BAC Calculator

Estimate blood alcohol concentration from body weight, sex, how many standard drinks you have had and how long ago you started. The estimate uses the Widmark formula and runs entirely in the page — nothing is uploaded and nothing is stored.

Important: Not Medical Advice

This page is an educational calculator, not a medical device, and it does not diagnose, treat or prevent any condition. Its output is a general estimate built from average population figures; it ignores individual factors such as age, body composition, pregnancy, medication and medical history. Always speak to a qualified clinician about your own situation before acting on any number shown here.

Estimating Alcohol in the Bloodstream

Blood alcohol concentration is the mass of alcohol per unit volume of blood, written as a percentage in the United States and in milligrams per 100 millilitres almost everywhere else. It cannot be measured without a breathalyser or a blood test, so everything a webpage can offer is a model. The model here follows the standard one: a fixed amount of alcohol per drink, distributed through a volume that scales with body weight, with a fixed rate of elimination over time.

• Four inputs. Sex and body weight determine how far a given amount of alcohol is diluted; the number of standard drinks determines how much was consumed; hours since the first drink determines how much has been cleared.
• Weight can be entered in pounds or kilograms. Pounds are converted at 0.453592 kilograms to the pound before the calculation.
• The result is a figure to three decimal places, an impairment level from four bands, and a plain statement of the method and its assumptions underneath.
• The whole calculation is a handful of arithmetic operations evaluated in the browser, which is why it works with no connection once the page has loaded.

The Widmark Formula Behind the Figure

1. Each drink is counted as 14 grams of pure alcohol — the United States standard drink, equivalent to a 12-ounce beer at 5%, a 5-ounce glass of wine at 12%, or a 1.5-ounce shot of 40% spirits. Enter a count in those units, not in glasses of whatever you actually poured.
2. Body weight is converted to grams, then multiplied by a distribution factor: 0.73 for a male body and 0.66 for a female body. That factor reflects the proportion of body mass in which alcohol distributes, and it is the main reason two people of the same weight can reach different concentrations from the same drinking.
3. Concentration is computed as drinks multiplied by 14 and by 100, divided by the product of body weight in grams and the distribution factor. Multiplying by 100 converts the fraction to a percentage.
4. Elapsed time is handled by subtracting 0.015 percentage points for every hour since the first drink, with the result floored at zero so the estimate cannot go negative.
5. The resulting figure is then classified: at or above 0.08 as High and legally impaired, at or above 0.05 as Moderate, at or above 0.02 as Mild, and below that as Low.

Sample Run: Four Drinks, Two Hours, 70 kg

Take a 70 kilogram male who has had four standard drinks over the course of an evening and is checking his estimate two hours after the first one.

• Four drinks is 56 grams of alcohol. Distributed across a body mass of 70,000 grams and a factor of 0.73, that gives 0.109589 — displayed as 0.110%, comfortably above the 0.08 limit.
• Two hours of elimination at 0.015 per hour removes 0.030, bringing the estimate to 0.079589.
• The page displays 0.080% but classifies the level as Moderate. This is not a bug in the display: the figure is rounded to three decimals for reading while the classification compares the unrounded value against 0.08, and 0.0796 is below it. It is a good illustration of why these estimates should not be read to the third decimal place.
• Two more hours brings it to 0.0496, displayed as 0.050% and classified as Mild. The limit is crossed somewhere around the three-hour mark, and the useful answer from an estimate like this is the approximate hour rather than the concentration.

Why the Level and the Number Can Disagree

The figure shown and the impairment band applied to it are computed from slightly different values. This is worth understanding because it happens near every threshold, and the thresholds are exactly where people look most carefully.

• The displayed figure is rounded to three decimal places; the band is decided by comparing the unrounded value. A reading of 0.080 can therefore be labelled Moderate, and a reading of 0.050 can be labelled Low.
• The elimination term is linear, but real elimination does not start at the first sip. Absorption continues for a while after drinking stops, which means a reading taken shortly after the last drink may still be rising.
• A negative result is impossible by construction. Elapsed time is subtracted and floored at zero, so a long gap reports zero rather than the small negative number the formula would otherwise produce.
• The output says nothing about peak concentration, which may already have passed, and nothing about when the estimate will reach a given level.
• The tool refuses to calculate if weight or drink count is zero, which prevents a division by zero from producing an infinite figure.

What No Calculator Can Account For

Alternative	Notes
Food in the stomach	Not represented at all. Eating slows absorption and lowers peak concentration, sometimes substantially. Two people with identical inputs can differ by a wide margin purely on whether they ate.
Speed of drinking	The formula treats drinks as a total and time as hours since the first one. Four drinks in one hour and four drinks across four hours produce very different concentrations at the halfway point, and only the second figure is well modelled here.
Body composition	The sex factor is a population average. Two people of the same weight with different proportions of fat and muscle tissue will not reach the same concentration.
Medication, illness and liver function	Anything affecting metabolism shifts the elimination rate, which is fixed at 0.015 per hour here. Some medications and conditions change this considerably.
Tolerance and measurement error	Neither changes the concentration, but tolerance changes how impaired someone feels, which is the opposite of helpful: a person who feels fine may be well over the limit. Breathalysers also carry their own error margins.

Questions About the Estimate

Can I use this to decide whether I am safe to drive?

No. It is an estimate built from five inputs and it omits food, drinking rate, body composition, medication and time since the last drink, all of which can move the real figure well outside the range this tool reports. Any legal limit is a line you stay far away from rather than approach with a calculator, and the only reliable answer comes from a calibrated measuring device operated by someone trained to use it. Nothing on this page should be used to make a driving decision.

Why do the limbs and weight matter so much?

Because the estimate is a ratio. A fixed quantity of alcohol is distributed through a volume that scales with body mass, so the same four drinks produce a higher concentration in a lighter body. The distribution factor does the same job for sex, on the basis that the proportion of body water differs between typical male and female bodies.

How long until the estimate reaches zero?

Divide the current figure by 0.015 to get the hours at the modelled elimination rate. At 0.08 that is about five and a half hours. This is the part of the model people most often underestimate, and it is the reason sleeping for a couple of hours does not reliably resolve the problem — time is the only variable that does, and a rough night's sleep is not the same as an equal number of waking hours.

What are the legal limits?

They vary and they change. The United States federal limit for drivers is 0.08, with Utah at 0.05 and a commercial driver's licence limit of 0.04. Most of Europe uses 0.05, Sweden and Norway use 0.02, and several countries including the Czech Republic and Hungary apply a zero limit. This page does not determine which limit applies to you, and none of its output is a legal measurement.

Is anything I enter stored or sent anywhere?

No. The inputs stay in the page and the arithmetic runs locally, with no account, no saved history and no server call. The trade-off is that the estimate is only as good as the numbers you type, and none of the context surrounding a real drinking session can be captured by a form.

Last verified: October 7, 2026

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Maintained by Huang Bin · Last updated October 3, 2026