Blood Alcohol Content (BAC) Calculator
Estimate blood alcohol content using the Widmark formula. Educational only — never use this to decide whether it's safe to drive.
1 standard drink = 12oz beer, 5oz wine, or 1.5oz spirits (all roughly equivalent in pure alcohol content).
This is a rough estimate for educational purposes only. It cannot account for food intake, metabolism, medication, hydration, or many other factors, and must never be used to decide whether it's safe to drive. If you've been drinking, do not drive — use a rideshare, taxi, or designated driver.
How is BAC calculated?
This calculator uses the Widmark formula, a long-standing method used to estimate blood alcohol content based on the amount of alcohol consumed, body weight, biological sex, and time elapsed. The formula converts your drinks into grams of pure alcohol (about 14 grams per US standard drink), then divides that by your body weight adjusted for a body water distribution constant, before subtracting the amount of alcohol your body has metabolized over time (roughly 0.015% per hour). The body water constant differs between men and women (commonly 0.68 for men and 0.55 for women) because body composition affects how alcohol is distributed and diluted in the body.
Why does the calculator ask for sex and weight?
Blood alcohol concentration depends heavily on how much water is available in the body to dilute the alcohol you've consumed. Because body composition differs on average between men and women, and because a larger body generally dilutes the same amount of alcohol more than a smaller one, both weight and the body water constant meaningfully change the estimate. None of this makes the result exact for any individual — actual BAC is also affected by food in your stomach, hydration, medications, liver function, and genetics.
Why can't this tool tell me if I'm safe to drive?
Even a BAC estimate calculated perfectly can't capture your real-time impairment, reaction time, or how alcohol interacts with your unique physiology in the moment. Legal limits are a legal threshold, not a safety guarantee — impairment begins well below 0.08% for most people. This tool is designed purely for education about how BAC estimation works, not as a decision-making tool for driving or operating machinery.
Frequently Asked Questions
Can this calculator tell me if it's safe to drive?
No. This tool cannot tell you whether it is safe to drive. BAC estimates are rough approximations and can't capture your real impairment level. The only safe choice if you've been drinking is not to drive — use a rideshare, taxi, or designated driver instead.
What formula does this calculator use?
It uses the Widmark formula, a widely referenced method for estimating BAC from grams of alcohol consumed, body weight, a sex-based body water constant, and hours elapsed since drinking began, with an adjustment for alcohol metabolized over time.
What counts as one standard drink?
In the US, one standard drink is roughly 12oz of regular beer, 5oz of wine, or 1.5oz of distilled spirits — each contains about 14 grams of pure alcohol, even though the serving sizes look very different.
Why does the calculator ask for biological sex?
The Widmark formula uses a body water distribution constant that differs on average between men and women due to differences in body composition, which affects how diluted the same amount of alcohol becomes in the bloodstream.
How accurate is this BAC estimate?
It's a rough approximation only. Real BAC is affected by food intake, hydration, medications, liver function, genetics, and the rate at which you drank — none of which this simplified calculator can measure. Never treat the output as a precise or reliable number.
What is the legal BAC limit in the US?
Most US states set 0.08% as the legal limit for driving for drivers 21 and older, with lower limits for commercial drivers and zero-tolerance limits for drivers under 21. This reference is shown for context only — it is not a safety threshold, and impairment can begin well below this number.
How long does it take for BAC to reach zero?
On average the body metabolizes alcohol at roughly 0.015% BAC per hour, but this varies by individual. This means even several hours after drinking, BAC may not be fully back to zero — coffee, cold showers, and food do not meaningfully speed up alcohol metabolism.
What should I do instead of relying on a BAC number?
If you have been drinking at all, the safest choice is to not drive. Arrange a rideshare, call a taxi, use public transit, or ask a sober friend for a ride — regardless of what any BAC calculator estimates.