This free BAC calculator gives you a rough estimate of your blood alcohol content using the widely-referenced Widmark formula, based on your biological sex, body weight, number of standard drinks, and hours since you started drinking. Enter your details and the calculator instantly shows an estimated blood alcohol percentage, how that compares with the 0.08% legal driving limit used in most US states, and roughly how many hours it would take for your body to metabolize the alcohol back down to zero.
Before you use the number, understand what it actually is: an educated guess built on averages, not a reading from your bloodstream. At Arb Digital we build tools and content that make useful information easy to access — this calculator is offered purely for education and awareness, never as a green light to get behind the wheel.
What This BAC Calculator Does
The tool takes four inputs — sex, weight, number of standard drinks, and elapsed time — and runs them through the Widmark equation, a formula developed in the 1930s by Swedish physiologist Erik Widmark and still used today by researchers, health educators, and even some breathalyzer training programs as a starting point for estimating blood alcohol concentration. It converts your drink count into grams of pure alcohol, spreads that alcohol across an estimated volume of body water (which differs by sex and weight), and then subtracts back out the alcohol your liver has already had time to process.
The output is a single percentage — your estimated BAC — plus a plain-language comparison against the 0.08% limit that defines legal intoxication for driving in nearly every US state, and an estimate of how many more hours it would take for that number to reach zero at a typical elimination rate. None of this replaces an actual breath or blood test, but it does illustrate, often quite starkly, how quickly a handful of drinks can push a person into territory where driving is dangerous and illegal.
How to Use the BAC Calculator
- Select your biological sex. The Widmark formula uses different average body-water constants for men and women, because body composition affects how alcohol is diluted in the bloodstream.
- Enter your body weight in pounds or kilograms using the unit toggle. A heavier person generally has a larger volume of body water to dilute the same amount of alcohol, which tends to lower BAC for the same number of drinks — but this varies enormously by individual.
- Enter the number of standard drinks you've had. Remember: a "standard drink" is a specific measure (about 14 grams of pure alcohol), not a specific glass — a tall craft beer or a generous wine pour can easily be 1.5 to 2 standard drinks on its own.
- Enter hours since your first drink. This lets the calculator subtract the alcohol your liver has already metabolized, since elimination starts as soon as alcohol enters your bloodstream.
- Click Calculate to see your estimated BAC, its status against the 0.08% limit, the drinks counted, and an estimate of when your BAC would reach zero.
The Widmark Formula Explained
The calculator converts your standard drinks into grams of alcohol (drinks × 14g), then applies the Widmark equation: BAC% = (grams of alcohol ÷ (body weight in grams × r)) × 100, minus an elimination adjustment of about 0.015% per hour that has passed. The constant "r" represents the widmark distribution ratio — roughly 0.68 for men and 0.55 for women — which approximates the fraction of total body weight that is water available to dilute alcohol. Because women have, on average, a higher proportion of body fat and lower proportion of body water than men of the same weight, the same number of drinks tends to produce a higher estimated BAC for a woman than for a man of identical weight. This is a documented physiological pattern, not a value judgment, and it is one of several reasons the formula is only ever an average.
The National Institute on Alcohol Abuse and Alcoholism explains how blood alcohol concentration relates to standard drink counts and impairment risk in detail — see the NIAAA's overview of alcohol consumption and BAC for the underlying research this calculator draws on.
Why the Widmark Formula Can Be Very Wrong for You
It is worth repeating clearly: the Widmark formula is a population average, and real individual BAC can deviate from it substantially in either direction. Whether you've eaten recently and how much food was in your stomach dramatically changes how quickly alcohol is absorbed — drinking on an empty stomach can push peak BAC significantly higher and faster than drinking with a full meal. Hydration status, certain medications, hormonal cycles, liver health, genetics that affect alcohol-metabolizing enzymes, how quickly you drank (a slow sip over three hours behaves very differently from the same total consumed in thirty minutes), and even altitude and body temperature can all shift the real number away from the formula's estimate. Two people of identical sex and weight who had the "same" number of drinks can end up with meaningfully different actual BACs.
This is exactly why BAC calculators — including this one — are educational tools, not legal or medical instruments, and why they are never a reliable way to decide whether it's okay to drive. Even a calculator that says you're comfortably under 0.08% cannot tell you what your true, in-the-moment BAC actually is.
A Standard Drink Is Smaller Than You Think
One of the most common reasons people underestimate their own BAC is miscounting "drinks." A standard drink in the US contains about 14 grams of pure alcohol — that's a 12oz regular beer at 5% ABV, a 5oz glass of wine at 12% ABV, or a 1.5oz shot of 80-proof spirits. A 16oz craft IPA at 7-8% ABV can be closer to two standard drinks. A generous restaurant pour of wine, or a strong mixed cocktail with two shots of liquor, can easily be two or three standard drinks in a single glass. If you're mentally counting "glasses" or "cups" rather than standard drinks, you are very likely underestimating your real alcohol intake — and therefore your real BAC.
