BAC Calculator

BAC calculator at a glance#

How is blood alcohol content calculated?#

The calculator uses the Widmark formula. It works out the grams of pure alcohol in your drinks, divides by your body weight times a body water factor (0.68 for men, 0.55 for women), and then subtracts 0.015% for every hour since your first drink. The result is an estimate in percent, the same unit as a legal limit like 0.08%.

How long does it take for BAC to go back to zero?#

The calculator assumes your body clears 0.015% per hour, the average rate, so a BAC of 0.051% takes about 3.4 hours to reach zero. Real rates in most people run from 0.010% to 0.025% per hour, so the true time can be shorter or much longer.

What BAC is legal for driving?#

In every US state it's illegal to drive with a BAC of 0.08% or higher, and Utah's limit is 0.05%. Commercial drivers and drivers under 21 face lower limits, and a driver can be arrested below 0.08% when their behavior shows impairment. Never use this calculator to decide whether you can drive.

This gives an estimate, not medical advice. A doctor can tell you what the number means for you. If you've been drinking, don't drive. Plan a ride home before you start.

What BAC means#

Blood alcohol content (BAC), also called blood alcohol concentration, is the amount of alcohol in your blood. In the US it's written as a percent: 0.08% means 0.08 grams of alcohol in every 100 milliliters (one deciliter) of blood.

BAC depends on more than the number of drinks. The National Highway Traffic Safety Administration lists five things that change how high it goes: how much you drink, how fast you drink, your sex, your body weight, and whether you've eaten. Women generally have less body water and more fat per pound than men, and alcohol doesn't go into fat easily, so more of it stays in the blood. That's also why body composition matters, which the body fat percentage calculator estimates.

One example runs through this page: a woman who weighs 140 lb (63.5 kg) has two 5 oz (150 mL) glasses of 12% wine, and 2 hours have passed since her first sip. The calculator puts her at about 0.051%.

The Widmark formula#

Forensic toxicology guidelines from the Organization of Scientific Area Committees for Forensic Science, hosted by NIST, give Widmark's formula as alcohol concentration = dose ÷ (volume of distribution × body weight). In the form this calculator uses:

• Grams of alcohol = volume in mL × ABV ÷ 100 × 0.789 (the density of alcohol in grams per mL).
• Peak BAC (%) = grams of alcohol ÷ (body weight in kg × r) ÷ 10, where r is 0.68 for men and 0.55 for women.
• BAC now (%) = peak BAC − 0.015 × hours since the first drink.
• Hours to zero = BAC now ÷ 0.015.

Count the hours from your first drink, not your last. If you need the gap between two clock times, the time duration calculator works it out.

Worked through for the example:

1. Alcohol: 2 × 150 mL × 12 ÷ 100 = 36 mL of pure alcohol, and 36 × 0.789 = 28.4 grams.
2. Body water: 63.5 kg × 0.55 = 34.9.
3. Peak: 28.4 ÷ 34.9 ÷ 10 = 0.081%.
4. After 2 hours: 0.081 − 0.015 × 2 = 0.051%.
5. Hours to zero: 0.051 ÷ 0.015 = 3.4 hours.

The percentage calculator can do the first step if you want to check the alcohol in a drink: 12% of 150 mL is 18 mL per glass.

What the estimate leaves out#

The formula treats everyone of one sex as the same and every hour as the same. Real bodies differ, and the NIST guidelines say a single body water factor isn't appropriate: they use a range of 0.40 to 0.80 L/kg instead. That one change can move an estimate a long way in either direction.

Absorption isn't instant either. The calculator counts all your drinks as if they hit your blood at once. In reality, peak BAC comes anywhere from 5 to 120 minutes after you stop drinking, about 60 minutes on average, and food in your stomach slows absorption. NIAAA, the National Institute on Alcohol Abuse and Alcoholism, warns that BAC can keep rising after someone stops drinking, even if they're asleep or passed out.

Elimination varies too. The 0.015% per hour the calculator uses is an average, and most people fall between 0.010% and 0.025% per hour. NIAAA notes that cold showers, hot coffee and walking don't reverse the effects of an alcohol overdose. Your BAC falls only as your body clears the alcohol over time.

The calculator's 12 oz options count 355 mL, the size of a US 12 oz can or bottle. Its other drink sizes pair ounce and milliliter labels that don't match exactly, such as 16 oz with 500 mL, and it uses the milliliter value.

Standard drinks#

In the US, a standard drink holds 14 grams of pure alcohol (0.6 fl oz), according to the NIAAA standard drink guide. That's the amount in a 12 oz beer at 5%, a 5 oz glass of wine at 12%, or a 1.5 oz shot of 40% spirits. A larger pour or a stronger drink counts as more than one: a 12 oz beer at 10% is two standard drinks.

Estimated BAC by body weight#

These tables use the calculator's formula with 14 grams of alcohol per standard drink, one hour after the first drink. They're averages for illustration. Your own BAC can be much higher than the table shows.

For men:

For women:

A number under 0.08% here doesn't mean it's safe or legal to drive.

Legal limits and what BAC does to driving#

The legal limit is not a safe limit. NHTSA's guide to BAC lists effects on driving that start well below 0.08%:

Limits by driver in the US:

• All drivers: every state makes it illegal to drive at 0.08% or higher, and Utah has set its limit at 0.05% since December 30, 2018.
• Drivers under 21: under federal rules, a state keeps its full highway funding only if its law treats a driver under 21 with a BAC of 0.02% or more as driving under the influence. A state can set a lower limit but not a higher one.
• Commercial drivers: federal rules bar truck and bus drivers from safety-sensitive work at an alcohol concentration of 0.04 or more.
• Any driver: you can be arrested below 0.08% when your behavior gives an officer probable cause.

Limits are different in other countries, so check the local law wherever you drive. The safest number for driving is zero.

Sources#

• National Highway Traffic Safety Administration, The ABCs of BAC (DOT HS 809 844)
• National Institute on Alcohol Abuse and Alcoholism, What Is A Standard Drink?
• OSAC 2020-S-0003, Guidelines for Performing Alcohol Calculations in Forensic Toxicology (NIST)

Frequently asked questions#

Is this BAC calculator accurate?#

It gives an average estimate, not a measurement. It can't see your real body water, how fast you absorbed the drinks, or how fast your liver clears alcohol, and each of those can shift the result by a lot. Only a breath or blood test measures BAC.

Can I use the result to decide whether to drive?#

No. Your real BAC can be higher than any calculator shows, and impairment starts below the legal limit. If you've been drinking, use a designated driver, a taxi or a ride service, or stay where you are.

What does the time to reach 0% mean?#

It's your current estimate divided by 0.015% per hour. For the example, 0.051% gives about 3.4 hours. At the slow end of the normal range, 0.010% per hour, the same BAC would take about 5.1 hours, so the real wait can be much longer than the calculator says.

Why do women reach a higher BAC than men from the same drinks?#

On average, women have less body water and more body fat per pound. Alcohol spreads through body water, so the same amount ends up more concentrated. That's why the calculator uses 0.55 for women and 0.68 for men.

Does food lower my BAC?#

Food in your stomach slows absorption, so your BAC tends to rise more slowly and peak lower. It doesn't remove alcohol, and the calculator doesn't account for food at all.

Does a breathalyzer measure the same thing?#

A breath test measures the alcohol in your breath, which tracks the alcohol in your blood. A calculator only estimates from what you tell it, so it can't replace a breath or blood test.