BAC Calculator: Blood Alcohol Estimate Over Time (Widmark Formula)

1. Choose your sex. Sex sets the Widmark ratio r, which is 0.68 for men and 0.55 for women on average.
2. Enter your body weight and pick pounds or kilograms.
3. Pick how you count drinks. Standard drinks (14 g of alcohol each) or a custom drink with its own volume and ABV.
4. Enter the number of drinks, and for custom mode the volume in mL and the ABV percent.
5. Enter the hours since your first drink. The tool subtracts alcohol your body has already removed.

This is an educational estimate, not medical or legal advice.

Blood alcohol concentration, or BAC, is the amount of alcohol in your blood. It is written as a percent, where 0.08 percent means 0.08 grams of alcohol per 100 millilitres of blood. In the United States 0.08 percent is the common legal threshold for driving, and Utah uses 0.05 percent.

BAC rises as you drink and falls as your liver breaks the alcohol down. The rise depends on how much pure alcohol you take in and how much body water you have to dilute it. The fall is slow and fairly steady, at about 0.015 percent per hour.

This calculator estimates BAC with the Widmark formula, a classic method from forensic toxicology. It is a model for an average person, so the figure it gives is a rough estimate, not a reading from a breathalyzer or a blood test.

Important: impairment starts before any legal limit. The NHTSA notes measurable effects on vision and tracking around 0.02 percent, well below 0.08. A number under a legal limit is never a green light to drive.

The tool uses the Widmark equation, the standard way to estimate BAC from the amount of alcohol consumed and body size.

1. Find the alcohol in grams. A standard drink is 14 g of pure alcohol (NIAAA). A custom drink is its volume in mL times its ABV fraction times the density of ethanol, 0.789 g/mL.
2. Convert weight to grams and multiply by r to get the volume of body water the alcohol spreads into.
3. Divide and scale. Divide grams of alcohol by that figure and multiply by 100 to get a percent.
4. Subtract elimination. Take away 0.015 percent for every hour since the first drink. The result cannot go below zero.

The ratio r is lower for women on average because they carry a higher body-fat and lower body-water fraction. More precise methods, such as the Watson equation, derive r from total body water (age, height and weight), so real r varies from person to person.

Take a man who weighs 180 lb and has had 4 standard drinks, 2 hours since his first drink.

1. Weight in grams: 180 x 453.592 = 81,646.6 g. The ratio r for a man is 0.68.
2. Alcohol: 4 standard drinks x 14 g = 56 g.
3. Peak-style term: (56 / (81,646.6 x 0.68)) x 100 = 0.1009 percent.
4. Elimination: 0.015 x 2 hours = 0.03 percent.
5. BAC = 0.1009 - 0.03 = 0.0709, shown as 0.071 percent.
6. Time to ~0.00: 0.0709 / 0.015 = about 4.7 hours, roughly 4 h 43 min from now.

This case is loaded as the default preset. A rough 0.071 percent is below the common 0.08 legal limit, yet it is still an estimate of real impairment, and not a signal that driving is safe. The only safe amount before driving is zero.

The arithmetic is exact, but a real BAC can differ a lot from this estimate, because the formula uses population averages.

Food in the Stomach

Drinking with food slows absorption and usually lowers the peak BAC. The formula assumes none of this.

Sex and Body Composition

The ratio r depends on body water. Two people of the same weight can have very different real r values, so a fixed 0.68 or 0.55 is only an average.

Drinking Rate

Four drinks over one hour and over four hours give different real curves. The formula uses only the total and the time since the first drink.

Medications and Health

Some medicines, and liver or metabolic conditions, change how alcohol is handled. The formula cannot see any of this.

Hydration and Genetics

Fluid balance and inherited differences in alcohol enzymes shift real BAC up or down.

Because of all this, treat the output as a rough guide only. For how alcohol fits a wider picture, see our calories in alcohol guide and the alcohol impact on recovery calculator.

You can enter alcohol two ways. Both end in grams of pure alcohol, which is what the Widmark formula needs.

Standard Drinks

In the United States one standard drink is 14 g of pure alcohol, which is 0.6 fl oz. By the NIAAA, that equals a 12 fl oz beer at 5% ABV, a 5 fl oz glass of wine at 12% ABV, or 1.5 fl oz of spirits at 40% ABV.

