Blood Alcohol Content, written as BAC and read as a percentage, is the mass of ethanol in grams per 100 mL of blood, and it is the number printed on a breathalyzer result and cited in every DUI statute. The standard way to calculate BAC from what you drank, your body weight, your sex, and the time that has passed is the Widmark equation, published in 1932 by Swedish researcher Erik M. P. Widmark and still the foundation of modern forensic alcohol estimates. The formula is BAC% = (number_of_standard_drinks × 14 g × 5.14) ÷ (body_weight_in_kg × Widmark_ratio) − (0.015 × hours_since_first_drink). A US standard drink contains 14 g of pure alcohol, the body-weight term controls how diluted the alcohol becomes inside the water of your tissues, and the Widmark ratio accounts for the fact that women carry a lower fraction of body water than men, so the same drinks push their BAC higher. The 0.015 factor per hour reflects the average rate at which a healthy liver clears alcohol from the bloodstream, and nothing speeds that up.
Project managers who run on tight deadlines often ask whether BAC has a meaning inside their field. In project management, BAC is an unrelated acronym for Budget at Completion, the total planned cost of all work scheduled on a project. That acronym and the Blood Alcohol Content calculation share three letters only by accident, so the calculator described here solves the alcohol question, not the cost-management question. A dedicated BAC Calculator applies the Widmark formula for you, and a parallel guide at How to Calculate BAC from Drinks, Weight, Sex and Time walks through the same arithmetic on paper.
The Widmark Formula Explained
The Widmark model treats the body as a single compartment of water in which alcohol distributes at a fixed ratio. For an average adult male that distribution ratio is about 0.68, meaning roughly 68% of his body mass behaves like water that alcohol can dissolve into. For an average adult female the ratio is lower, near 0.55, because women tend to carry a smaller share of body water and a larger share of fat for a given body weight. Plugging those constants into the equation is what produces the sex difference you see in any breathalyzer estimate.
| Input | Value or convention |
|---|---|
| Standard drink (US) | 14 g of pure alcohol |
| Conversion constant | 5.14 (g/dL to % w/v units) |
| Widmark ratio, male | 0.68 |
| Widmark ratio, female | 0.55 |
| Average elimination rate | 0.015% BAC per hour |
| Body weight input | kilograms (pounds must be converted first) |
The 14 g drink size is defined by the US National Institute on Alcohol Abuse and Alcoholism, the 5.14 conversion constant reconciles metric grams with the percent-units that appear on a BAC readout, and the 0.015-per-hour elimination rate is the long-cited population average used in forensic toxicology. These constants are the reason a calculator can produce a defensible estimate in a fraction of a second.
How to Use the BAC Calculator
The BAC Calculator applies the Widmark formula with the official constants, so the only thing you have to do is enter four values and read the result. The steps below line up with the inputs the tool expects.
- Pick your sex with the Male / Female toggle so the correct Widmark ratio (0.68 or 0.55) is used for the calculation.
- Enter the number of US standard drinks you have had, where each drink counts as 14 g of pure alcohol regardless of the glass size.
- Enter your body weight in kilograms, converting from pounds first if needed (1 lb = 0.4536 kg).
- Enter the number of hours since your first drink, which is used to subtract 0.015% per hour of liver elimination.
- Read the estimated Blood Alcohol Content, which updates instantly as you change any input, and never use that number to decide whether to drive.
Because the result is recomputed on every keystroke, you can also use the tool to compare scenarios: a second drink in the same hour shifts the peak BAC upward, and each additional hour drops it by roughly 0.015% as the liver clears alcohol at its fixed pace.
