BAC Calculator — Blood Alcohol Content

Blood alcohol content (BAC) measures the concentration of alcohol in your bloodstream, expressed as a percentage — a BAC of 0.08%, for instance, means 0.08 grams of alcohol per 100 milliliters of blood. This calculator estimates BAC using the Widmark formula, a method developed nearly a century ago and still the basis for most BAC estimation today. Before anything else, it's worth being direct about what this tool is for: understanding roughly how alcohol affects your body, not deciding whether it's safe to drive. The only BAC that's genuinely safe for driving is 0.00% — a number below any legal limit still means measurable impairment.

The Widmark formula

Swedish physician and forensic toxicologist Erik Widmark developed the formula behind this calculator in the 1930s, and — with only minor refinements — it remains the standard method used by researchers, medical professionals, and often law enforcement to estimate BAC from alcohol consumption.

Widmark’s original research was genuinely pioneering for its time: he ran controlled experiments giving known quantities of alcohol to volunteers and directly measuring their resulting blood alcohol levels over time — one of the first systematic attempts to mathematically model how the body processes alcohol, published in his 1932 monograph on the subject. The core relationship he identified — that BAC depends predictably on dose, body weight, sex-specific distribution, and a roughly constant elimination rate — has held up remarkably well against nearly a century of subsequent research, even as the exact average values for the distribution factor and elimination rate have been refined somewhat by later studies with larger and more diverse samples than Widmark’s original volunteers.

The distribution factor “r” accounts for how alcohol disperses through body water rather than through fat tissue, which barely absorbs alcohol at all. Widmark’s original research established different average values for men (r ≈ 0.73) and women (r ≈ 0.66), reflecting that women typically have a higher percentage of body fat and lower total body water than men at an equivalent weight — meaning the same amount of alcohol, in the same body weight, distributes into a smaller water volume and produces a higher BAC for a woman than for a man, on average. Beyond body composition, women also tend to produce less alcohol dehydrogenase — the stomach enzyme that begins breaking down alcohol before it even reaches the bloodstream — a further contributor to the same pattern.

Worked example

Using this calculator’s own default: a 160 lb man, 2 hours since his first drink, having consumed about 28 grams of alcohol (roughly two standard 12oz beers at 5% ABV):

This example lands well below the 0.08% legal marker used in most US states — but “below the legal marker” and “safe to drive” are not the same claim, and this article returns to that distinction more than once, deliberately.

Standard drinks and alcohol content

A “standard drink” in the US is defined as roughly 14 grams of pure alcohol — the amount found in a 12oz regular beer (about 5% ABV), a 5oz glass of wine (about 12% ABV), or a 1.5oz shot of distilled spirits (about 40% ABV). These three serving sizes were specifically chosen because they all contain approximately the same amount of pure alcohol despite looking very different in the glass, which is exactly why a “drink” can mean such different volumes depending on what’s being poured.

This is worth knowing because casual descriptions of “how much I drank” often undercount real alcohol content — a large glass of wine poured at home, a strong craft beer well above 5% ABV, or a full 750ml bottle of wine (which contains roughly five standard drinks, not one) can each add up to meaningfully more alcohol than an equivalent number of “standard” servings might suggest.

Craft beer is a particularly common source of this miscalculation. Many popular craft styles — imperial stouts, double IPAs, barleywines — commonly run 8–12% ABV or higher, meaning a single 12oz pour can contain nearly twice the alcohol of a standard 5% ABV beer, while looking and being poured exactly like “one beer.” Someone tracking “three beers” who drank three double IPAs at 9% ABV has actually consumed alcohol equivalent to roughly five to six standard drinks, not three — a gap large enough to meaningfully change an estimated BAC if the stronger ABV isn’t accounted for. Checking the actual ABV printed on the can or menu, rather than assuming a standard 5%, meaningfully improves the accuracy of any BAC estimate built from a drink count.

How the body eliminates alcohol

The liver metabolizes alcohol at a fairly constant rate, commonly cited as roughly 0.015–0.020% BAC per hour, with 0.015% used as a representative average in this calculator. This rate is a genuine biological constant for a given person — unlike the rate of alcohol absorption, which food, drink strength, and other factors can meaningfully speed up or slow down, the elimination rate itself doesn’t meaningfully respond to coffee, cold showers, exercise, or any other commonly suggested “sobering up” trick. A BAC of 0.10% takes roughly 6–7 hours to reach 0.00% purely through normal liver metabolism; there is no reliable way to accelerate this.

