Blood Alcohol Content is estimated with the Widmark formula: BAC% = (grams of pure alcohol ÷ (r × body weight in kilograms × 1000)) × 100 − 0.015 × hours since the first drink, where r is the body-water distribution ratio (0.68 for men, 0.55 for women). The word "backing" in "back-end BAC" refers to the elimination phase, the period after drinking stops when the liver metabolizes alcohol at roughly 0.015 percentage points per hour. This free BAC calculator applies that equation the moment you type. Pick the sex toggle, enter standard US drinks (14 g of pure alcohol each), your weight in kilograms, and the hours since you started drinking, and the estimate updates instantly. The number you see is a rough educational figure, not a medical or legal measurement, and it must never be used to decide whether you are fit to drive. If you have been drinking, the safe choice is to arrange a taxi, rideshare, or designated driver instead.
What "Backing" Means in Back-End BAC
The phrase "how to calculate backing for a quilt" overlaps with the BAC term "back-end" in a way that often confuses searchers. In blood alcohol work, back-end BAC refers to the declining phase of alcohol in your bloodstream, the part of the curve after your last drink, when the liver is steadily metabolizing ethanol into acetaldehyde and then acetate. A back-end calculation estimates what your BAC is right now, given how long it has been since your first drink. The Widmark equation handles both the rising peak and the falling tail in a single line of algebra, which is why the same calculator produces a number you can read as either a "peak" or a "backing" estimate depending on the hours input. Three hours of elapsed time does not magically turn the math into something else; it simply subtracts three times 0.015 from the peak figure. The result is clamped at zero because a real blood alcohol reading cannot fall below nothing.
The Widmark Formula Explained
The Widmark formula was developed by Swedish chemist Erik M. P. Widmark in the 1920s and 1930s, and a version of it is still the starting point for back-of-the-envelope BAC estimates nearly a century later, as documented in the Wikipedia entry on blood alcohol content. The implementation used in this tool is:
BAC% = (grams of alcohol / (r × body weight in kg × 1000)) × 100 − 0.015 × hours
Where:
- grams of alcohol = standard drinks × 14 (one US standard drink contains 14 g of pure alcohol)
- r = 0.68 for men, 0.55 for women
- the 0.015 term is the average elimination rate in percentage points per hour
- the result is clamped at zero
A worked example makes the math concrete. Three standard drinks for an 80 kg man one hour after starting:
- Grams of alcohol: 3 × 14 = 42 g
- Peak BAC: (42 / (0.68 × 80 × 1000)) × 100 = (42 / 54,400) × 100 ≈ 0.0772%
- Elimination: 0.015 × 1 = 0.015
- Final estimate: 0.0772 − 0.015 ≈ 0.062%
Two drinks for a 60 kg woman two hours in works out to about 0.055%. Adjusting any single input moves the answer in the direction you would expect: more drinks push it up, a longer wait pulls it down as your body metabolizes the alcohol, a heavier body dilutes the same alcohol across more water, and switching from the male to the female ratio raises the estimate because women on average carry a smaller proportion of body water.
How to Use the BAC Calculator
Using the BAC Calculator is a three-step process that takes less than a minute:
- Pick your sex with the Male / Female toggle so the correct Widmark ratio is used.
- Enter the number of US standard drinks (14 g alcohol each), your body weight in kilograms, and the hours since your first drink.
- Read the estimated Blood Alcohol Content, which updates instantly — and never use it to decide whether to drive.
The calculator runs entirely in your browser. Nothing you type is ever uploaded to a server, so you can experiment with different drink counts and elapsed times without worrying about data leaving your device. For a quick back-end estimate, start with the actual drinks you consumed, set the hours field to the time since your first sip, and read the result. If you want to see how the number falls over the next few hours, keep the drink count fixed and increase the hours field; the BAC will drop at roughly 0.015 per hour until it hits zero, reflecting the body's average metabolic elimination rate.
US Standard Drink Sizes
In the United States, one standard drink contains about 14 grams of pure alcohol, regardless of the container it comes from. The table below shows three common ways to reach that 14 g mark.
| Beverage | Typical serving | Typical ABV | Pure alcohol |
|---|---|---|---|
| Beer | 12 fl oz | ~5% | ~14 g |
| Wine | 5 fl oz | ~12% | ~14 g |
| Spirits (80 proof) | 1.5 fl oz | ~40% | ~14 g |
A 16 oz "tallboy" beer at 5% is closer to 1.3 standard drinks, and a generous 6 oz wine pour is closer to 1.2 standard drinks. Counting accurately matters because the calculator multiplies your drink number by 14 g, so every half-drink miscount shifts the BAC estimate noticeably in either direction.
What the Formula Leaves Out
The Widmark model is deliberately simple, and that simplicity is both its strength and its weakness. The fixed r values (0.68 for men, 0.55 for women) are population averages; real body-water ratios vary with age, fitness, and body composition. The single elimination rate of 0.015 per hour ignores individual liver metabolism, which can range from roughly 0.010 to 0.025 percentage points per hour. Food in the stomach slows absorption; drinking on an empty stomach pushes the peak higher and faster. Hydration, medications, illness, sleep deprivation, and genetics all nudge the real number up or down. Two people with identical inputs into this calculator can end up with very different actual readings, and the only devices that measure true BAC are a calibrated breathalyzer or a laboratory blood test. For a deeper look at the factors the Widmark equation cannot capture, see the guide on what affects your real BAC beyond the Widmark formula.
Why You Must Never Use This to Drive
This calculator is an educational estimate only. It must never be used to judge whether you are under a legal limit or fit to drive. Legal limits vary by country, state, and circumstance, and they are enforced with breathalyzers and blood tests, not with a website. The Widmark estimate cannot account for the many factors that change real BAC, and a reading that looks "safe" on screen can easily be wrong by a factor of two or more. If you have been drinking, the safe answer is simple: do not drive. Arrange a taxi, rideshare, public transit, or a sober driver instead. This tool is not medical or legal advice, and it does not replace professional judgment or a calibrated measurement.
For a deeper look, see Accurate BAC Calculator: How Reliable Is the Result?.