🍺 Blood Alcohol Content (BAC) Calculator
By ToolNimba Health Team · Reviewed by ToolNimba Editorial Review, consumer health content · Updated 2026-06-19
This calculator gives a rough estimate only and is not a medical or legal measurement. Real blood alcohol content depends on many factors this tool cannot capture, including food in your stomach, medication, metabolism, body composition, drink strength, and how fast you drank. Never use this estimate to decide whether you are safe to drive or operate machinery. If you have been drinking, do not drive. This is not medical advice.
One US standard drink = 14 grams of pure alcohol (a 12 oz beer at 5%, a 5 oz glass of wine at 12%, or a 1.5 oz shot at 40%).
This blood alcohol calculator estimates your BAC, the percentage of alcohol in your bloodstream, using the well known Widmark formula. Enter your sex, body weight, the number of US standard drinks you have had, and how many hours have passed since your first drink. The result is a single estimate to help you understand roughly how alcohol adds up. It is a learning tool, not a breathalyzer, and it must never be used to judge whether you are fit to drive.
What is the Blood Alcohol Calculator?
Blood alcohol content (BAC) is the proportion of alcohol in your blood, written as a percentage. A BAC of 0.08% means 0.08 grams of alcohol per 100 millilitres of blood, which is the legal driving limit in many places. BAC rises as you drink and falls as your liver breaks the alcohol down at a fairly steady rate, roughly 0.015% per hour for most adults.
The Widmark formula is the classic way to estimate BAC. It divides the grams of pure alcohol you have consumed by your body water (your weight multiplied by a distribution ratio r), converts that to a percentage, then subtracts the alcohol your body has already cleared based on the hours elapsed. The ratio r reflects the share of the body that is water: it is taken as about 0.68 for men and 0.55 for women on average, because body composition differs between the sexes. The grams of alcohol come from your drink count, where one US standard drink contains 14 grams of pure alcohol.
The big caveat is that Widmark is a population average, not a personal reading. It assumes one fixed clearance rate and one body water ratio, but real people vary widely. Eating before or while drinking slows absorption, some medications interact with alcohol, and metabolism, hydration, and body fat all shift the true figure. Two people of the same weight and sex drinking the same amount can land at noticeably different real BAC values. That is exactly why the only safe rule after drinking is to not drive at all.
When to use it
- Understanding roughly how several drinks over an evening can add up in your bloodstream.
- Seeing how body weight and sex change the estimated effect of the same number of drinks.
- Learning how the body clears alcohol over time at about 0.015% per hour.
- Teaching or studying the Widmark formula in a health, biology, or driver education class.
How to use the Blood Alcohol Calculator
- Select your sex, which sets the body water ratio used in the formula.
- Choose your weight unit, then enter your body weight.
- Enter the number of US standard drinks you have consumed (each is 14 grams of alcohol).
- Enter how many hours have passed since your first drink.
- Read the estimated BAC and the impairment band, remembering it is only a rough guide.
Formula & method
Worked examples
A man weighing 80 kg has had 3 standard drinks, and 2 hours have passed since the first.
- Alcohol grams = 3 × 14 = 42 g
- Weight in grams = 80 × 1000 = 80,000 g, and r = 0.68 for men
- Body water term = 80,000 × 0.68 = 54,400
- Raw fraction = 42 ÷ 54,400 = 0.00077206
- As a percent = 0.00077206 × 100 = 0.077206%
- Subtract clearance = 0.077206 − (0.015 × 2) = 0.077206 − 0.03 = 0.047206
Result: Estimated BAC ≈ 0.047%
A woman weighing 60 kg has had 2 standard drinks, and 1 hour has passed since the first.
