Blood Alcohol (BAC) Calculator
An estimate of blood alcohol concentration built on the body-water method NHTSA documented step by step, using NIAAA’s definition of a standard drink. It is an educational estimate based on population averages — never a test of whether you are safe to drive.
- Published sources, named on the page
- Method and formula shown
- Limits stated with the result
Blood alcohol concentration depends on how much pure alcohol you drank, your body water, and how much time has passed. Alcohol leaves the blood at roughly 0.012 to 0.020 percentage points per hour, and only time lowers it. Any calculated figure is an estimate from population averages, not a measurement, and cannot tell you it is safe to drive.
One standard drink = 14 g of pure alcohol (NIAAA). Convert your drinks.
0.037%
Below 0.04%
- Peak before metabolism
- 0.071%
- 3 standard drinks · 42 g of alcohol
- Time to reach 0.00%
- 2.2 more hours
- Burnoff applied: 0.017 per hour
Only time lowers blood alcohol. Coffee, food, water, exercise and cold showers do not change the rate.
This is an educational estimate built on population averages, not a measurement of your blood. Real values vary with food, medications, liver function, body composition, drinking speed, and individual metabolism, and forensic guidance treats these calculations as having genuine uncertainty. It cannot tell you that you are under a legal limit or safe to drive. If you have been drinking, do not drive.
How This Estimate Is Calculated
BAC % = (grams of alcohol ÷ body water in mL) × 0.806 × 100 − (burnoff rate × hours)
Body water in mL = (weight in lb ÷ 2.2046) × 0.58 for men or 0.49 for women × 1000. Grams of alcohol = standard drinks × 14.
Alcohol distributes through the water in your body, so the calculation starts with body water. Weight in pounds is converted to kilograms, multiplied by the mean total body water fraction NHTSA publishes — 58% for men and 49% for women — and the resulting volume is used to dilute the alcohol consumed. Because blood is about 80.6% water, that concentration is multiplied by 0.806 to express it as alcohol per volume of blood. Finally, metabolism is subtracted: NHTSA gives 0.017 per hour as the average for moderate drinkers, 0.020 for heavy drinkers, and 0.012 as a conservative figure fewer than one in five people fall below.
The alcohol itself is counted in NIAAA standard drinks, each containing 0.6 fluid ounces or 14 grams of pure alcohol. A poured drink is frequently larger than one standard drink, which is the single most common reason a real BAC comes in above an estimate.
What Each BAC Level Typically Looks Like
NHTSA publishes the typical effects and driving effects associated with each blood alcohol level. Individual response varies, and impairment begins below every number in this table.
| BAC | Typical effects | Predictable effects on driving |
|---|---|---|
| 0.02% | Some loss of judgment, relaxation, slight body warmth, altered mood. | Decline in visual functions such as rapid tracking of a moving target, and decline in the ability to perform two tasks at once. |
| 0.05% | Exaggerated behavior, possible loss of small-muscle control such as focusing the eyes, impaired judgment, lowered alertness, release of inhibition. | Reduced coordination, reduced ability to track moving objects, difficulty steering, reduced response to emergency driving situations. |
| 0.08% | Poor muscle coordination affecting balance, speech, vision, reaction time and hearing; harder to detect danger; judgment, self-control, reasoning and memory impaired. | Impaired concentration, short-term memory loss, reduced speed control, reduced information-processing capability, impaired perception. |
| 0.10% | Clear deterioration of reaction time and control, slurred speech, poor coordination, slowed thinking. | Reduced ability to maintain lane position and to brake appropriately. |
| 0.15% | Far less muscle control than normal, vomiting may occur, major loss of balance. | Substantial impairment in vehicle control, in attention to the driving task, and in necessary visual and auditory information processing. |
Source: NHTSA, The ABCs of BAC (DOT HS 812 519).
Why an Estimate Differs From a Breath or Blood Test
Absorption is not instant
Alcohol takes time to move from the stomach and small intestine into the blood. Food slows it; drinking on an empty stomach speeds it up. A calculation assumes everything consumed is already absorbed, which overstates BAC early in a session and understates it if drinking is still in progress.
Body composition is not just weight
The method uses average total body water as a fraction of weight. Two people at the same weight with different muscle and fat proportions hold different amounts of water, and the concentration differs accordingly.
Metabolism varies by person and by history
NHTSA gives a range rather than one rate for a reason. Liver function, regular heavy drinking, medications and genetics all shift how fast alcohol clears.
A poured drink is rarely one standard drink
Most underestimates trace back to the count of drinks, not the formula. A pint of craft beer or a generous restaurant wine pour is close to two standard drinks.
Forensic guidance from the NIST-hosted Organization of Scientific Area Committees treats Widmark-type calculations as carrying real uncertainty, which is why they are reported as ranges rather than single values in casework.
Legal Limits, and Why They Are Not a Safety Line
Driving with a blood alcohol concentration of 0.08% or higher is a per se offense in most U.S. states; Utah’s limit is 0.05%. Commercial drivers are generally held to 0.04%, and every state has zero-tolerance provisions for drivers under 21. Those are enforcement thresholds, not points at which impairment begins — reaction time, judgment, and tracking degrade well below them, and a driver can be charged for impairment at a lower number. State law varies, so treat the figures above as general information rather than legal advice.
When Drinking Levels Become a Health Question
NIAAA defines binge drinking as the pattern that brings blood alcohol concentration to 0.08% or higher — typically about four drinks for women or five for men within roughly two hours. Repeated episodes matter clinically, but a number from a calculator does not diagnose anything. What clinicians look at is the pattern around it: control, craving, tolerance, withdrawal, and continued drinking despite harm.
One thing is worth stating plainly. If stopping alcohol after regular heavy drinking brings on tremor, sweating, agitation, vomiting, hallucinations, or confusion, that is a medical emergency risk, not a willpower problem. Alcohol withdrawal is one of the few withdrawals that can be fatal, and it is treatable under medical supervision.
Blood Alcohol: Frequently Asked Questions
How accurate is a BAC calculator?
It is an estimate, not a measurement. The calculation uses population averages for body water and alcohol metabolism, and real people vary on both. Food in the stomach, medications, liver function, body composition, and how quickly the drinks were consumed all shift the true number. Forensic guidance treats these calculations as ranges with real uncertainty, which is why no estimate should ever be used to decide whether it is safe to drive.
How long does it take for BAC to reach zero?
Only time lowers blood alcohol concentration. Using the metabolism rates documented by NHTSA, blood alcohol falls roughly 0.012 to 0.020 percentage points per hour depending on the person. Coffee, cold showers, food, exercise, and water do not speed this up.
What BAC is legally impaired?
In most U.S. states it is a per se offense to drive with a blood alcohol concentration of 0.08% or higher. Utah's limit is 0.05%. Commercial drivers are generally held to 0.04%, and drivers under 21 face zero-tolerance laws. State law varies, and a driver can be charged for impairment below any of these numbers.
Does body weight change how alcohol affects you?
Yes. The same amount of alcohol is distributed through body water, so a smaller person reaches a higher concentration from the same number of drinks. Total body water also averages lower in women than in men, which is why the calculation uses different fractions.
Is regularly reaching a high BAC a sign of a problem?
Repeated heavy drinking episodes are one of the patterns clinicians look at, but a diagnosis of alcohol use disorder is based on criteria such as loss of control, cravings, tolerance, withdrawal, and continued drinking despite harm — not on a number from a calculator. If stopping alcohol after regular heavy drinking causes shaking, sweating, nausea, or confusion, that is a medical situation: alcohol withdrawal can become life-threatening and should be managed with a clinician.
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