THE NUMORIX GUIDE
How to use the Body Fat Calculator
Last reviewed September 14, 2026
What this calculator does
For the male option the engine uses 86.
Formula and method
For the male option the engine uses 86.010 * log10(waist - neck) - 70.041 * log10(height) + 36.76. For the female option it uses 163.205 * log10(waist + hip - neck) - 97.684 * log10(height) - 78.387. It clamps the result to 0-60%, then multiplies weight by that percentage to estimate fat and lean weight.
Variables and inputs
The UI uses pounds and inches for weight, waist, neck, height, and the female-only hip measurement; age is displayed but not used. Male inputs require waist > neck, female inputs require waist + hip - neck > 0, and invalid nonpositive inputs return zero values.
Worked example
For the male defaults of 180 lb, 36 in waist, 16 in neck, and 70 in height: body fat = 86.010 * log10(20) - 70.041 * log10(70) + 36.76 = 19.4%; fat weight = 180 * 0.194 = 35.0 lb; lean weight = 145.0 lb.
How to interpret the result
The result is a tape-measure estimate, not a direct scan or laboratory measurement. Fat weight and lean weight are partitions of the entered body weight using the estimated percentage; lean weight should not be read as skeletal muscle alone.
Common mistakes to avoid
Keep all measurements in inches and weight in pounds, use the same waist landmark each time, and include hip measurement only when using the female branch. Do not let age or a visually judged body shape substitute for the required measurements.
Assumptions and limitations
The equations are sensitive to tape placement, posture, hydration, body shape, and the population from which they were derived. Age is unused, outputs are clamped to 0-60%, and invalid inputs can silently produce zeros. Different methods such as DXA or bioimpedance may disagree, so compare trends measured with the same method.