BAC Calculator — Free Online Blood Alcohol Estimator

How to Use the BAC Calculator

1. Select your unit system: Choose between Metric (kilograms) or Imperial (pounds) for your body weight input. The calculator converts internally regardless of your choice.
2. Enter your body weight: Type your current body weight. Accuracy matters here — the Widmark formula uses weight to determine how alcohol distributes through your body water. A difference of 20 pounds can change your estimated BAC by 0.01% or more.
3. Select your biological sex: Choose male or female. This determines the Widmark distribution factor: 0.68 for males (higher body water percentage) and 0.55 for females. This biological difference means females typically reach higher BAC levels than males of similar weight after consuming the same amount of alcohol.
4. Enter standard drinks consumed: Enter the total number of standard drinks you have consumed. One standard drink equals 12 oz beer (5%), 5 oz wine (12%), or 1.5 oz spirits (40%). If you had a 16 oz craft beer at 7%, that counts as approximately 1.9 standard drinks. You can enter decimal values for partial drinks.
5. Enter hours since first drink: Enter the number of hours since you started drinking. The calculator subtracts the amount of alcohol your body has metabolized at the average rate of 0.015% per hour. You can enter quarter-hour increments (0.25, 0.5, 0.75).

This calculator provides estimates only. Never use it to determine whether you are safe to drive. If you have consumed any alcohol, do not drive.

BAC Formula (Widmark Method)

Variables Explained

• Alcohol in grams: Each standard drink contains approximately 0.6 oz (17 g) of pure ethanol. Total alcohol = number of standard drinks x 0.6 oz x 28.35 g/oz.
• Body weight in grams: Your body weight converted to grams (kg x 1000). A heavier body has more water for alcohol to distribute into, resulting in lower BAC.
• Widmark factor (r): A gender-specific constant representing the proportion of body weight that is water. Males: 0.68. Females: 0.55. This accounts for the biological difference in body water composition.
• Metabolism rate (0.015): The average rate at which the liver breaks down alcohol, approximately 0.015% BAC per hour. This is relatively constant and cannot be accelerated by any method.
• Hours: Time elapsed since the first drink. Alcohol absorption typically peaks 30-90 minutes after the last drink, so BAC may still be rising shortly after drinking stops.

Step-by-Step Calculation Example

Suppose a 170 lb (77.1 kg) male consumed 3 standard drinks over 2 hours:

1. Alcohol in grams: 3 x 0.6 x 28.35 = 51.03 g
2. Body weight in grams: 77.1 x 1000 = 77,100 g
3. Widmark factor (male): 0.68
4. Raw BAC: (51.03 / (77,100 x 0.68)) x 100 = (51.03 / 52,428) x 100 = 0.0973%
5. Metabolism deduction: 0.015 x 2 = 0.030%
6. Estimated BAC: 0.0973 - 0.030 = 0.0673%

At 0.0673%, this person is below the US legal limit of 0.08% but is still experiencing impaired coordination and judgment. At least 4.5 more hours would be needed to reach 0.00% BAC.

Practical Examples

Example 1: Tom at a Work Happy Hour

Tom weighs 190 lbs and had 2 beers (12 oz each, 5% ABV) over 1.5 hours at a work event. He is wondering whether he should drive home or take a rideshare:

• Standard drinks: 2
• Weight: 190 lbs (86.2 kg)
• Estimated BAC: approximately 0.029%
• Status: Minimal impairment
• Time to 0.00%: approximately 1.9 hours

Although Tom's BAC is well below the legal limit, any amount of alcohol can impair reaction time. The safest choice is always to use a rideshare or designated driver after consuming alcohol.