This is also why sleep doesn’t guarantee sobriety by morning. Someone who stops drinking at 2 AM with a BAC of 0.15% is still metabolizing alcohol at the same fixed rate while asleep, and depending on how much time has actually passed by morning, may still have a measurable — and potentially illegal — BAC when they wake up, even after a full night’s sleep and feeling rested.

Why BAC differs between men and women

At an identical body weight and identical alcohol consumption, women on average reach a higher BAC than men — a pattern the Widmark formula captures through its different distribution factors (0.73 for men, 0.66 for women). The underlying physiology comes down to body composition and enzyme activity: women typically carry a higher percentage of body fat and a lower percentage of body water at an equivalent weight, and since alcohol distributes through body water rather than fat tissue, less water to dilute into means a higher resulting concentration for the same dose. Reduced gastric alcohol dehydrogenase activity in women compounds this further, since less alcohol is broken down before it reaches the bloodstream in the first place.

Legal limits: what they are, and what they aren't

Legal BAC limits vary by jurisdiction and driver category, and it’s worth knowing the current landscape — while keeping the more important point front and center: a legal limit describes what’s prosecutable, not what’s safe.

Utah remains the only US state with a 0.05% limit as of this writing, though the National Transportation Safety Board has recommended a nationwide 0.05% standard, and research following Utah’s change has been associated with improved road safety outcomes there. Laws in this area do shift over time and by jurisdiction — always verify the current limit for your specific location rather than assuming a number from any single source, including this one.

Internationally, the picture varies further: many European countries use a 0.05% standard rather than the 0.08% common across most of the US, and a number of countries — including several with especially strict enforcement — set the limit at 0.02% or effectively 0.00% for all drivers. This wide international variation is itself a useful reminder that “the legal limit” isn’t a fixed physiological fact about when impairment becomes dangerous — it’s a policy choice that different jurisdictions have made differently, informed by the same underlying research but weighted toward different levels of caution.

More importantly: measurable impairment to reaction time, coordination, and divided attention begins well before 0.08%, and research points to noticeable effects on driving-relevant skills starting around 0.02–0.05%. This is precisely why “I’m under the legal limit” is not the same claim as “I’m safe to drive” — legal limits exist as an enforceable line for prosecution, not as a scientifically identified point where impairment suddenly begins. The only BAC with zero added risk is 0.00%.

Why this is an estimate, not a measurement

The Widmark formula is a population-average model, and several real factors it can’t account for meaningfully affect actual BAC for any specific person and occasion: how much food was eaten and when (food slows alcohol absorption, lowering peak BAC and spreading it over a longer period), medications (many interact with alcohol metabolism), individual differences in liver function and body composition beyond the simple male/female split, and how quickly drinks were actually consumed (rapid consumption produces a higher peak BAC than the same total spread over a longer period, something a simple hours-elapsed input can’t fully capture). A breathalyzer or blood test measures actual BAC directly; this calculator estimates what BAC is likely to be, and should be treated with a meaningfully wider margin of uncertainty than a direct measurement would carry.

The timing of absorption specifically deserves attention, since it’s one of the biggest gaps between a simple formula and real physiology. Alcohol consumed on an empty stomach is absorbed considerably faster than alcohol consumed alongside food, since food in the stomach slows the rate at which alcohol passes into the small intestine (where the majority of absorption actually happens). This means two people who consume the identical total amount of alcohol over the identical number of hours can have meaningfully different peak BAC — and different impairment at any given moment — purely based on whether they ate beforehand or during. A formula built around total grams and total hours elapsed necessarily averages over this variation rather than capturing it, which is exactly why it estimates a plausible BAC rather than measuring an actual one.

Body composition beyond simple sex differences also matters more than the formula can capture. Two people of the same sex, weight, and drink count can still have genuinely different actual BAC if one carries meaningfully more muscle and less fat than the other, since — as with the male/female distribution factor — a higher proportion of body water relative to total weight means more dilution for the same alcohol dose. The Widmark formula’s single distribution factor per sex is a population average, not a personalized figure, and individual variation around that average is real and can meaningfully shift results in either direction.

The only genuinely safe choice

Given everything above — that estimates carry real uncertainty, that impairment begins well before any legal threshold, and that individual factors can shift actual BAC in either direction from what a formula predicts — the most reliable safety guidance is also the simplest: decide how you’re getting home before you start drinking, not after. A designated driver, rideshare, taxi, or staying overnight all remove the need to estimate anything at all. If driving is even a possibility after drinking, the only BAC that removes all uncertainty and all risk is 0.00%.