BAC calculator

Read this first. The number above is a rough estimate from a textbook formula of 1932 and a few averages. Real blood alcohol depends on food, medicines, how fast you drank, your metabolism and how big the pours really were, and it can be well above or below this range. It must never decide whether it is safe to drive. The only safe amount to drink before driving is none: if you are drinking, do not drive, and arrange a ride before the first drink.

What the estimate is and how it works

Blood alcohol concentration (BAC) is the weight of alcohol in a volume of blood, in the United States grams per deciliter, written as a percent: 0.08% means 0.08 g of alcohol in 100 mL of blood. The Widmark equation estimates it from three things: how much alcohol went in, how much body it spread through and how long the liver has had to clear it. The formula is C = A ÷ (r × W) − β × t. A is grams of pure alcohol, W is body weight in kilograms, r is the distribution factor, β is the elimination rate in percent per hour and t is hours since the first drink. The result cannot go below zero.

The midpoint uses Widmark's own factors and the 0.015% an hour that Jones's 2010 review calls a good average for moderate drinkers. The range pairs the lowest r with the slowest elimination for the top and the highest r with the fastest for the bottom. The men's r range runs from Widmark's 0.68 − 0.17 up to the highest value Seidl and colleagues measured, and the women's from 0.55 − 0.11 up to their 0.80.

Worked example

A 160-lb man (72.6 kg) drinks three 12-fl-oz beers at 5% over two hours. Each beer is one standard drink, so A = 3 × 14 = 42 g. If all of it were absorbed and none cleared, C = 42 ÷ (0.68 × 72.6 × 10) = 0.085%. Two hours of elimination take off 2 × 0.015 = 0.030, leaving 0.055% as the midpoint. The range is 0.027% to 0.093%. The same drinks reach zero 5.7 hours after the first one at the midpoint, between 3.3 and 11.3 hours across the range. The pounds-and-ounces form gives 0.055% for the midpoint, the same figure to rounding.

A second case shows the sex difference. A 140-lb woman has two standard drinks in an hour: midpoint 0.065%, range 0.035% to 0.090%. A man of the same weight with the same drinks gets 0.050% at the midpoint, because his r is 0.68 instead of 0.55.

Limits of this method

Absorption. The equation assumes the alcohol is completely absorbed. A slide set by the forensic toxicologist Ashraf Mozayani, hosted by NIST, notes that Widmark calculations rest on absorption being complete between 30 minutes and 2 hours after drinking, and are little help within the first hour. The hour-by-hour table above starts from the peak at hour 0 for that reason. In practice BAC rises during the first hour or more, and faster on an empty stomach.

Spread of r. The factor stands for the ratio of body water to blood water. Seidl and colleagues found it varies from 0.44 to 0.80 in women and from 0.60 to 0.87 in men, so two people of the same sex and weight can differ widely. The Washington State Patrol breath-test section writes that, even when a BAC has been measured, a better estimate of the two-standard-deviation error in the number of drinks back-calculated from it is close to 20%. A forward estimate like this one has no measurement to lean on and is looser still.

Elimination. The ordinary range is 10 to 20 mg/100 mL an hour, while Jones reports 25 to 35 in alcoholics during detoxification and 10 to 35 as the physiological extreme. The line in the table is straight for everyone; real curves are not.

Counting drinks. Pours are rarely standard. NIAAA says the standard drink amounts are helpful for health guidelines but may not reflect customary serving sizes, and a 16-oz craft beer at 7% is 1.9 standard drinks. NHTSA's website describes a standard drink as about 0.54 ounces of alcohol; this calculator follows NIAAA's 0.6 fl oz (14 g). Using 0.54 would lower the amount of alcohol, and the peak, by about 10%, which is small next to the spread above. Use the standard drink calculator to count a specific glass.

Common mistakes

Treating a midpoint as a measurement is the biggest one; the tool gives a range because no one can pin the number down without a breath or blood test. Others: counting from the last drink instead of the first, forgetting that a double pour is two drinks, assuming food lowers BAC (it slows absorption, but the liver clears alcohol at its own pace), and reading "below the limit" as "safe". NHTSA notes that even a small amount of alcohol can affect driving ability, and that drivers at 0.08 are about four times more likely to crash than drivers at zero.

Can I use a BAC calculator to decide whether I can drive?

No. This is a rough estimate, and the real number can be far from it. NHTSA says that because so many factors affect blood alcohol concentration, it is very difficult to assess your own BAC or impairment, and that a person will likely be too impaired to drive before looking, or even feeling, drunk. NHTSA's advice is plain: if you are drinking, do not drive. Plan a sober ride before the first drink.

