Free BAC Calculator Online

BAC Calculator: Estimate Blood Alcohol Concentration with the Widmark Formula

Read this first: the number this calculator produces is an estimate, not a measurement, and it must never be used to decide whether it is safe to get behind the wheel. A BAC calculator takes a count of standard drinks, a body weight, hours elapsed, and a biological sex, and runs them through the Widmark formula — a decades-old equation used to approximate blood alcohol concentration from those inputs alone. It cannot see your actual liver function, your food intake, your medications, or dozens of other factors that change how a real body processes alcohol, so the percentage it returns is a rough approximation, not a substitute for a breathalyzer, a blood test, or your own judgment.

The calculation runs as JavaScript directly in your browser — nothing you type is uploaded anywhere — and the same Widmark math is also available as a metered API endpoint for anyone building it into a separate application. Either way, the output carries the same warning every time: this is a rough estimate of a number that varies enormously between individuals, and if you've been drinking, the only safe choice is not to drive, regardless of what any calculator says.

Every figure on this page is an informational, educational estimate, not medical advice, and this tool has no way to diagnose alcohol dependence or evaluate anyone's drinking habits. If you're worried about your own or someone else's drinking, that conversation belongs with a doctor or an addiction-medicine professional, not a percentage produced by a decades-old formula.

How to Use This BAC Calculator

The tool takes four pieces of information, one per line:

1. Enter the number of standard drinks consumed. A standard drink is treated as roughly 14 grams of pure ethanol — the equivalent of a typical 12 oz beer, a 5 oz glass of wine, or a 1.5 oz shot of spirits, though real-world pours frequently contain more alcohol than the "standard" reference amount.
2. Enter body weight in kilograms.
3. Enter the number of hours elapsed since the first drink — this factors in the roughly 0.015% per hour the body metabolizes away over time.
4. Enter biological sex (male or female), which sets the Widmark distribution ratio used in the formula.
5. The result shows an estimated BAC percentage, a comparison against the 0.08% legal limit used in most U.S. states, and a standing reminder that the number is an estimate only and is never a basis for deciding whether driving is safe.

There's no drink-by-drink timeline, no accounting for food eaten alongside the drinks, and no adjustment for tolerance, medication interactions, or overall health — the formula is a simplification, and the tool is upfront about that rather than presenting a false sense of precision.

Why the Widmark Formula Exists and What It Actually Estimates

Erik Widmark, a Swedish forensic scientist, published the formula in the 1930s as a way to work backward from a measured blood sample to estimate how much alcohol a person had consumed, or forward from consumption to an estimated concentration. The core idea is that alcohol distributes through body water rather than through fat or bone, so a heavier person with more body water dilutes the same amount of alcohol into a lower estimated concentration than a lighter person drinking the identical amount. The formula this tool uses is:

Term	Meaning
Grams of alcohol	Standard drinks x 14 grams of pure ethanol per drink
Distribution ratio (r)	0.68 for men, 0.55 for women — reflects average differences in body water proportion
Body weight in grams	Weight in kilograms x 1000
Elimination rate	Roughly 0.015% per hour subtracted for time elapsed since the first drink

Put together: BAC% equals grams of alcohol divided by body weight in grams times the distribution ratio, multiplied by 100, minus 0.015 times hours elapsed — and the result is clamped so it never shows a negative number once enough time has passed to metabolize everything consumed.

Why the Distribution Ratio Differs by Sex, and Why That's Still Just an Average

The 0.68 and 0.55 ratios used here reflect population averages in the proportion of body water to total body weight, which on average runs higher in men than in women. But those are averages across large groups, not fixed biological constants — actual body composition varies widely within each sex based on muscle mass, body fat percentage, age, and individual physiology, so two people of the same weight and sex can have meaningfully different real body-water proportions and therefore different true BAC even after drinking identical amounts. The formula has no way to see that variation; it only ever applies the population-average ratio.

What This Calculator Cannot Account For

The Widmark formula, and this tool by extension, leaves out a long list of factors that genuinely change blood alcohol levels in the real world: whether the drinks were consumed on an empty stomach or with food, which slows absorption; the actual alcohol content and pour size of what was actually drunk, versus the idealized 14-gram "standard drink"; medications that interact with alcohol metabolism; liver health and hydration; how quickly the drinks were consumed rather than spread over the stated hours; and individual metabolic rate, which varies from person to person and can differ from the 0.015%-per-hour average used here. None of these are things a weight-and-drink-count formula from the 1930s can measure, which is exactly why every result on this page is labeled an estimate rather than a reading.

