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Blood Alcohol (BAC) Calculator

See how the Widmark formula estimates blood alcohol from drinks, body weight, sex and time. For classroom and lab lessons only.

Level: Grades 6–7

  • Common Core standard 6.EE.A.2.cEvaluate expressions at specific values of their variables. Include expressions that arise from formulas used in real-world problems. Perform arithmetic operations, including those involving whole-number exponents, in the conventional order when there are no parentheses to specify a particular order (Order of Operations).

For educational lab use only. Never use this to decide whether to drive.

This calculator is for educational and classroom lab use only. Its result is a rough estimate. Do not rely on it to decide whether you can drive after drinking alcohol.

After even one alcoholic drink, the safe choice is not to drive. Call a cab, use a rideshare or have a designated sober driver.

Blood Alcohol (BAC) Calculator

Inputs
Count the alcohol by

One US standard drink is 14 g of pure alcohol.

Sex

Sets the body-water ratio the formula uses.

Result

Estimated blood alcohol concentration (BAC)

0.055%

The estimated BAC is about 0.055%. This number is a rough educational estimate and must not be used to decide whether to drive. After any alcohol, call a cab, use a rideshare or have a designated sober driver.

Same estimate in grams per liter
0.55 g/L
Starting estimate, before any alcohol is removed
0.085%
Estimated amount removed over that time
0.03%
Pure alcohol counted
42 g

See the step-by-step solution ↓

Step-by-step solution

  1. Find the grams of pure alcohol. Each US standard drink holds 14 g

    3×14 g=42 g

  2. Change body weight to grams, so it uses the same unit as the alcohol (1 lb is 453.59237 g)

    160 lb×453.59237=72,574.78 g

  3. Spread the alcohol through the body's water. Divide by weight times r (0.68 for males), then times 100 for a percent

    42 g(72,574.78 g×0.68)×100=0.0851%

  4. Estimate how much the body has removed: about 0.015 for every hour since the first drink

    0.015×2 h=0.03%

  5. Subtract the amount removed from the starting estimate

    0.0851%−0.03%=0.0551%

Please note: This calculator gives a general screening estimate, not a diagnosis. Talk to a qualified healthcare professional about your individual health.

What this calculator does

See how scientists estimate blood alcohol from a few numbers. This page uses the Widmark formula, a classic model taught in health, chemistry and forensic science classes. You enter how much alcohol was drunk, body weight, sex and the hours since the first drink. The calculator shows each step, from grams of alcohol to an estimated blood alcohol concentration (BAC). BAC is the amount of alcohol in the blood, written as a percent.

This is a teaching tool, not a measuring device. Real blood alcohol depends on food, drinking speed, health, medicines and each person's own body. The estimate can be far from the true value, in either direction. Never use it to decide whether you can drive. After any alcohol, call a cab, use a rideshare or have a designated sober driver.

Where it's used

Why this math matters outside the classroom:

  • Forensic science class. Students put 42 g of alcohol, a 72,575 g body weight and r = 0.68 into the formula and get a starting estimate of about 0.085%.
  • Unit conversion. A class changes 160 lb into 72,574.8 g (160 × 453.59237) so body weight and alcohol are in the same unit.
  • Comparing the r values. Swapping r = 0.68 for r = 0.55 raises the estimate by about 24%, since 0.68 ÷ 0.55 is about 1.24.
  • Rates over time. Subtracting 0.015 for each of 3 hours shows a model estimate dropping by 0.045 percentage points.
  • Health education. A health teacher shows that the same 3 drinks give a higher estimate for a 120 lb adult than for a 200 lb adult.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.EE.A.2.c. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose how to count the alcohol: as US standard drinks, or from the size and ABV (alcohol by volume) of a drink.
  2. Enter the body weight and pick pounds or kilograms.
  3. Choose the sex, which sets the body-water ratio the formula uses.
  4. Enter the hours since the first drink, then read each step of the working.

What the inputs mean

Number of US standard drinks
A US standard drink holds 14 g of pure alcohol. That is roughly a 12 fl oz beer at 5%, a 5 fl oz glass of wine at 12% or 1.5 fl oz of 40% spirits. Many real pours are larger.
Size of one drink
Enter the size of one drink. The calculator multiplies by the ABV and by the density of alcohol, 0.789 g per mL, to get grams.
Alcohol by volume (ABV)
Printed on the label.
Sex
Widmark measured a lower average body-water ratio in females (0.55) than in males (0.68). The same alcohol is spread through less water, so the estimate comes out higher.
Hours since the first drink
Count from the start of the first drink. The model assumes the body removes about 0.015 percentage points of BAC each hour from that moment.

