How BAC is calculated
The calculator works the estimate two ways and leads with the higher. The first is NHTSA's 1994 paper Computing a BAC Estimate, which sets out the method from the Department of Transportation's 1992 report to Congress on alcohol limits:
BAC (g/dL) = alcohol (g) ÷ body water (mL) × 0.806 × 100 − burn-off × hours
- Body water is weight × 58% for men and × 49% for women, NHTSA's averages. Alcohol spreads through the body's water, and women carry less water per pound.
- 0.806 turns the concentration in body water into one in blood: blood is on average 80.6% water.
- Burn-off counts from the first drink. NHTSA gives .017 an hour for moderate drinkers, .02 for heavy drinkers and .012 as a conservative rate that fewer than one person in five is slower than.
- A standard drink is 14 g of alcohol.
The second is Widmark's original formula, alcohol ÷ (body weight × r), with his set values r = 0.68 for men and 0.55 for women. It puts the same drinks at a higher peak than NHTSA's body-water figures, and researchers note it tends to overestimate.
The result is deliberately a conservative upper estimate: the higher of the two peaks, less NHTSA's slow .012 an hour, the rate NHTSA suggests for judging when BAC is back to zero. Time to zero uses the same inputs. Beside it are NHTSA's central estimate (its body water and the average .017 an hour) and a range running from the lower peak at the heavy-drinker .02 up to the headline figure.
NHTSA's own example: a 128 lb man drinks one 12 oz beer at 4.5%, which holds 0.54 oz of alcohol. If it were all absorbed at once his BAC would be .03; an hour later, at .012 an hour, it is .018. The calculator's NHTSA arithmetic gives the same figures, and its tests check them.
What is a standard drink?
The National Institute on Alcohol Abuse and Alcoholism (NIAAA) defines a US standard drink as about 14 grams of pure alcohol, about 0.6 fl oz. Each of these is about one:
| Drink | Amount | Typical ABV |
|---|---|---|
| Regular beer | 12 oz | 5% |
| Malt liquor or hard seltzer | 8–10 oz | about 7% |
| Table wine | 5 oz | 12% |
| Distilled spirits | 1.5 oz | 40% |
Craft beers, generous wine pours and cocktails often hold more than one standard drink, so count carefully.
BAC levels and their effects
NHTSA's table of typical effects, and what they do to driving:
| BAC (g/dL) | Typical effects | Effects on driving |
|---|---|---|
| .02 | Some loss of judgment; relaxation, slight body warmth, altered mood | Worse at tracking a moving target and at doing two things at once |
| .05 | Exaggerated behavior, impaired judgment, lowered alertness, release of inhibition | Reduced coordination, difficulty steering, slower response to emergencies |
| .08 | Poor muscle coordination (balance, speech, vision, reaction time, hearing); judgment, self-control, reasoning and memory impaired | Poorer concentration, speed control and perception; slower information processing |
| .10 | Clear deterioration of reaction time and control, slurred speech, slowed thinking | Less able to hold a lane and brake appropriately |
| .15 | Far less muscle control than normal, possible vomiting, major loss of balance | Substantial impairment in vehicle control and attention |
Impairment starts at the first row, well below any legal limit. NHTSA counted 2,028 people killed in 2024 in crashes where a driver's BAC was between .01 and .07. At high levels, the NIAAA warns, blackouts, loss of consciousness and death can occur, and BAC can keep rising after drinking stops.
Legal limits
- .08: the limit in all 50 states, DC and Puerto Rico, except Utah (NHTSA).
- .05: Utah's limit (NHTSA).
- .04: commercial motor vehicle drivers on duty (federal rule 49 CFR 382.201).
- .02 or lower: drivers under 21. Federal law (23 U.S.C. 161) requires states to treat .02 or more as driving under the influence, and some set a lower figure.
You can be charged with impaired driving below these numbers if your driving is affected. Limits differ between countries; check the law where you are.
Sobering up takes time, and only time
At NHTSA's average rate of .017 an hour, a BAC of .08 takes about 4.7 hours to reach zero; at the conservative .012, about 6.7 hours. Nothing reliably speeds that up.
The NIAAA says cold showers, hot coffee and walking do not reverse the effects of alcohol. They may make you feel more awake while you are still impaired, which is especially dangerous.
Limitations
- An estimate, not a measurement. Only a breath or blood test measures BAC.
- Built on averages. NHTSA's body-water and burn-off figures are population means. Younger and leaner people carry more water, older and heavier people less, and burn-off runs from below .010 to above .040 an hour.
- Drink strength varies. One "drink" can be far more than one standard drink.
- Never use it to decide to drive. If you have had any alcohol, do not drive.
FAQs
How is blood alcohol content (BAC) calculated?
Two ways, keeping the higher. NHTSA's 1994 procedure spreads the alcohol through the body's water, taken as 58% of a man's weight and 49% of a woman's; Widmark's original formula uses his factors of 0.68 and 0.55 instead. From the first drink the liver removes alcohol at a fairly steady rate, taken here at NHTSA's slow .012 an hour. Every input is a population average, so the answer is an estimate, pitched high on purpose.
What counts as one standard drink?
The NIAAA defines a US standard drink as about 14 grams (about 0.6 fluid ounces) of pure alcohol. A regular 12 oz beer at 5%, a 5 oz glass of 12% wine and a 1.5 oz shot of 40% spirits each hold about that much; 8 to 10 oz of malt liquor or hard seltzer at about 7% does too. Craft beers, large pours and mixed drinks often hold more than one.
What is the legal BAC limit for driving?
NHTSA: driving at .08 g/dL or higher is illegal in every state, DC and Puerto Rico, except Utah, whose limit is .05. Federal rules hold commercial drivers to .04, and federal law requires states to treat a driver under 21 at .02 or more as driving under the influence; many states set that lower still. Police can charge impaired driving below any of these figures.
How long does it take for alcohol to leave your system?
NHTSA puts the average fall for moderate drinkers at .017 an hour and uses .012 as a conservative figure that fewer than one in five people are slower than. At those rates a BAC of .08 takes about 4.7 to 6.7 hours to reach zero. Nothing you drink or do makes the liver work faster.
How accurate is an online BAC calculator?
Not accurate enough to act on. Food, drinking pace, medication, liver health, body composition and the real strength of each drink all move the true figure, in either direction, and two people who drink the same can end up far apart. Only a breath or blood test measures BAC, and even a low reading is no reason to drive.
Can drinking water or coffee lower my BAC?
No. The NIAAA lists hot coffee, cold showers and walking among things that do not reverse the effects of alcohol. Coffee can make you feel more awake, which is worse: you feel able to drive while your blood alcohol is unchanged. Only time brings it down.
What factors affect BAC the most?
NHTSA lists five: how many drinks, how fast you drink them, your sex, your weight and whether you have eaten. Women generally carry less water and more fat per pound, so more alcohol stays in the blood; heavier people have more water to dilute it; and food in the stomach slows absorption.
How many drinks does it take to reach 0.08%?
Fewer than many people think. On the calculator's upper estimate, a 160 lb man drinking over one hour reaches .08 at 4 standard drinks and a 140 lb woman at 3. That is not a number anyone can drink up to: judgment and vision are already affected at .02, and a real BAC can be higher than the estimate.