Alcohol Units Calculator

What counts as one standard drink

A standard drink is a measuring unit, not a serving size. In the United States it is any drink that contains 0.6 fluid ounces of pure alcohol, which is 14 grams. The National Institute on Alcohol Abuse and Alcoholism sets that definition and uses it for every guideline that follows.

Three ordinary pours happen to land on the same number:

• 12 fluid ounces of regular beer at 5% alcohol by volume
• 5 fluid ounces of table wine at 12% alcohol by volume
• 1.5 fluid ounces of 80-proof distilled spirits at 40% alcohol by volume

The formula behind those three is simple: ounces of drink times the alcohol percentage equals ounces of pure alcohol. A 12 ounce beer at 5% gives 0.6 ounces. A 5 ounce glass of wine at 12% gives 0.6 ounces. Divide by 0.6 and you have the count of standard drinks.

The catch is that real pours rarely match the reference. A pint glass holds 16 ounces, not 12. A restaurant wine pour is often 6 or 9 ounces. A mixed drink can carry 2 or 3 ounces of spirits. The label on the bottle gives you the percentage, and the glass in your hand gives you the volume, so the arithmetic is available even when the pour is generous.

Converting what you pour into standard drinks

The table converts common American servings into standard drinks. The number in the last column is what counts toward a weekly total, not the number of glasses.

A pint of 7% craft beer is nearly two standard drinks, so four pints is closer to seven and a half drinks than to four. A 40 ounce bottle of malt liquor is four drinks in one container. A margarita served in a 10 ounce glass at 12% is two drinks, and a second one puts the evening at four.

Bottles are the other place counts drift. A 750 milliliter bottle of 12% wine holds 90 milliliters of alcohol, which is 3.04 fluid ounces, or 5.07 standard drinks. Two people splitting that bottle have 2.5 drinks each, not one. A fifth of 40% spirits is 750 milliliters and holds 6.76 standard drinks, so a cocktail with 2 ounces of whiskey is 1.33 drinks before the mixer is counted.

Estimating blood alcohol from weight, drinks and time

Blood alcohol concentration, or BAC, is the weight of alcohol per volume of blood, written as grams per deciliter. A BAC of 0.08 means 0.08 grams of alcohol in 100 milliliters of blood. The estimate on this page uses the Widmark formula, which has been the standard back of the envelope method for close to a century.

The formula has three inputs: the ounces of pure alcohol consumed, your body weight in pounds, and a distribution factor that differs by sex. Men use 0.73 and women use 0.66, because body water dilutes alcohol and average body composition differs. The constant 5.14 converts ounces and pounds into the BAC scale.

BAC = (ounces of alcohol x 5.14) / (body weight x distribution factor) - (0.015 x hours)

The last term is the part people forget. The liver clears roughly 0.015 of BAC per hour, and that rate does not speed up with coffee, food, a cold shower or a run around the block. Time is the only thing that removes alcohol, and it removes it at a fixed pace.

Two limits on the math: the formula assumes alcohol spread evenly through body water, and it ignores how fast you drank. Five drinks in one hour and five drinks over five hours give the same peak estimate at the same elapsed time, which is wrong in practice. Treat the output as a rough estimate with a margin of error near 0.02 either way.

Worked example: three beers in two hours

Take a 180 pound man who drinks three 12 ounce beers at 5% over two hours.

Each beer carries 0.6 ounces of pure alcohol, so the total is 1.8 ounces, which is 3.0 standard drinks. The Widmark side is 1.8 times 5.14, or 9.252, divided by 180 times 0.73, or 131.4. That gives 0.0704. Subtract 0.015 for each of the two hours, or 0.030, and the estimate is 0.040.

At 0.040 he is at half the 0.08 legal limit, and the estimate clears to zero in about 2.7 hours. Change one input and the answer moves a lot. The same three beers in one hour instead of two gives 0.055. The same three beers for a 140 pound woman gives 0.070, which is 74% higher than the 180 pound man.

That spread is the reason a count of drinks alone says little. Two people can leave the same table with the same number of empties and widely different BAC readings, because weight, sex and the clock all move the number.

The 0.08 limit arrives sooner than most people expect

Every state except Utah sets the illegal per se limit at 0.08 grams per deciliter. Utah uses 0.05. The National Highway Traffic Safety Administration notes that crash risk rises sharply at 0.08 and that impairment starts well below it, with measurable losses in tracking and attention at 0.05 and some at 0.02.

Read the 160 pound row: four drinks in two hours is 0.076, just under the line, and five drinks is 0.102, over it. A 120 pound man passes 0.08 at four drinks. A 200 pound man needs six. For women the same drinks produce higher readings, because the 0.66 factor raises the result: a 140 pound woman at four drinks in two hours is at 0.104.

Food slows absorption and can lower a peak, but it does not change the total amount of alcohol in the body or the rate at which the liver clears it. That is why a big dinner makes someone feel steadier without making them legal to drive.

Why the estimate misses: food, genetics, medication

The Widmark formula is a population average applied to one person, and several factors push the real number up or down.

• Body composition. Muscle holds water and fat does not, so two people at the same weight can read differently. A lean 180 pounds and a 180 pounds with more body fat do not clear alcohol at the same rate.
• Drinking speed. A rapid rise in BAC is more impairing than the same peak reached slowly, and the formula does not model the curve.
• Food. A full stomach slows absorption and can cut the peak, which is why the estimate tends to run high on an empty stomach and low after a large meal.
• Medication. Sedatives, antihistamines, sleep aids and some pain relievers add to impairment without changing BAC at all.
• Liver health and genetics. Enzyme variants change how fast alcohol is broken down, and liver conditions change it more.
• Hydration and altitude. Dehydration concentrates the reading, and altitude changes how the body handles alcohol.

For all of those reasons, no calculator can tell you whether you are under the limit for legal purposes. A breath test measures what it measures, and a court uses that number, not an estimate from a web page. The calculator is useful for seeing how fast a night adds up, not for deciding whether to drive.

Weekly totals: what 15 drinks looks like

NIAAA defines heavy drinking as 5 or more drinks on any day or 15 or more per week for men, and 4 or more on any day or 8 or more per week for women. Binge drinking is the same daily threshold reached within about two hours, which is the pattern that pushes BAC to 0.08 or above.

The last row is the one that hides. Twelve drinks spread over two nights sits under the men's weekly mark, but both nights clear the five-drink daily threshold, so the pattern is heavy drinking by the NIAAA definition even though the weekly total looks modest.

Count in standard drinks, not glasses. A four pint evening of 7% craft beer is 7.5 standard drinks, which passes the daily mark on its own. A bottle of wine shared over dinner is 2.5 drinks each, and two of those evenings a week is 5 drinks a week from wine alone.

When to stop counting and call a ride

A calculator is a planning tool, not a decision tool. The practical rules are simpler than the formula.

• If you have been drinking, do not drive. A 0.040 estimate is impairment, not safety, and the estimate can be wrong by 0.02 in either direction.
• Plan the ride before the first drink, because the decision is worse after four.
• Watch the pace, not just the total. Three drinks in one hour is a different risk from three drinks over three hours.
• Never let a number on a phone settle an argument about who drives.

For anyone who wants to check their own pattern, NIAAA publishes a standard drink guide with pictures of common servings, and the NHTSA impaired driving page lists how BAC levels map to driving effects. The NIAAA definition of heavy drinking is the source for the weekly thresholds above.

This is an educational estimate, not medical or legal advice. Alcohol affects people differently, and only a breath or blood test measures BAC. If you or someone else may have alcohol poisoning, call 911.