Body Fat Calculator

Body fat percentage is not BMI

Body fat percentage describes the share of body weight estimated to be fat. BMI is weight divided by squared height, not a percentage. The CDC explains that BMI does not directly measure body fat. This page calculates a body-fat estimate using a named equation and labels BMI separately.

Two adult estimation methods

The circumference option uses the historical equations commonly called the US Navy method. Both need height, neck and a body circumference; the female equation also needs hip circumference. Weight determines the displayed mass breakdown, not the circumference percentage. For the male equation, abdomen refers to the measurement at the navel; the female equation uses waist at its narrowest point and hip at its fullest point. Keep abdomen/waist and hip measurements horizontal. Measure neck below the larynx, keeping circumferences snug without compressing skin. A circumference-method study in men describes the navel and neck sites. Do not mix measurement sites between equations.

The historical inch-based equations are reproduced in a 2022 Naval Postgraduate School research report, page 17 of the report (PDF page 33). This citation documents the equations, not current Navy policy or an endorsement. No official tape-rounding, pass/fail standards or military certification is applied here.

Male: BF% = 86.010 x log10(abdomen - neck) - 70.041 x log10(height) + 36.76
Female: BF% = 163.205 x log10(waist + hip - neck) - 97.684 x log10(height) - 78.387

These coefficients require inches. Metric entries are converted to inches before the same equation is evaluated, so changing units does not select a different model. Every log10 argument must be positive. Impossible percentages are rejected rather than silently clamped to a plausible-looking number.

BMI-based body fat estimate

The alternative uses the adult equation from Deurenberg, Weststrate and Seidell (1991):

BF% = 1.20 x BMI + 0.23 x age - 10.8 x s - 5.4
BMI = weight in kilograms / height in meters squared

Here s is 1 for the male coefficient and 0 for the female coefficient used in the original study. The selection describes the published equation, not gender identity or a personalized assessment of which equation is appropriate. This tool supports adults aged 18-83 and does not apply the paper's separate child equation. Outside that age range it returns a limitation rather than extrapolating.

The study reported an adult standard error of estimate of about 4.1 percentage points and some overestimation among people with obesity. That group-level statistic is not a personal confidence interval or a promise that your error is within that amount. Predictions can differ from measured body composition; a clinician can advise whether an assessment is useful in your circumstances.

The sampled BMI range in the 1991 paper was 13.9-40.9. This tool does not extrapolate the BMI-based prediction outside that range; a circumference result may still appear, with the BMI-based cross-check marked unavailable.

Fat mass and fat-free mass

fat mass = weight x BF% / 100
fat-free mass = weight - fat mass

Fat-free mass includes more than muscle, so it must not be read as measured muscle mass. The chart and mass values follow the selected estimate, not an average of the two methods. A BMI-based cross-check is shown separately when it produces a usable percentage. It is not used to validate the circumference estimate.

Worked example and measurement consistency

For the example male circumference entries of 70.5 inches height, 37.5 inches abdomen and 19.5 inches neck, the equation gives about 15.3% body fat. At 152 pounds, that corresponds to about 23.2 pounds of estimated fat and 128.8 pounds of fat-free mass. Changing weight alone changes those mass amounts, not the circumference percentage. The age-25 BMI-based estimate is about 15.4%; agreement in one example does not prove accuracy for another person.

Repeat measurements at consistent sites and under similar conditions when comparing your own readings. A small tape-placement difference can move the equation's answer. Switching between BMI-based and circumference methods changes the model, so it should not be interpreted as a physical change in body fat.

Limits and related tools

This is an educational estimate, not a diagnosis, pregnancy assessment, recommended target or instruction to lose weight. It does not determine an ideal body-fat percentage, prescribe a diet, label a fitness category or certify military readiness. Neither method directly measures body composition. Do not make medical or eating decisions from a single calculator value.

For a distinct height-weight screening calculation, use the adult BMI calculator. It answers a different question and cannot substitute for body-composition measurement.

Frequently asked questions

Can I calculate without a tape measure?

The BMI-based option uses height, weight, age and the chosen coefficient, without neck, abdomen or hip inputs. It is still a statistical prediction, not a direct measurement.

Why is hip hidden for the male equation?

The historical male equation does not use it. Hidden inputs do not affect the result or validation.

Does the chart show measured fat?

No. It illustrates the selected equation's estimated split of body weight into fat and fat-free mass.

Why did the calculator reject my inputs?

Check the units, age range and measurement sites. Neck and the relevant circumference combination must permit a positive logarithm argument. Results outside 0-100% are not displayed as valid estimates.