Body fat calculator

Estimating body fat without a scale

Body-fat percentage tells you how much of your weight is fat versus lean tissue — muscle, bone, organs and water. It is a far more useful health signal than weight alone, because two people of the same weight can have very different body compositions. The catch is that measuring it precisely usually means a DEXA scan or skinfold callipers. The US Navy circumference method sidesteps that: with a soft tape measure and a few readings, it gives a solid estimate you can repeat at home.

How the math works

The method feeds your circumferences and height into a logarithmic regression that the US Navy fitted to real body-composition data. All measurements are converted to centimetres first, and the logarithms are base-10. The equations are:

• Men: %fat = 495 / (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450
• Women: %fat = 495 / (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450

The waist-minus-neck term captures how much your midsection exceeds your frame, while the height term normalises for body size. Because the formula takes a logarithm of waist − neck, your waist must be larger than your neck (and waist + hip larger than neck for women) — otherwise the result is undefined, and this tool shows a friendly error instead.

A worked example

Take a man who is 175 cm tall with a 38 cm neck and an 85 cm waist. Then log₁₀(85 − 38) = log₁₀(47) ≈ 1.672 and log₁₀(175) ≈ 2.243. Plugging in: 495 / (1.0324 − 0.19077 × 1.672 + 0.15456 × 2.243) − 450 ≈ 16.9% — placing him in the fitness range on the table below.

Body-fat ranges (American Council on Exercise)

Ranges are general guidance for adults and vary with age, ethnicity and athletic background. The circumference method is an estimate; expect a margin of roughly ±3–4 percentage points versus a lab measurement.