight because they carry proportionally less body water to dilute the alcohol.
What is a standard drink?
One US standard drink contains about 14 grams of pure alcohol. These are each roughly one standard drink:
| Drink | Amount | Typical ABV |
|---|---|---|
| Regular beer | 12 oz | 5% |
| Wine | 5 oz | 12% |
| Spirits (80-proof) | 1.5 oz | 40% |
Craft beers, generous wine pours, and cocktails often hold more than one standard drink, so count carefully — it's easy to underestimate.
BAC levels and their effects
| BAC | Typical effects |
|---|---|
| 0.02–0.03% | Slight relaxation; judgment and reaction time already start to decline |
| 0.05–0.07% | Lowered alertness and coordination; impaired driving ability |
| 0.08–0.12% | Legally intoxicated in most states; poor balance, slurred speech |
| 0.15–0.30% | Severe impairment; nausea, confusion, risk of blackout |
| 0.30%+ | Potentially life-threatening; risk of alcohol poisoning — seek help |
Notice that impairment begins far below the 0.08% legal limit. There is no BAC at which driving is "safe."
Legal limits
- 0.08% — the limit for drivers 21+ in most US states.
- 0.05% — Utah's lower limit.
- 0.04% — typical limit for commercial drivers.
- 0.01–0.02% — zero-tolerance limits for drivers under 21.
You can be charged with impaired driving even below these numbers if your driving is affected. Limits vary by state and country — check your local law.
Sobering up takes time — only time
Your body clears alcohol at roughly 0.015% per hour, and nothing reliably speeds that up. A BAC of 0.08% takes about 5–6 hours to reach zero.
Coffee, water, food, a cold shower, and exercise do not lower your BAC. They may make you feel more awake while you're still impaired, which is especially dangerous. The only cure is time.
Limitations
- An estimate, not a measurement. Only a breathalyzer or blood test measures actual BAC.
- Individual variation is large. Food, metabolism, medications, liver health, and body composition all change the real number.
- Drink strength varies. One "drink" can be far more than one standard drink.
- Never use it to decide to drive. If you've consumed any alcohol, don't drive — full stop.
FAQs
How is blood alcohol content (BAC) calculated?
This calculator uses the Widmark formula: BAC = (grams of alcohol ÷ (body weight in grams × r)) × 100 − 0.015 × hours, where r is a body-water constant (about 0.68 for men, 0.55 for women) and 0.015 is the average rate alcohol leaves the blood each hour. One standard US drink contains roughly 14 grams of pure alcohol. The result is only an estimate — actual BAC varies between people and situations.
What counts as one standard drink?
In the US, one standard drink contains about 14 grams of pure alcohol. That's roughly a 12 oz regular beer (5% ABV), a 5 oz glass of wine (12% ABV), or 1.5 oz of 80-proof spirits (40% ABV). Stronger craft beers, large wine pours, and mixed drinks often contain more than one standard drink, so it's easy to underestimate how much you've actually had.
What is the legal BAC limit for driving?
In most US states the limit is 0.08% for drivers 21 and over. Utah's limit is lower at 0.05%. Commercial drivers are typically held to 0.04%, and drivers under 21 face zero-tolerance laws (often 0.01–0.02%). Importantly, driving is impaired well below any legal limit — reaction time and judgment decline starting around 0.02%. The only safe amount before driving is none.
How long does it take for alcohol to leave your system?
The body removes alcohol at a fairly fixed rate of about 0.015% BAC per hour, and nothing reliably speeds that up. If your BAC is 0.08%, it takes roughly 5–6 hours to return to zero. Coffee, cold showers, exercise, and water do not sober you up faster — they may make you feel more alert while still being impaired, which is dangerous.
How accurate is an online BAC calculator?
It's a rough estimate, not a measurement. Real BAC depends on many factors a formula can't capture — whether you ate, your body composition, medications, liver health, drink strength, and individual metabolism. Two people drinking identically can end up with very different BACs. Never use a calculator result to decide whether it's safe to drive; only a breathalyzer or blood test measures actual BAC, and even then, don't drive after drinking.
Can drinking water or coffee lower my BAC?
No. Only time lowers your BAC, because your liver metabolizes alcohol at a steady rate. Water can help with hydration and a hangover, and coffee can make you feel more awake, but neither removes alcohol from your blood any faster. Feeling alert while still intoxicated is one of the most dangerous combinations behind the wheel.
What factors affect BAC the most?
The biggest drivers are how much alcohol you drink and over what time period, your body weight, and your sex (women generally reach higher BAC than men of the same weight due to differences in body water). Eating before and while drinking slows alcohol absorption, while drinking on an empty stomach raises BAC faster. Medications and health conditions can also change how alcohol affects you.
How many drinks does it take to reach 0.08%?
It varies a lot by weight, sex, and time, but as a rough guide a 160 lb man might reach around 0.08% after about 4 standard drinks in an hour, and a 140 lb woman after about 3. These are estimates only — the threshold can be much lower with less food, faster drinking, or higher-strength drinks. Because the line is so individual and unpredictable, the safest plan is to not drive after any drinking.