Blood Alcohol Content (BAC) Calculator
Quick Answer
Blood Alcohol Content (BAC) measures the percentage of alcohol in your bloodstream; 0.08% is the legal driving limit in most US states per the NHTSA, and 0.05% in Utah and most European countries. A 180-lb male drinking 2 standard drinks in 1 hour reaches approximately 0.04% BAC using the Widmark formula.
Also searched as: BAC calculator, alcohol level calculator, drinks to BAC, DUI calculator
Estimated BAC
—
US legal driving status (0.08%)
—
Hours until BAC = 0
—
Standard drinks consumed
—
How BAC Works
Blood Alcohol Content is the proportion of alcohol, by volume or mass, present in a person's bloodstream at a given moment. When you drink, alcohol passes through the stomach and small intestine into the blood, reaching peak concentration 30–90 minutes after the last drink. The liver then metabolizes it at a roughly fixed rate of about one standard drink per hour, lowering BAC by approximately 0.015% per hour regardless of how much is in your system, according to the US National Highway Traffic Safety Administration. Because the decline is constant, no amount of coffee, water, or fresh air speeds it up.
In the United States, driving with a BAC of 0.08% or higher is illegal in every state except Utah, where the limit is 0.05%. Commercial drivers face a stricter 0.04% limit, and drivers under 21 are subject to zero-tolerance laws that ban any measurable alcohol. According to the Centers for Disease Control and Prevention, alcohol-impaired crashes killed 13,524 people in the US in 2022 — one fatality every 39 minutes. Impairment begins well below the legal limit: at 0.05% BAC, reaction time slows by about 8% and crash risk doubles. See also our BMI calculator and calorie calculator.
The Widmark Formula
The Widmark formula, introduced in 1932 and still the basis of forensic BAC work, is: BAC (%) = (A × 5.14 ÷ (W × r)) − 0.015 × h, where A is ounces of pure alcohol consumed, W is body weight in pounds, r is the alcohol distribution ratio (0.73 for males, 0.66 for females), and h is hours since the first drink. One 12-oz beer at 5% ABV contains 0.6 oz of pure alcohol, the same as a 5-oz glass of 12% wine or a 1.5-oz shot of 40% spirits — the US NIAAA definition of a standard drink. Plugging in a 180-lb male who has had 2 beers (1.2 oz alcohol) over 1 hour: BAC = (1.2 × 5.14 ÷ (180 × 0.73)) − 0.015 ≈ 0.047% − 0.015 ≈ 0.032%. Real BAC varies with food, genetics, and health.
Key Terms You Should Know
Standard drink (US): any drink containing about 14 grams (0.6 oz) of pure alcohol. ABV: alcohol by volume, the percentage of alcohol in a beverage. Widmark r factor: the fraction of body weight that contains water and thus distributes alcohol (0.73 male, 0.66 female). Metabolism rate: the speed at which the liver processes alcohol, typically 0.015% BAC per hour. Per-se limit: a BAC level at which driving is legally impaired regardless of observed behavior. Zero tolerance: laws that make any detectable BAC illegal for drivers under 21 in the US.
BAC Effects and Legal Limits — Reference Data
The table below shows typical physiological effects and legal status at different BAC levels, compiled from NHTSA, CDC, and NIAAA materials. Impairment is measurable at 0.02% and becomes severe well before the 0.08% per-se limit. Most European countries set their driving limit at 0.05%, and some (Norway, Sweden, Poland) at 0.02%.