Formula and method

Starting estimate (%) = A ÷ (W × r) × 100
Estimated BAC (%) = starting estimate − 0.015 × t
A = grams of pure alcohol   W = body weight in grams
r = 0.68 (male) or 0.55 (female)   t = hours since the first drink

Alcohol spreads into the water in the body, not into fat. So the formula divides the grams of alcohol by the body weight times r. The letter r is the Widmark factor, the share of body weight that acts like water for alcohol. Multiplying by 100 turns the answer into a percent, read as grams of alcohol per 100 mL of blood.

The body also removes alcohol over time, mostly in the liver. The formula takes off a fixed 0.015 for each hour. If that subtraction goes below zero, the calculator shows 0, because a blood level can't be negative.

This is the simplified textbook form, first published by the Swedish scientist Erik Widmark in 1932. It ignores the small difference between the weight and the volume of blood, which shifts the result by a few percent.

Worked example

A class works through 4 US standard drinks for a 140 lb female, 3 hours after the first drink.

  1. Grams of alcohol: 4 × 14 g = 56 g.
  2. Body weight in grams: 140 × 453.59237 = 63,502.93 g.
  3. Starting estimate: 56 ÷ (63,502.93 × 0.55) × 100 = 0.1603%.
  4. Amount removed: 0.015 × 3 = 0.045%.
  5. Estimated BAC: 0.1603% − 0.045% = 0.1153%.

Result: The model estimates a BAC of about 0.115%. It is a classroom estimate only and says nothing about whether anyone can drive.

Understanding the result

A BAC of 0.10% means about 0.10 g of alcohol in every 100 mL of blood. That is the same as 1 g per liter, which is the second figure the calculator shows.

Many places set a legal blood alcohol limit for drivers. Some use 0.08% and others use lower figures, such as 0.05%. Slower reactions and weaker judgment begin well below any of these limits.

Use the result to see how each input changes the answer. Doubling the drinks doubles the starting estimate. A heavier body or a larger r lowers it. Each hour takes off the same fixed amount.

Assumptions and limitations

  • All the alcohol is absorbed into the blood at once, at the moment of the first drink.
  • The body-water ratio is Widmark's average: 0.68 for males and 0.55 for females.
  • The body removes 0.015 percentage points of BAC every hour, at a steady rate.
  • A US standard drink holds 14 g of pure alcohol, and ethanol weighs 0.789 g per mL.
  • It is an educational model, not a breath or blood test. It must never be used to decide whether to drive, operate machinery or do anything else risky.
  • Real removal rates vary from person to person. Published averages range from about 0.010 to 0.020 per hour.
  • Food, drinking speed, medicines, health conditions and body build all change real results, and the model ignores them.
  • In the first hour or so after drinking, real BAC can still be rising while the model shows it falling.

Frequently asked questions

Can I use this to decide if I can drive?

No. The result is a rough estimate for learning, and real blood alcohol can be much higher than it shows. After even one alcoholic drink, the safe choice is not to drive. Call a cab, use a rideshare or have a designated sober driver.

Why does the formula ask for sex?

Because Widmark found different average body-water ratios. On average, females have a little less body water for their weight than males. The same alcohol in less water gives a higher concentration, so r is 0.55 instead of 0.68.

How accurate is the estimate?

Not very, for any one person. The formula uses averages for body water and for how fast alcohol is removed. Real people differ from those averages, and food and drinking speed change absorption too. That is why it is used to teach the idea, not to measure anyone.

Why might real BAC keep rising after the last drink?

Alcohol takes time to pass from the stomach and gut into the blood. The formula pretends it all arrives at once. So for a while after drinking, the true level can be climbing even though the model shows it dropping.

Do coffee or a cold shower lower BAC?

No. Coffee, cold water and exercise don't speed up how fast the body removes alcohol. They may make someone feel more alert while their blood alcohol stays the same.

How this calculator is checked

Method: Widmark formula (E. M. P. Widmark, 1932) in its simplified textbook form: BAC (%) = A ÷ (W × r) × 100 − β × t, with A in grams of alcohol, W in grams of body weight, Widmark's average r of 0.68 (male) and 0.55 (female), and a commonly used average elimination rate β of 0.015 g/dL per hour. Alcohol in grams from volume × ABV × 0.789 g/mL; US standard drink = 14 g. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-26. Found a problem? Tell us.

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