This BAC calculator estimates blood alcohol concentration with a Widmark style formula. Defaults are weight 70 kg, male, 3 standard drinks, and 2 hours, which return about 0.058%. The result is educational only. It is not legal advice, not medical advice, and never a claim that anyone is safe to drive.
Body size conversions can use the BMI calculator or the ideal weight calculator, but those tools do not measure impairment.
How the formula works
Alcohol grams = drinks × 14. For males r = 0.68. BAC% = (alcohol grams / (weight kg × 1000 × r)) × 100 – 0.015 × hours, floored at zero. Females use r = 0.55 with the same structure.
Worked example
Drinks 3 gives alcohol grams = 42. Denominator = 70 × 1000 × 0.68 = 47600. Raw percent = (42 / 47600) × 100 ≈ 0.088235%. Subtract 0.015 × 2 = 0.03. Estimated BAC ≈ 0.058235%, shown as about 0.058%.
| Input | Value |
|---|---|
| Weight | 70 kg |
| Sex | male (r = 0.68) |
| Standard drinks | 3 |
| Hours | 2 |
| Estimated BAC | about 0.058% |
How to use the fields
- Body weight is in kilograms.
- Sex selects the Widmark r factor.
- Standard drinks counts drinks at 14 g alcohol each.
- Hours since first drink scales the elimination subtraction.
Why this is never a driving green light
Impairment depends on more than a classroom formula. Absorption timing, food, medications, drink size, and individual biology vary. Laws also use specific testing methods. A model output of about 0.058% does not authorize driving, and a lower model output does not either.
If the question is transportation, arrange a sober ride. Do not negotiate with an estimate.
Common mistakes
- Treating the estimate as a roadside legal result
- Using pounds in the weight field without converting to kilograms
- Assuming every beverage equals one standard drink
- Ignoring that elimination is a simplified constant here
Standard drink discipline
A “drink” in conversation may be larger than the 14 g model. If a pour is stronger or taller, the alcohol grams term rises and the estimate rises. Classroom honesty requires matching the 14 g definition or rescaling drinks before entry.
Count drinks carefully. The model is linear in alcohol grams before elimination.
Sex factor r
Changing r changes the denominator. Smaller r increases the percent for the same alcohol mass and weight. That is why the tool asks for sex even when weight is identical across examples.
Keep r stated beside every reported BAC so reviewers know which path ran.
Hours and the 0.015 term
Each hour subtracts 0.015 from the percent term in this sketch. Longer time lowers the estimate if alcohol grams stay fixed. Real elimination rates vary; the constant exists for reproducible teaching math, not personalized physiology.
Compare hours 0 versus hours 2 with the default drinks to see the subtraction clearly.
Educational framing only
Health decisions about alcohol belong with qualified clinicians and personal risk context. Nutrition style tools such as the protein calculator and the TDEE calculator do not measure intoxication.
Share weight, drinks, hours, and the disclaimer whenever you discuss a sample output in class.
Classroom ethics
Use the default 70 kg, 3 drinks, and 2 hours path to practice algebra, not to plan a night out. Require every submitted solution to include the sentence that the estimate is not legal advice and does not justify driving.
If a student asks whether 0.058% is “under the limit,” redirect to local law and professional sources. This page will not certify compliance.
Compare male and female r values with identical weight and drinks only to show the formula sensitivity. Do not turn that comparison into stereotypes or jokes about tolerance.
Close every lab with a reminder that arranging safe transport beats arguing about decimal places. The calculator exists to teach Widmark arithmetic, not to negotiate risk.
For the default path, keep about 0.058% tied to 70 kg, male, 3 drinks, and 2 hours whenever you cite the example.
Limitations
No breathalyzer calibration, no legal limit advisory by jurisdiction, and no safe driving threshold. Widmark style educational estimate only. Not legal advice. Not medical advice. Never a reason to drive after drinking.