| BAC | Typical Effect | US Legal Status |
|---|---|---|
| 0.02% | Relaxation, slight mood change | Legal (underage: illegal) |
| 0.05% | Lowered alertness, impaired judgment | Illegal in Utah |
| 0.08% | Impaired muscle coordination, reaction time | Illegal to drive (49 states) |
| 0.10% | Clear slurring of speech, poor balance | Illegal |
| 0.15% | Major loss of balance, vomiting likely | Aggravated DUI in many states |
| 0.30% | Stupor, loss of consciousness | Illegal, medically dangerous |
| 0.40%+ | Coma, potentially fatal | Medical emergency |
Practical Examples
Example 1 — Happy hour. A 160-lb female has 2 glasses of wine in 1 hour. Alcohol consumed = 2 × 0.6 = 1.2 oz. BAC = (1.2 × 5.14 ÷ (160 × 0.66)) − 0.015 ≈ 0.058% − 0.015 ≈ 0.043%. She is under the US 0.08% limit but impaired; reaction time is measurably slower. Example 2 — Dinner out. A 200-lb male has 1 beer, 1 glass of wine, and 1 shot over 3 hours (1.8 oz alcohol). BAC = (1.8 × 5.14 ÷ (200 × 0.73)) − 0.045 ≈ 0.063% − 0.045 ≈ 0.018%. Still measurable, still not a safe level to drive. Example 3 — Time to sober up. A 180-lb male at 0.09% BAC: (0.09 ÷ 0.015) = 6 hours until his liver has fully cleared the alcohol. No shortcut exists.
Tips and Best Practices
Never use BAC calculators as a green light to drive. They are educational estimates only, not legal evidence, and real BAC can be higher than predicted. Alternate with water. Hydration slows absorption slightly and reduces hangover severity, per NIAAA guidance. Eat before and during drinking. Food in the stomach slows alcohol transfer into the blood, lowering peak BAC by 20–30%. Pace yourself. One standard drink per hour roughly matches the liver's metabolism rate and keeps BAC from climbing rapidly. Plan transportation before you drink. Decide on a designated driver, rideshare, or taxi at the start of the night, not after you have had several drinks.
Frequently Asked Questions
What is BAC?
Blood Alcohol Content, or BAC, is the percentage of alcohol in a person's bloodstream. A BAC of 0.08% means 0.08 grams of alcohol per 100 milliliters of blood. The US National Highway Traffic Safety Administration (NHTSA) reports that 0.08% is the illegal per-se driving limit in every US state except Utah, where the limit is 0.05%. BAC rises with consumption and declines over time as the liver metabolizes alcohol at about 0.015% per hour.
What is the Widmark formula?
The Widmark formula, published by Swedish physician Erik Widmark in 1932, estimates BAC from body weight, alcohol consumed, and time. It is: BAC = (alcohol grams ÷ (body weight grams × r)) × 100 − (0.015 × hours), where r is the distribution ratio (0.73 for men, 0.66 for women). The formula is still the basis for most forensic and educational BAC estimates and is cited in the US Department of Justice's driving-under-the-influence training materials.
How long does alcohol stay in your system?
The liver metabolizes alcohol at a roughly constant rate of about 0.015% BAC per hour, according to the NHTSA. That means a BAC of 0.08% takes about 5 hours and 20 minutes to return to zero, and a BAC of 0.12% takes about 8 hours. Unlike caffeine, alcohol cannot be sped up by coffee, cold showers, or exercise. Only time lowers BAC.
What is considered one standard drink?
The US definition of a standard drink is 14 grams (about 0.6 fluid ounces) of pure alcohol, which is the amount contained in a 12-ounce beer at 5% alcohol, a 5-ounce glass of wine at 12%, or a 1.5-ounce shot of distilled spirits at 40%. These are the figures published by the US National Institute on Alcohol Abuse and Alcoholism. The UK uses a smaller 8-gram 'unit,' and Australia uses a 10-gram standard drink.
Why do men and women have different BACs?
For the same weight and the same number of drinks, women typically reach a higher BAC than men because they have a smaller volume of water in their bodies to dilute the alcohol, and lower levels of the stomach enzyme alcohol dehydrogenase. The Widmark distribution ratio r is 0.73 for men and 0.66 for women, reflecting this difference. Hormones, body composition, and food also affect absorption, which is why BAC calculators can only provide rough estimates.
Does food affect BAC?
Yes. Drinking on a full stomach slows alcohol absorption and lowers peak BAC by roughly 20 to 30 percent compared with drinking on an empty stomach, according to studies cited by the NIAAA. Food delays the transfer of alcohol from the stomach to the small intestine, where most absorption happens. However, food does not change the total amount of alcohol absorbed, only the speed. Eventually BAC converges and the same number of drinks reach the same peak plateau.