Impairment Starts Long Before 0.08%
The 0.08% threshold is the legal limit for driving in most US states, but it is not the point where impairment begins — it's simply the line where impairment is legally presumed for everyone. Research summarized by the CDC's impaired driving program shows that measurable declines in reaction time, coordination, judgment, and divided attention — all critical for safe driving — begin at BAC levels well below 0.08%, sometimes as low as 0.02%. Vision tracking, reaction to sudden events, and the ability to multitask (like monitoring mirrors while braking) are all measurably worse at 0.05% than at 0.00%, long before anyone would be considered legally "over the limit." That's why "under the limit" and "safe to drive" are not the same thing, and this calculator should never be read as the latter.
The only BAC that carries zero impairment risk and zero legal risk is 0.00%. If you plan to drink at all, the safest plan is always arranged in advance: a designated sober driver, rideshare, taxi, or staying where you are until the next day.
What Affects How Fast Your Body Clears Alcohol
The liver metabolizes alcohol at a fairly steady rate for a given person — this calculator uses a commonly cited average of about 0.015% BAC per hour, which for many people works out to roughly one standard drink's worth of alcohol per hour. That rate is relatively fixed; unlike what popular myths suggest, coffee, cold showers, exercise, or "sweating it out" do not meaningfully speed up alcohol elimination. Time is the only reliable way to lower BAC. If your estimated hours-to-zero is several hours away, that time doesn't shrink no matter how you spend it — which is exactly why "I'll just wait it out with some coffee" is a dangerous plan, not a safe one.
- Food in your stomach slows alcohol absorption but does not change total alcohol consumed.
- Sleep does not accelerate liver metabolism of alcohol.
- Caffeine can mask sleepiness without reducing actual impairment — a dangerous combination.
- Only time allows BAC to decline; there is no verified shortcut.
Arb Digital builds fast, informative tools like this one to help people make better everyday decisions — explore more free calculators below, including our alcohol units calculator and supportive sobriety tracker.
Alcohol Units Calculator All Free ToolsCommon Mistakes to Avoid
- Treating the result as exact. It's an estimate built on averages — real BAC can be higher or lower for you personally.
- Undercounting drinks. Large pours, strong craft beers, and mixed cocktails often equal 1.5–3 standard drinks each, not one.
- Assuming "under 0.08%" means "safe to drive." Measurable impairment begins well below the legal limit.
- Trying to "speed up" sobriety. Coffee, cold water, and exercise do not lower BAC faster — only time does.
- Ignoring food, medication, and hydration. These can shift your real BAC significantly away from the formula's estimate.
- Using the calculator mid-drinking to decide "one more is fine." BAC continues rising for a while after your last drink as alcohol is still being absorbed.
Related Free Tools From Arb Digital
Curious how your total weekly drinking compares to health guidelines? Try our Alcohol Units Calculator. Tracking time away from alcohol? Our supportive Sobriety Calculator celebrates every milestone. If smoking is also part of the picture, see the Smoking Cost Calculator. Curious about your daily caffeine intake instead, try the Caffeine Calculator, or burn some energy with the Swimming Calories Burned Calculator. Browse everything in our free online tools hub.
Frequently Asked Questions
No. It provides a rough population-average estimate using the Widmark formula. Real BAC varies with food, hydration, medication, genetics, and how fast you drank. The only safe BAC for driving is 0.00% — never use this tool, or any estimate, to decide it's okay to drive after drinking.
In the US, one standard drink contains about 14 grams of pure alcohol — roughly a 12oz regular beer (5% ABV), a 5oz glass of wine (12% ABV), or a 1.5oz shot of 80-proof spirits. Many restaurant and bar pours are larger than this and count as more than one standard drink.
The Widmark formula uses different average body-water ratios for men (about 0.68) and women (about 0.55), reflecting typical differences in body composition. This is an average pattern from population research, not a statement about any individual.
On average, the body eliminates alcohol at roughly 0.015% BAC per hour, which is often summarized as about one standard drink per hour. This rate is fairly fixed for a given person and cannot be meaningfully sped up.
No. Neither coffee, cold showers, exercise, nor food after drinking speeds up how quickly the liver metabolizes alcohol. They may make you feel more alert, but your actual BAC and impairment level are unchanged.
Yes. Reaction time, coordination, and judgment can be measurably affected starting at BAC levels well below 0.08%, according to research summarized by the CDC. "Under the limit" is not the same as "unimpaired" or "safe to drive."
This tool provides general estimates for educational purposes only and is not medical advice. Estimates are approximate — never use them to decide whether it is safe to drive.