- Alcohol grams = 2 × 14 = 28 g
- Weight in grams = 60 × 1000 = 60,000 g, and r = 0.55 for women
- Body water term = 60,000 × 0.55 = 33,000
- Raw fraction = 28 ÷ 33,000 = 0.00084848
- As a percent = 0.00084848 × 100 = 0.084848%
- Subtract clearance = 0.084848 − (0.015 × 1) = 0.084848 − 0.015 = 0.069848
Result: Estimated BAC ≈ 0.070%
Estimated BAC for a 70 kg (154 lb) man, 1 hour after the first drink
| Standard drinks | Alcohol (grams) | Estimated BAC |
|---|---|---|
| 1 | 14 g | 0.014% |
| 2 | 28 g | 0.044% |
| 3 | 42 g | 0.073% |
| 4 | 56 g | 0.103% |
| 5 | 70 g | 0.132% |
General impairment guide by BAC band (effects vary widely between people)
| BAC range | Typical effects |
|---|---|
| 0.01% to 0.03% | Little to no obvious effect for most people |
| 0.03% to 0.08% | Reduced judgment, coordination, and reaction time |
| 0.08% to 0.15% | At or above the common legal driving limit, clear impairment |
| 0.15% to 0.30% | Heavy impairment, confusion, possible nausea |
| 0.30% and above | Potentially life threatening, risk of unconsciousness |
Common mistakes to avoid
- Treating the estimate as a real reading. The Widmark formula is a population average, not a breathalyzer. Your true BAC depends on food, medication, metabolism, and body composition, so the number here can be well off for any individual.
- Miscounting standard drinks. A large cocktail, a strong craft beer, or a generous pour can each be two or more standard drinks. One US standard drink is 14 grams of alcohol, so heavier pours add up faster than the count suggests.
- Assuming coffee or a shower lowers BAC. Only time clears alcohol from your blood, at roughly 0.015% per hour. Coffee, cold showers, and food after the fact do not speed up the liver, they just may make you feel more alert while still impaired.
- Using the result to decide whether to drive. No estimate is accurate enough to clear you to drive. If you have been drinking, the only safe choice is not to drive, regardless of what any calculator shows.
Glossary
- BAC
- Blood alcohol content, the percentage of alcohol in your blood, for example 0.08%.
- Standard drink
- A defined amount of alcohol. In the United States one standard drink contains 14 grams of pure alcohol.
- Widmark formula
- A classic equation that estimates BAC from alcohol consumed, body weight, sex, and time elapsed.
- Distribution ratio (r)
- The fraction of body weight that is water, taken as about 0.68 for men and 0.55 for women.
- Elimination rate
- How fast the body clears alcohol, on average about 0.015% BAC per hour.
Frequently asked questions
How is BAC calculated?
This tool uses the Widmark formula: BAC% = (grams of alcohol ÷ (body weight in grams × r)) × 100 minus 0.015 times the hours elapsed. The grams of alcohol come from your drink count (14 grams per US standard drink), and r is 0.68 for men or 0.55 for women.
Is this BAC calculator accurate?
No calculator can give an accurate personal BAC. The Widmark formula is a population average that ignores food, medication, metabolism, drink strength, and body composition. Treat the result as a rough learning estimate only, never as a real measurement.
Can I use this to decide if I am safe to drive?
Never. The estimate is not precise enough to clear anyone to drive, and your true BAC could be much higher than shown. If you have been drinking, do not drive. Use a taxi, rideshare, public transport, or a sober driver instead.
How long does it take for BAC to reach zero?
The body clears alcohol at roughly 0.015% per hour, so a BAC of 0.06% would take about four hours to fall to zero. This rate varies between people, and nothing but time reliably lowers it.
What counts as one standard drink?
In the United States one standard drink is 14 grams of pure alcohol, which is about a 12 oz beer at 5%, a 5 oz glass of wine at 12%, or a 1.5 oz shot of 40% spirits. Larger or stronger servings count as more than one.
Why does sex change the result?
The formula uses a body water ratio that differs by sex on average, about 0.68 for men and 0.55 for women, because body composition differs. A lower ratio means alcohol is spread through less body water, which tends to raise the estimated BAC.
Sources
- Understanding the Dangers of Alcohol Overdose , National Institute on Alcohol Abuse and Alcoholism (NIAAA)
- Impaired Driving: Get the Facts , U.S. Centers for Disease Control and Prevention (CDC)