Why "Standard Drink" Sizes Vary So Much in Practice

The 14-gram standard drink used in this calculator is a U.S. reference figure, and it's already a source of error before any other factor comes into play. A home-poured glass of wine or a generously mixed cocktail routinely contains well more than one standard drink's worth of alcohol, and beer strength varies widely between a session lager and a strong craft ale, even though both might get counted as "one drink" when entering a number here. Other countries define a standard drink differently — for example, the unit used in the United Kingdom represents a smaller amount of pure alcohol than the U.S. standard drink this tool assumes — so anyone converting a night of drinking measured in a different country's units needs to convert to the roughly-14-gram U.S. figure first, or the drink count entered won't mean what the formula assumes it means. This is one more reason the resulting percentage is a rough estimate rather than a precise reading: the input itself, "number of standard drinks," is already an approximation of what was actually consumed.

Legal Limits Are Not One Fixed Number

The 0.08% figure shown for comparison is the general legal limit for adult drivers in most U.S. states, but it is not the only limit that applies in practice. Drivers under 21 face a zero-tolerance or near-zero threshold in most states, commercial drivers holding a CDL are typically held to a stricter 0.04% limit, and many countries outside the U.S. set their general limit lower still, commonly around 0.05% or even 0.02%. A calculator that only checks a result against 0.08% can make an estimate look "fine" for a driver who is actually over the limit that legally applies to them. None of these thresholds change the underlying point: an estimate anywhere above zero means alcohol is still in your system, and the only universally safe choice is not to drive at all after drinking.

Why This Number Must Never Guide a Decision to Drive

This bears repeating in plain terms: a calculated BAC estimate is not a breathalyzer result, is not admissible as a measurement of anything, and should never be treated as a green light to drive. Because the underlying formula ignores food, absorption speed, medication, individual metabolism, and pour size, the true BAC for two people who enter identical numbers into this calculator can differ meaningfully from the estimate and from each other. A result that reads under the 0.08% legal threshold does not mean impairment is absent — reaction time, judgment, and coordination can all be measurably affected well below that number, and legal limits vary by location and by circumstance (commercial drivers and drivers under 21 typically face stricter limits in the U.S., for example). If you have been drinking at all, the only reliably safe choice is not to drive — arrange a designated driver, call a taxi or rideshare, or stay where you are until you are certain you're sober, rather than consulting any calculator, this one included, to make that call.

Manual Estimation vs. Using a Calculator

Even setting aside the driving-safety warning above, there are practical reasons a dedicated tool beats doing the Widmark math by hand:

• Working the formula manually. Converting drinks to grams, weight to grams, applying the right distribution ratio, and subtracting the time-based elimination term is several steps of arithmetic that are easy to fumble, especially after actually having a few drinks.
• Rule-of-thumb charts. Printed "one drink per hour" charts posted in some bars are a rough simplification of the same underlying idea, but they don't account for individual weight or sex the way the full formula does.
• A real breathalyzer. A consumer or professional breathalyzer measures actual breath alcohol rather than estimating from inputs, which makes it a meaningfully different (and more direct) kind of tool than any calculator — though even a breathalyzer reading isn't a substitute for simply not driving after drinking.
• This calculator's API. Developers building a responsible-drinking app or an event check-in tool who need the same Widmark math server-side, rather than re-implementing the formula themselves, can call it as a metered endpoint; the docs page covers accepted parameters and pricing, and any such integration should carry the same never-drive-on-this-estimate warning shown here.

Is this calculator as accurate as a breathalyzer?

No. A breathalyzer measures actual alcohol in a breath sample; this calculator only estimates a number from drinks, weight, time, and sex using a formula from the 1930s. Treat the output as a rough approximation, never as a measurement, and never as grounds for deciding it's safe to drive.

If my estimated BAC is under 0.08%, does that mean I'm safe to drive?

No. A result under the legal limit does not mean impairment is absent, and the formula's known blind spots — food, absorption speed, individual metabolism, medication, and more — mean your real BAC could be higher than the estimate. If you've had any alcohol, don't drive; arrange another way home.

Why does the calculator use a different ratio for men and women?

The distribution ratio reflects an average difference in the proportion of body water to total weight between the sexes, since alcohol disperses through body water rather than fat. It's a population average, not a measurement of any individual, so real results still vary from person to person within each group.

Does eating food before or during drinking change the result?

The formula doesn't account for food intake at all, even though eating substantially slows alcohol absorption in the real world. That's one of several reasons the number shown here should be read as a rough estimate rather than a precise reading of your actual blood alcohol level.

Does it matter whether the drinks were beer, wine, or spirits?

Not to the formula — every drink entered is treated as an identical ~14 grams of pure ethanol regardless of whether it came from a beer, a glass of wine, or a shot of liquor. In reality, pour sizes and actual alcohol content vary a great deal by beverage and by how it was poured, so two very different drinking sessions with the same "drink count" can involve noticeably different real amounts of alcohol.

Related Health Tools

Alcohol close to bedtime can disrupt the sleep-stage pattern behind the sleep calculator, so it's worth checking there if a night of drinking is affecting how rested you feel. If weight management or daily energy needs are also on your mind, the TDEE calculator and calorie calculator estimate how many calories a day you burn and need, including the extra calories alcoholic drinks themselves contribute.