A Worked Example You Can Verify
Take a 75 kg male who has had 3 standard drinks (42 g of alcohol) over the last 2 hours. The Widmark calculation works out as BAC% = (3 × 14 × 5.14) ÷ (75 × 0.68) − (0.015 × 2). The numerator is 3 × 14 × 5.14 = 215.88, the denominator is 75 × 0.68 = 51.00, so 215.88 ÷ 51.00 = 0.2329% before any time adjustment. Subtracting the elimination term, 0.015 × 2 = 0.030%, gives a final estimate of 0.2329% − 0.030% = 0.203% BAC. The same person four hours later, after no additional drinks, would fall by another 4 × 0.015 = 0.060%, landing near 0.143%.
Switch the scenario to a 60 kg female with the same 3 drinks and the same 2 hours, and the only change is the Widmark ratio. BAC% = (3 × 14 × 5.14) ÷ (60 × 0.55) − (0.015 × 2). The denominator becomes 60 × 0.55 = 33.00, so 215.88 ÷ 33.00 = 0.6541%, and subtracting 0.030% leaves 0.624% BAC. The estimate is roughly three times the male result for identical drinks, which matches the practical observation that lower body weight and a smaller body-water fraction push the same dose of alcohol to a much higher peak.
What the BAC Number Means in Practice
Most US states set the per-se DUI threshold at 0.08% BAC for drivers 21 and older, with lower limits such as 0.04% for commercial drivers and 0.02% for under-21 drivers. Below 0.02% most people show no detectable impairment, while the 0.03% to 0.12% band brings measurable changes in reaction time, judgment, and coordination. Above 0.15% gross motor control and balance are usually compromised, and past 0.30% the risk of alcohol poisoning becomes medically significant. These bands are population averages from peer-reviewed toxicology, not personal guarantees, which is exactly why estimates should never replace a certified breath test.
Limits of the Widmark Estimate
The Widmark formula is a population model, so it cannot account for individual factors that change how alcohol distributes and clears. Recent food in the stomach slows absorption and can lower the peak BAC reached from the same drinks. Hydration, sleep debt, certain medications, liver disease, and habitual heavy drinking can shift the elimination rate above or below the 0.015-per-hour average. Beer poured generously into a tall glass can quietly contain more than 14 g of alcohol, and mixed drinks often carry two or three times the standard drink amount. The calculator treats every input as exact, so an honest estimate depends on honest counts.
How Long Until You Are Back to Zero
A useful shortcut once you have a BAC number is to divide it by 0.015 to get the number of hours required to reach zero at the average elimination rate. A peak of 0.12% therefore needs about 8 hours of full metabolism, which is why morning-after driving can still produce a positive reading after a late-night bar visit. Because the 0.015 figure is itself an average, real people sometimes eliminate slightly faster and sometimes slower, and only a certified chemical test can confirm that you are below a legal limit.
Frequently Used Conversions Around the Calculation
Two conversions show up every time the formula is run by hand. Pounds to kilograms is 1 lb = 0.453592 kg, so a 170 lb person weighs about 77.11 kg. Fluid ounces of beer to grams of alcohol depends on the alcohol by volume: 12 fl oz of 5% beer contains roughly 14 g of pure ethanol, the same 14 g that defines a US standard drink. Keeping those two conversions in mind lets you reproduce any calculator result on a piece of paper if you ever need to double-check the arithmetic.
For readers who want to compare the alcohol estimate with other body metrics, the BMI Calculator covers weight in the same kilograms input, and the Calculate Your BMR in Seconds with Two Trusted Formulas guide explains the resting metabolism side of why bodies process alcohol differently. The How to Calculate Your Blood Alcohol Content Accurately article adds a longer-form walkthrough of the same Widmark inputs.
Safety Note
A BAC estimate, however carefully computed, is an approximation and not a legal or medical measurement. Real BAC varies with food intake, hydration, fatigue, medications, and individual liver function, and a calculated zero can still correspond to a real reading above zero. The only safe choice after drinking is to arrange a sober ride, a taxi, or a rideshare, and the only reading that matters in court is the one from an approved evidential device.
Related reading: How to Calculate Online Percentage in Your Browser.