Example 2: Megan at a Dinner Party

Megan weighs 130 lbs and had 3 glasses of wine (6 oz each, 12% ABV) over 3 hours with dinner:

• Standard drinks: 3 x (6/5) = 3.6 standard drinks
• Weight: 130 lbs (59 kg)
• Estimated BAC: approximately 0.091%
• Status: Over legal limit
• Time to 0.00%: approximately 6.1 hours

Megan is above the 0.08% legal limit despite drinking over 3 hours. The larger-than-standard wine pours increased her total consumption significantly. She arranges a ride home and plans to pick up her car the next morning.

Example 3: Understanding the Morning After

Derek weighs 160 lbs and had 6 standard drinks between 8 PM and midnight (4 hours). He wants to know if he is safe to drive at 7 AM the next morning (11 hours after first drink):

• Standard drinks: 6
• Weight: 160 lbs (72.6 kg)
• BAC at midnight (4 hours): approximately 0.108%
• BAC at 7 AM (11 hours): approximately 0.108% - (0.015 x 11) = approximately 0.00% (after 7.2 hours)

Based on the estimate, Derek would reach 0.00% BAC approximately 7.2 hours after midnight, or around 7:12 AM. However, residual effects of heavy drinking (fatigue, dehydration, impaired concentration) can persist even after BAC reaches zero. These effects can still impair driving ability.

BAC Impairment Level Reference Table

Tips and Complete Guide to Understanding BAC

How Food Affects Alcohol Absorption

Eating before or while drinking significantly slows alcohol absorption into the bloodstream. Food, particularly high-protein and high-fat foods, delays gastric emptying, keeping alcohol in the stomach longer where it is absorbed more slowly. Studies show that drinking on a full stomach can reduce peak BAC by 20% to 30% compared to drinking on an empty stomach. However, food does not prevent alcohol from being absorbed — it only delays the process. Your total BAC will eventually reach a similar level, but the peak will be lower and occur later, giving your liver more time to metabolize the alcohol.

Why You Cannot Speed Up Sobering

The liver metabolizes alcohol at a nearly constant rate of approximately 0.015% BAC per hour through the enzyme alcohol dehydrogenase (ADH). Common myths suggest that coffee, cold showers, exercise, or eating can speed up sobering — none of these work. Coffee may make you feel more alert but does not reduce BAC or impairment. Cold showers and exercise may make you feel different but have no effect on metabolism rate. The only factor that reduces BAC is time. This is why planning ahead — designating a sober driver, booking a rideshare, or limiting consumption — is essential.

Alcohol and Health Risks

Beyond the immediate risks of impaired driving and acute intoxication, regular heavy alcohol consumption is linked to numerous long-term health consequences including liver disease, cardiovascular disease, increased cancer risk (breast, liver, colon, throat), weakened immune function, and mental health disorders. The CDC defines heavy drinking as 15 or more drinks per week for men and 8 or more for women. If you are concerned about your alcohol consumption, speak with your healthcare provider or contact SAMHSA's National Helpline at 1-800-662-4357 for free, confidential support.

Common Mistakes to Avoid

• Using this calculator to decide whether to drive: This tool provides rough estimates only. Individual BAC can vary significantly based on factors not included in the formula. If you have consumed any alcohol, the safest choice is to not drive.
• Underestimating drink counts: Many beverages contain more than one standard drink. A large craft beer, a generous wine pour, or a strong cocktail can easily equal 2 to 3 standard drinks. Count standard drink equivalents, not number of glasses.
• Assuming you feel fine means you are safe: Tolerance affects how intoxicated you feel but does not reduce BAC or actual impairment of reaction time and decision-making. A person with high tolerance may feel relatively normal at a BAC that significantly impairs driving ability.
• Believing coffee or food can sober you up: Nothing accelerates alcohol metabolism. Only time reduces BAC. Coffee may mask the feeling of intoxication while leaving impairment unchanged — this is arguably more dangerous than feeling drunk.
• Ignoring the morning after: BAC can still be above zero the morning after heavy drinking. A person who had 8 drinks and stopped at midnight may still have a BAC of 0.04% or higher at 7 AM, which impairs driving even though they feel mostly sober.

Frequently Asked Questions