What the BAC calculator does
This Blood Alcohol Content calculator estimates the concentration of alcohol in your bloodstream from four things: your sex, your body weight, how many standard drinks you have had, and how many hours have passed since your first drink. It uses the Widmark formula, the equation that has underpinned consumer and forensic BAC estimation for almost a century.
BAC is reported as a percentage. A reading of 0.08% means there are 0.08 grams of alcohol per 100 millilitres of blood — the standard adult drink-driving limit in many US states. This tool exists to help you understand roughly how drinking, body size and time interact, not to tell you whether you are legally clear to do anything.
How the Widmark formula works
The estimate is built in two simple steps:
alcoholGrams = standardDrinks × 14
BAC(%) = max(0, (alcoholGrams ÷ (weightKg × 1000 × r)) × 100 − 0.015 × hours)
- 14 g per drink — the NIAAA definition of one US standard drink (a 12 oz beer at 5% ABV, a 5 oz glass of wine, or a 1.5 oz shot of spirits).
- r, the distribution factor — the share of your body mass that alcohol spreads into: 0.68 for men and 0.55 for women, on average.
- 0.015% per hour — the rate your liver removes alcohol, subtracted for every hour since your first drink. The result is floored at 0%, since BAC cannot go negative.
Worked examples
These rows come straight from the same engine that powers the calculator above, so the figures can never drift from the tool:
| Person | Drinks | Hours | Pure alcohol | Estimated BAC |
|---|---|---|---|---|
| Male, 80 kg | 2 | 1 | 28 g | 0.036% |
| Female, 60 kg | 3 | 2 | 42 g | 0.097% |
| Male, 90 kg | 1 | 0 | 14 g | 0.023% |
| Female, 50 kg | 6 | 4 | 84 g | 0.245% |
Notice how the 50 kg person who had six drinks over four hours reaches a far higher BAC than the 90 kg person who had a single drink — weight, drink count and time all move the number at once.
Reading your result: impairment stages
The gauge places your estimate on a 0% to 0.20% scale banded by the effects alcohol typically has. These ranges are educational guides drawn from public-health materials — they describe how impairment usually builds, not a legal threshold in any jurisdiction:
| BAC range | Stage | Typical effects |
|---|---|---|
| 0.00 – 0.02% | Little to none | Below the level at which most people show measurable impairment. |
| 0.02 – 0.05% | Mild | Relaxation, mild mood change and the first small losses of judgement. |
| 0.05 – 0.08% | Increasing | Reduced coordination, alertness and reaction time; impaired reasoning. |
| 0.08 – 0.15% | Significant | Clear loss of muscle control, balance, speech and reaction time. |
| 0.15 – 0.30% | Severe | Major loss of balance and control; risk of nausea, blackout and vomiting. |
| 0.30%+ | Life-threatening | Risk of unconsciousness, alcohol poisoning and death — seek help. |
If you want to think about the whole picture of what you drink and how it fits your day, the TDEE calculator estimates your daily calorie burn (alcohol adds roughly 7 calories per gram), the water intake calculator helps you plan hydration, and the BMI calculator puts your body weight in context.
Assumptions and limitations
The Widmark formula is deliberately simple, and this calculator inherits every one of its simplifications. Read the result as an order-of-magnitude estimate, not a measurement:
- It assumes all your alcohol is already fully absorbed at the elapsed time you enter, ignoring the absorption lag and any drinks still in progress.
- It models elimination as a constant 0.015% per hour from time zero. Real rates vary from roughly 0.010% to 0.020% per hour between people.
- The distribution factor is a fixed sex-based constant (0.68 / 0.55), not one adjusted for your individual body composition — results can be less accurate at very high or very low body-fat percentages.
- It does not account for food, drinking speed, medications, health conditions, hydration, hormonal state or tolerance, all of which measurably change real BAC.
- It does not evaluate legality anywhere — limits differ by country, state, age and licence type.
Frequently asked questions
What is Blood Alcohol Content (BAC)?+
Blood Alcohol Content (BAC) is the concentration of alcohol in a person's bloodstream, expressed as a percentage - for example, 0.08% means 0.08 grams of alcohol per 100 millilitres of blood. It is the standard measure used in medicine, research and drink-driving law to describe how much alcohol is currently in someone's system.
How does this calculator estimate my BAC?+
It uses the Widmark formula, the classic and most widely cited method for estimating BAC from the amount of alcohol consumed, body weight, sex and time elapsed. It converts your standard drinks into grams of pure alcohol, divides that by your body weight adjusted for a sex-specific distribution factor, and then subtracts the alcohol your body has likely metabolised over the elapsed hours.
