Body Fat Calculator

What this calculator does

This calculator estimates body fat percentage using the U.S. Navy circumference method, a tape-measure formula that needs only a few body measurements. You select your sex, choose metric or imperial units, and enter your height plus neck and waist circumference — and, for women, hip circumference. The tool returns an estimated body fat percentage, a descriptive category, and, once you add your body weight, an estimate of fat mass and lean mass. Every value updates the moment you change an input, and all of it is calculated inside your browser.

Why you might need it

Body fat percentage describes the proportion of body weight made up of fat tissue, which scales such as the bathroom variety cannot show. Laboratory methods — underwater weighing, air-displacement plethysmography, DXA scanning — are accurate but require equipment and appointments. The Navy method exists as a practical middle ground: with a soft tape measure you can produce a repeatable estimate at home. People use it to put a number on a measurement they already take, to compare readings taken weeks apart, or simply to understand how circumference figures translate into a percentage. Because this tool shows the formula and the categories openly, it is also a way to see exactly how that translation works.

How to use it

1. Choose your sex. This selects the correct equation.
2. Pick metric (centimetres, kilograms) or imperial (inches, pounds).
3. Enter your height and your neck and waist circumference. Women also enter hip circumference.
4. Enter your body weight so the calculator can derive fat mass and lean mass.
5. Read the estimated body fat percentage, its category, and the mass breakdown. Use Reset to return to the defaults.

Measure the neck just below the larynx, the waist at the navel for men and at the narrowest point for women, and the hips at the widest point — keep the tape level and snug but not compressing the skin.

How it’s calculated

The tool uses the Hodgdon-Beckett equations adopted by the U.S. Navy. All measurements are converted to centimetres first; imperial inputs are multiplied by 2.54. For men the percentage is

BF% = 495 / (1.0324 − 0.19077 · log10(waist − neck) + 0.15456 · log10(height)) − 450.

For women it is

BF% = 495 / (1.29579 − 0.35004 · log10(waist + hip − neck) + 0.22100 · log10(height)) − 450.

Both formulas use base-10 logarithms. Because log10 is only defined for positive numbers, the waist (for men) or the waist plus hip minus neck (for women) must be greater than the neck measurement — otherwise the calculator shows a prompt instead of a number. Fat mass is then body weight multiplied by the percentage, and lean mass is body weight minus fat mass. The descriptive categories — essential, athletic, fitness, average and above average — follow the commonly cited American Council on Exercise ranges, which differ between adult men and women.

Common pitfalls

The biggest source of error is the tape measure itself. Pulling it tight, holding it at an angle, or measuring at a slightly different spot can shift the percentage by a point or more, so consistency matters more than precision on any single reading. Mixing units is another trap — entering inches while the tool is set to metric will produce a wildly wrong figure; the unit toggle changes how every field is interpreted. Finally, remember the method is a statistical model fitted to a population. Very muscular builds and unusual body proportions sit at the edges of that model, where the estimate is least reliable.

Tips and related calculations

For the most useful trend, measure at the same time of day under the same conditions and re-run the calculator periodically rather than reading a single result. Pairing body fat percentage with body mass index gives two different views of the same body — one a ratio of weight to height, the other a tissue proportion. Because all the logarithms and divisions run locally, you can adjust any measurement and watch the result change instantly, and none of the figures ever leave your browser.