How is blood alcohol concentration estimated?

With the Widmark equation, first published in 1932 by the Swedish physician E. M. P. Widmark: BAC = A ÷ (r × W) − β × t. A is the grams of pure alcohol, W is body weight, r is a distribution factor that stands for how much of the body the alcohol spreads into (average 0.68 for men and 0.55 for women), β is how fast the body clears alcohol (0.015% per hour here) and t is the time since the first drink. In the pounds-and-ounces form you often see online, BAC = A × 5.14 ÷ (W × r) − 0.015 × t, with A in fluid ounces of pure alcohol and W in pounds; the 5.14 is just the unit conversion. This calculator works in grams and kilograms, and takes one US standard drink as 14 g (0.6 fl oz) of pure alcohol, the NIAAA definition.

Why does the calculator show a range instead of one number?

Because the inputs are averages. The Widmark factor r is given as 0.68 ± 0.17 for men and 0.55 ± 0.11 for women in slides by the forensic toxicologist Ashraf Mozayani hosted by NIST, and a 2000 study of 273 men and 256 women (Seidl and colleagues) measured the underlying body-water ratio from 0.60 to 0.87 in men and from 0.44 to 0.80 in women. Elimination runs about 10 to 15 mg/100 mL an hour on an empty stomach and 15 to 20 after eating, according to a review by Jones (2010). The low end of the range here uses the highest r and 0.020% an hour, the high end the lowest r and 0.010% an hour. For the example below that is 0.03% to 0.09%, a range wide enough to contain both a person who would feel fine and one who is impaired.

How long until my BAC is back to 0.00?

Divide the peak by the elimination rate. In the example below the peak is 0.085%, so at 0.015% an hour it takes 5.7 hours from the first drink; with the slowest and fastest assumptions it is 3.3 to 11.3 hours. Nothing speeds this up: the liver clears alcohol at a roughly steady pace, which is why the estimate is a straight line downward. The estimate cannot tell you when it is safe to drive: if you may need to drive, plan not to drink at all.

How many drinks does it take to reach 0.08?

NIAAA defines binge drinking as a pattern that brings blood alcohol concentration to 0.08% or higher, and says that for a typical adult this takes about five drinks for a man or four for a woman in about two hours. Smaller people reach it with fewer drinks, and NIAAA says youth reach the same level with three to five drinks. That is a statement about a typical adult, not about you, and 0.08% is the legal driving limit in every state except Utah, where it is 0.05% (NHTSA, read October 27, 2026). Crash risk is already raised well below those limits.

What are the legal limits?

NHTSA states that it is illegal in all 50 states, the District of Columbia and Puerto Rico to drive with a BAC of 0.08 or higher, except in Utah, where the limit is 0.05. Drivers of commercial motor vehicles are barred from safety-sensitive duty at 0.04 or greater (49 CFR 382.201). Federal regulations on zero-tolerance laws (23 CFR 1210.4) call for a state law that covers everyone under 21, sets a limit no higher than 0.02 and makes exceeding it an offense in itself; many states use 0.00, so check yours. A driver can also be arrested below these numbers if the officer has probable cause, and NHTSA counted 2,028 deaths in 2024 in crashes where a driver's BAC was between 0.01 and 0.07.

Do food, medicines or sex change the result?

Yes, and only some of it is in the formula. Sex and weight are: women generally have less body water and more body fat per pound, so the same drinks leave more alcohol in the blood (NHTSA). Food is not: eating slows absorption, and drinking quickly gives a higher BAC than drinking the same amount over a longer time (NHTSA). Medicines do not change your BAC, according to NHTSA's brochure The ABCs of BAC, but alcohol mixed with certain medicines can make you more impaired at the same number. The calculator knows none of this.

Related: the standard drink calculator counts drinks and grams of alcohol, the calorie calculator helps you see what alcohol adds to a day's energy, and the water intake calculator covers fluid needs.

For general information only — not medical or legal advice, and not a way to judge fitness to drive. If you are worried about your drinking, talk with a doctor. Sources, read October 27, 2026: NHTSA, Drunk Driving and The ABCs of BAC (DOT HS 809 844, 2005); NIAAA, What Is A Standard Drink? and Understanding Binge Drinking; A. Mozayani, slides on estimating blood alcohol with the Widmark formula (NIST); Washington State Patrol, Widmark's Equation (2002); Jones AW, Forensic Sci Int 2010;200:1–20; Seidl S, Jensen U, Alt A, Int J Legal Med 2000;114:71–77; 49 CFR 382.201; 23 CFR 1210.4. Laws change, so check your state's current limits. How we check our calculators.