What counts as one "standard drink" in this calculator?+
This calculator uses the US definition from the National Institute on Alcohol Abuse and Alcoholism (NIAAA): one standard drink contains about 14 grams of pure alcohol - roughly a 12 oz (355 mL) regular beer at 5% ABV, a 5 oz (148 mL) glass of wine at 12% ABV, or a 1.5 oz (44 mL) shot of 80-proof spirits.
Why does the calculator ask for my sex?+
The Widmark formula uses a different distribution factor for men (0.68) and women (0.55), reflecting that, on average, alcohol distributes into a somewhat smaller share of body mass in women due to typical differences in body water and fat proportion. This is a population-average constant, not an individual measurement, so it will not perfectly fit every body.
Why do two people who drink the same amount get different BAC results?+
Body weight and sex both change the denominator in the Widmark formula, so the same number of drinks spreads across a different effective volume for each person. A lighter person or someone with a smaller distribution factor will show a higher estimated BAC than a heavier person who drank the exact same amount over the same time.
How long does it take for BAC to return to zero?+
This calculator assumes alcohol is eliminated at a fixed rate of about 0.015% per hour, so you can roughly divide your current estimated BAC by 0.015 to see how many more hours it would take to reach zero. In reality, individual elimination rates commonly range from about 0.010% to 0.020% per hour, so actual time to zero varies from person to person.
Is this calculator accurate enough to know if I'm okay to drive?+
No. This tool gives an educational estimate only - it does not account for food intake, drinking speed, medications, health conditions, hydration or your individual metabolism, all of which measurably change real BAC. It must never be used to decide whether it is safe or legal to drive, operate machinery, or do anything safety-critical; only a certified breath or blood test can determine that.
What is the legal BAC limit for driving?+
Legal limits vary by country, and even within a country by state/province, driver age and license type. Many U.S. states set 0.08% as the standard adult limit, with lower limits for commercial drivers and near-zero limits for drivers under 21; other countries set different thresholds entirely. Always check the specific rules where you are - this calculator does not evaluate legality anywhere.
Does eating food before or while drinking lower my BAC?+
Food in the stomach slows the rate alcohol is absorbed into the bloodstream, which can lower and delay your peak BAC compared to drinking on an empty stomach. This calculator assumes alcohol is already fully absorbed by the time you read the result, so it does not separately model the food effect - real peak BAC after eating is often somewhat lower than the estimate.
Why might my actual BAC differ from this calculator's estimate?+
The Widmark formula is a population-average model. Real BAC also depends on factors the formula doesn't fully capture - body composition (muscle vs fat), drinking speed, whether you've eaten, hormonal state, hydration, certain medications, and genuine person-to-person variation in metabolism. Treat the result as a rough, order-of-magnitude estimate, not a precise measurement.
What is the Widmark formula?+
The Widmark formula, developed by Swedish physician Erik Widmark in the 1930s, is the foundational equation for estimating BAC from alcohol consumed, body weight and time. In its common form, BAC = (alcohol mass / (body weight x distribution factor)) - (elimination rate x hours), and it remains the basis for most consumer and forensic BAC estimation methods used today.
Can this calculator be used for someone under the legal drinking age?+
The underlying Widmark math works the same regardless of age, but this tool is intended for general educational use by adults of legal drinking age. Many jurisdictions apply zero-tolerance or near-zero BAC limits for drivers under 21, and alcohol affects younger and lighter bodies more strongly - the estimate does not adjust for this, so results should be treated with extra caution.
Does coffee, a cold shower, or exercise sober you up faster?+
No. None of these change how quickly your liver metabolises alcohol, so none of them lower your actual BAC any faster than time alone does. Coffee and a cold shower can make you feel more alert, which is exactly the risk - that alertness can mask how impaired you still are while your BAC has not actually moved. The only thing that reduces BAC is waiting for your body to finish processing the alcohol already in your system.
Disclaimer
Sources
- National Institute on Alcohol Abuse and Alcoholism (NIAAA) - What Is A Standard Drink?
- PMC (National Library of Medicine) - Estimating blood alcohol concentration via the Widmark equation
- Wikipedia - Blood alcohol content (Widmark equation)
- National Institute on Alcohol Abuse and Alcoholism (NIAAA) - Hangovers (only time clears alcohol from the body)
Formula and data last reviewed by the TheCalculatorHive team on 11 July 2026. Figures are for general information, not professional advice.
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