Body fat calculator

The US Navy method (Hodgdon & Beckett, 1984) regresses body density on the log of body circumferences, then converts density to fat percentage. Men use neck and waist; women add the hip. All measurements are in centimeters — imperial inputs are converted before the formula runs.

Men:
  %BF = 495 / (1.0324 - 0.19077·log10(waist - neck)
                      + 0.15456·log10(height)) - 450

Women:
  %BF = 495 / (1.29579 - 0.35004·log10(waist + hip - neck)
                       + 0.22100·log10(height)) - 450

(all circumferences and height in cm; log10 is base-10)

How to measure. Use a flexible tape, pulled snug but not compressing the skin, with the body relaxed. Neck: just below the larynx. Waist: at the navel for men, at the narrowest point for women. Hip (women): the widest point of the buttocks. Measurement error is the biggest source of inaccuracy here, so take each reading twice and keep your method consistent between check-ins.

Fat mass and lean mass. If you enter your body weight, the calculator multiplies it by the body-fat percentage to split your weight into fat mass and fat-free (lean) mass. Lean mass is what you're trying to keep while cutting, so watching it hold steady is more useful than watching the scale alone.

Accuracy. Validation studies put the circumference method at roughly ±3-4% body fat compared with hydrostatic weighing or DEXA for typical adults. It tends to lose accuracy at the extremes — very lean or very high body-fat individuals — and for heavily muscled physiques where waist size doesn't track fat. Treat the number as a screening estimate and a trend line, not a clinical readout.