Blood Alcohol Content (BAC) Calculator
Estimate your Blood Alcohol Content (BAC) using the Widmark formula. See if you are over the legal driving limit.
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The BAC calculator estimates your blood alcohol content using the Widmark formula — the same mathematical model used by law enforcement and health researchers worldwide. Enter the number of standard drinks consumed, your body weight, biological sex, and how long ago you started drinking — the result is an estimated BAC percentage, colour-coded against the 0.08% legal driving limit used in most countries. Understanding your BAC matters for safety, not just legality. At 0.05% BAC, reaction time slows and judgment is impaired — even if you feel fine. At 0.08% (the legal driving limit in the US, UK, Canada, and Australia), risk of a fatal crash is approximately 11 times higher than at 0.00%. Many countries have stricter limits: Australia and much of Europe set the legal limit at 0.05%, and some countries enforce zero tolerance (0.00%). This calculator is a planning tool — it helps you understand how alcohol metabolises and how different factors affect your BAC. It is not a substitute for a breathalyser or medical test. Never make a driving decision based solely on a calculator estimate. If you have consumed alcohol, the safest choice is always not to drive.
How to Use the Blood Alcohol Content (BAC) Calculator
The Blood Alcohol Content (BAC) Calculator is designed to give you an accurate answer in seconds. Follow these steps:
- 1Enter your number of standard drinks in the Number of Standard Drinks field. The valid range is 0 to 20. The default is 2. Adjust this to match your specific situation.
- 2Enter your body weight (kg) in the Body Weight (kg) field. The valid range is 30 to 200. The default is 70 kg. Adjust this to match your specific situation.
- 3Choose your biological sex from the Biological Sex dropdown. Select the option that most accurately reflects your current situation — this value feeds directly into the calculation.
- 4Enter your hours since drinking started in the Hours Since Drinking Started field. The valid range is 0 to 12. The default is 1. Adjust this to match your specific situation.
- 5Click Calculate to see your results instantly. The output updates as soon as you submit.
No account or sign-up required. All calculations run locally in your browser — nothing is stored or transmitted to any server.
Example Calculation
Here is what the Blood Alcohol Content (BAC) Calculator produces with its default values. Change any input above to recalculate instantly for your own figures.
Inputs
- Number of Standard Drinks2
- Body Weight (kg)70 kg
- Biological Sex0.68
- Hours Since Drinking Started1
Results
- Blood Alcohol Content (%)0.572 %BAC
How It Works
BAC = (Drinks × 14 ÷ (Weight × Gender Factor)) − (0.016 × Hours)
Formula: BAC = (Drinks × 14 ÷ (Weight × Gender Factor)) − (0.016 × Hours) The Widmark formula calculates BAC based on: - Alcohol consumed: each standard drink contains 14 grams of pure alcohol (the US standard; one 355ml beer, 148ml wine, or 44ml spirits at standard strength) - Body weight: heavier people have more body water to dilute alcohol - Gender factor: males use 0.68 (68% of body weight is water); females use 0.55 (due to proportionally lower total body water) - Metabolism: the body metabolises alcohol at approximately 0.016% BAC per hour regardless of body weight Example: 75 kg male, 3 drinks over 2 hours. BAC = (3 × 14 / (75 × 0.68)) − (0.016 × 2) = 42 / 51 − 0.032 = 0.824 − 0.032 = 0.792... wait, that is BAC in g/dL ÷ 10 = 0.079% Actual factors that affect BAC beyond this formula include: food consumed (slows absorption by ~50%), drinking speed (faster drinking raises peak BAC), medication, and individual metabolic variation of ±20%.
Frequently Asked Questions
What is the legal BAC limit for driving?
The legal limit varies by country: United States: 0.08% (federal standard; some states use 0.05% for commercial drivers). United Kingdom: 0.08% (80 mg/100ml blood). Australia: 0.05% for fully licensed drivers; 0.00% for learners and probationary drivers. Germany and most of mainland Europe: 0.05%. Sweden, Norway: 0.02%. Japan: 0.03%. China, Czech Republic: 0.00%. Always check the current local limit — enforcement varies and limits can change.
How long does it take to sober up after drinking?
The body metabolises approximately 0.016% BAC per hour. If your BAC peaks at 0.10%, it takes around 6 hours to return to 0.00%. There is no way to speed up this process — coffee, cold showers, and food do not accelerate alcohol metabolism. Food slows absorption (reducing peak BAC) but does not change the rate of elimination. The only remedy for elevated BAC is time.
What counts as a standard drink?
A standard drink contains approximately 14 grams of pure alcohol in the US. This equals: one 355 ml can of regular beer (5% ABV), one 148 ml glass of wine (12% ABV), or one 44 ml shot of spirits (40% ABV). Note that standard drink definitions vary by country: the UK unit is 8 grams of alcohol; Australia's standard drink is 10 grams. Craft beers and strong wines often contain 1.5–2+ standard drinks per serving despite being a single glass.
Does eating before drinking lower your BAC?
Yes — significantly. Eating before or during drinking slows gastric emptying, which delays alcohol absorption and reduces peak BAC by up to 50%. High-fat and high-protein foods are most effective at slowing absorption. Eating does not reduce total alcohol processed or prevent eventual intoxication — it just spreads the absorption out over a longer period, producing a lower and later peak BAC.
Is the BAC calculator accurate?
The Widmark formula produces estimates accurate to within ±15–25% for most people. Individual variation in metabolism, tolerance, medication interactions, and hydration level all affect actual BAC. This calculator should never be used to make a driving decision. The only reliable way to know your BAC is a certified breathalyser or blood test. If you have consumed any alcohol, the safest choice is always not to drive.