BAC Calculator
Calculate Blood Alcohol Concentration
BAC Calculator tool
BAC Calculator
BAC Calculator: key facts
- What it does
- Calculate Blood Alcohol Concentration
- Category
- Health & Fitness Calculators
- Cost
- Free, with no account, sign-up, or install.
- Your data
- Runs entirely in your browser — the files and text you enter are never uploaded to a server.
- Last reviewed
- . Report an incorrect result.
About the BAC Calculator
Blood alcohol concentration estimates how much alcohol is in your bloodstream, expressed as a percentage. This calculator uses the well-established Widmark formula, factoring in your sex, body weight, the number of standard drinks, and the time elapsed since you started drinking, and flags when the estimate crosses the common 0.08% legal driving limit.
People use it to understand how drinks accumulate and how slowly alcohol clears — useful for planning a night out or appreciating why "just a couple" can still register hours later. It is strictly an educational estimate: individual metabolism, food, medication, and many other factors mean your true BAC can differ significantly, and no calculator can tell you it is safe to drive.
How to use it
- Select your sex (it sets the body-water distribution constant in the formula).
- Provide your body weight in kilograms.
- Enter the number of standard drinks consumed (each counted as 14 grams of alcohol).
- Enter the hours elapsed since you began drinking.
- Read your estimated BAC, with a warning shown if it is at or above 0.08%.
The Widmark equation
Blood alcohol concentration is estimated here using the Widmark formula, published in the 1930s and still the basis of most BAC arithmetic. It divides the alcohol consumed by the volume of body water available to dilute it, then subtracts what the liver has cleared over the elapsed time.
Each standard drink is counted as 14 grams of pure alcohol, the US definition — equivalent to a 350 ml beer at 5%, 150 ml of wine at 12%, or 45 ml of spirits at 40%. Drinks poured at home are very often larger than this, and a single craft beer or a generous glass of wine can be one and a half or two standard drinks.
The distribution ratio r accounts for alcohol dissolving in body water rather than in fat: 0.68 is applied for men and 0.55 for women, reflecting average differences in body composition. Elimination is subtracted at 0.015 percentage points per hour, which is a population average rate.
- The default entry — 70 kg, 3 drinks, 2 hours, male — estimates 0.0582%.
- The same intake for a 70 kg woman estimates 0.0791%, because the smaller distribution ratio concentrates the same alcohol in less body water.
- Both figures sit under the 0.08% threshold used in much of the United States — and both are close enough to it that the error in the estimate easily spans the limit.
Why use this version
- It uses the Widmark method with sex-specific distribution constants (0.68 for men, 0.55 for women), the standard scientific approach to estimating BAC.
- It accounts for elimination over time, subtracting roughly 0.015% per hour, so it reflects how alcohol clears rather than just how much you drank.
- It defines a drink as 14 grams of pure alcohol, the standard-drink convention, so counts are consistent.
- It runs entirely in your browser — nothing about your drinking is uploaded.
Never use this to decide whether to drive
This is the most important limitation on any tool on this site. A Widmark estimate can be wrong by a wide margin in either direction, and the inputs it lacks are exactly the ones that matter: whether you have eaten, what and when; how quickly the drinks were consumed; your sex, age, liver health, and body composition beyond total weight; medications, some of which interact seriously with alcohol; and your individual elimination rate, which varies substantially between people. The 0.68 and 0.55 ratios are averages that fit many individuals poorly.
The formula also assumes all the alcohol has been absorbed, which is not true in the first hour or so of drinking — during absorption your real BAC may be climbing well above what this reports. Elimination does not begin neatly either, so short elapsed times are the least reliable case.
Legal limits vary by country and by driver category, and many jurisdictions set them at 0.05% or 0.02%, or at zero for novice and professional drivers. Enforcement uses calibrated breath or blood testing, not arithmetic, and an estimate below a limit is no defence and no evidence of fitness to drive. Impairment of reaction time and judgement begins below every legal threshold.
If you have been drinking, do not drive. Use a taxi, public transport, or a sober driver. Nothing on this page should inform that decision, and for concerns about alcohol consumption or dependence, speak to a doctor or a support service.
Frequently Asked Questions
How accurate is this BAC estimate?
It is an approximation. The Widmark formula uses population averages and cannot account for your individual metabolism, food in your stomach, medications, hydration, or health conditions. Your actual BAC may be higher or lower, so never treat the result as a green light to drive.
What counts as one standard drink?
This calculator uses the common standard of 14 grams of pure alcohol per drink — roughly a 12 oz regular beer, a 5 oz glass of wine, or a 1.5 oz shot of spirits. Stronger or larger servings count as more than one drink.
How long does it take for BAC to return to zero?
Alcohol leaves the bloodstream at roughly 0.015% per hour, and that rate cannot be sped up by coffee, water, or food. Depending on how much you drank, it can take many hours to reach zero, which is why a BAC can still be significant the morning after.
Is it safe to drive below 0.08%?
Not necessarily. Impairment begins well before 0.08%, the limit is lower in many places and for commercial or young drivers, and any amount of alcohol can affect reaction time. This tool is educational only and is not a measure of fitness to drive.
