THE NUMORIX GUIDE
How to use the Body Composition Calculator
Last reviewed September 14, 2026
What this calculator does
BMI is weight / (height in meters squared).
Formula and method
BMI is weight / (height in meters squared). For body fat, the male branch uses 495 / (1.0324 - 0.19077 * log10(waist - neck) + 0.15456 * log10(height)) - 450; the female branch substitutes waist + 15 - neck and its own constants. Fat mass is weight * body-fat percentage / 100, and lean mass is weight minus fat mass.
Variables and inputs
The UI takes height, weight, waist, and neck in centimeters and kilograms, plus gender and age. Age is stored and displayed as an input but is not used by this engine. The result reports BMI, body-fat percentage, fat mass, lean body mass, and a gender-specific category.
Worked example
For a male input of 170 cm, 70 kg, 80 cm waist, and 38 cm neck: BMI = 70 / 1.7^2 = 24.2; the engine's logarithmic branch gives 13.7% body fat; fat mass = 70 * 0.137 = 9.6 kg; lean mass = 60.4 kg.
How to interpret the result
The outputs are estimates of different quantities: BMI uses total mass, while fat and lean mass are inferred from tape measurements. The category is based on hardcoded body-fat bands, not a diagnosis or a complete assessment of health or fitness.
Common mistakes to avoid
Use centimeters and kilograms as the labels require, measure waist and neck consistently, and do not expect changing age alone to change the result because age is unused. Do not confuse lean body mass with muscle mass or BMI with body-fat percentage.
Assumptions and limitations
The body-fat equations are empirical and sensitive to tape placement, body shape, and unit calibration. The engine directly applies constants to the UI's centimeter inputs, rounds intermediate outputs, does not validate logarithm domains, and ignores age. BMI and body-fat categories may be misleading for athletes, pregnancy, older adults, or medical conditions.