Custom Drinks

If your drink is not a standard size, switch to custom mode and enter the volume in mL and the ABV. The tool finds the grams of alcohol as volume x (ABV/100) x 0.789, the density of ethanol.

To convert a drink to standard-drink units yourself, try our ABV calculator.

Learning and Curiosity

It is useful for understanding how weight, sex, intake and time shape a BAC estimate, and how slowly alcohol leaves the body.

Planning Ahead

Seeing that a few drinks can take many hours to clear can help you plan a sober ride in advance, as the CDC advises.

Never for a Driving Decision

Do not use this, or any calculator, to judge whether you can drive. It is an estimate, not a measurement, and it does not know your real state. If you have been drinking, do not drive.

Not for Medical or Legal Use

This figure is not valid for any medical, workplace or legal purpose. Only a calibrated breath or blood test gives a real BAC.

For other wellbeing measures, see our health calculators hub.

1. Treating the Estimate as a Measurement

A formula is not a breathalyzer. Your real BAC can be meaningfully higher or lower.

2. Reading a Legal Limit as a Safe Level

Impairment starts below 0.08 percent. A number under a limit is not permission to drive.

3. Forgetting Drink Size

A large beer or a strong cocktail can be two or more standard drinks. Use custom mode for real sizes.

4. Ignoring Time Since the First Drink

Elimination only removes about 0.015 percent per hour. You cannot rush it with coffee, food or a shower.

5. Assuming Everyone Clears Alcohol the Same

Rate varies by person. Averages can be wrong for you.

6. Using It to Decide to Drive

The biggest mistake of all. The only safe amount of alcohol before driving is zero.

The calculator reproduces the Widmark formula and the standard constants exactly. The limits lie in what a simple formula can say about one real person on one night.

Because of these limits, this is an educational estimate only. It is never a legal or safety judgment, and never a reason to drive.

No. It is educational only and gives a rough estimate, not a real measurement. Never use it, or any calculator, to decide whether to drive. The only safe amount of alcohol before driving is zero.

It is a rough model built on population averages. Your real BAC can be meaningfully higher or lower because of food, drinking rate, body composition, medications, hydration, health and genetics. Only a breath or blood test measures a real BAC.

In the United States one standard drink is 14 grams of pure alcohol (0.6 fl oz), per the NIAAA. That is about a 12 fl oz beer at 5% ABV, a 5 fl oz glass of wine at 12% ABV, or 1.5 fl oz of spirits at 40% ABV.

Alcohol leaves the blood at an average of about 0.015 percent per hour, which is roughly one standard drink per hour for an average man. You cannot speed it up with coffee, food, water or a cold shower.

Sex sets the Widmark ratio r, which is about 0.68 for men and 0.55 for women on average. Women tend to have a higher body-fat and lower body-water fraction, which concentrates alcohol more for the same intake.

No. Impairment begins before any legal limit. The NHTSA reports effects on vision and tracking around 0.02 percent, well below the common 0.08 threshold. A number under a limit is never a green light to drive.

Switch to custom mode and enter the drink volume in mL and its ABV. The tool finds the grams of alcohol as volume times the ABV fraction times 0.789, the density of ethanol, so a big beer or strong cocktail counts as more than one standard drink.

Yes. For the same alcohol, a heavier person usually reaches a lower BAC because there is more body water to dilute it. Weight is converted to grams and multiplied by r in the formula.

The tool divides your current estimated BAC by 0.015 to get the hours until it reaches about 0.00. This is only an estimate from the current figure and should never be used to plan driving.

No. The math runs in your browser. Inputs are not sent to our servers, and anything you Save stays only in your own browser storage.

• What is a standard drink? (14 g pure alcohol = 0.6 fl oz; beer/wine/spirits equivalents) (NIAAA; accessed 2026-10-05).
• Alcohol metabolism: elimination at about 0.015 percent per hour (BGSU; accessed 2026-10-05).
• Impaired driving: impairment starts below the legal limit; plan ahead so you do not drive (CDC; accessed 2026-10-05).
• Drunk driving: measurable impairment below 0.08; effects at 0.02 BAC (NHTSA; accessed 2026-10-05).