📏 Body Fat Percentage Calculator

Estimate body fat percentage using the U.S. Navy circumference method — just a tape measure needed. Accuracy is roughly ±3-4% compared to lab methods, but it's a solid free estimate.

Units

Metric (cm) Imperial (in)

Sex

Height (cm)

Neck (cm)

Waist (cm)

Hip (cm, women only)

The Navy circumference formula

This is the Hodgdon-Beckett equation the U.S. Navy uses for body composition screening. It fits a curve through tape measurements, so every input is converted to inches first (centimetres are divided by 2.54):

men:   BF% = 86.010 × log₁₀(waist - neck) - 70.041 × log₁₀(height) + 36.76
women: BF% = 163.205 × log₁₀(waist + hip - neck) - 97.684 × log₁₀(height) - 78.387

A man at 170 cm with a 38 cm neck and 85 cm waist becomes 66.93 in, 14.96 in and 33.47 in, and the equation returns 17.9% - the "Fitness" band. The hip field appears only for women, because their equation is the one that uses it.

The tape is the whole measurement

Half an inch of sloppy tape work moves the answer by more than a percentage point, so measure the same way every time:

Neck just below the larynx, tape sloping slightly down at the front. Waist at the navel for men, at the narrowest point for women. Hip at the widest part of the buttocks.
Snug, not tight, on bare skin, at the end of a normal breath out.

Expect roughly ±3-4 percentage points against a DEXA scan. The formula assumes fat sits where it usually does, so it misreads people who carry weight unusually. Track the trend across weeks rather than one reading.

Frequently asked questions

What is a good body fat percentage?

The tool uses the common fitness bands: for men, under 14% athletic, 14-18% fitness, 18-25% acceptable. For women, under 21% athletic, 21-25% fitness, 25-32% acceptable. Women carry more essential fat, so the bands sit higher.

Why does it say my waist must be greater than my neck?

The formula takes the logarithm of waist minus neck, which has no answer at zero or below. It almost always means a measurement was typed in the wrong box or in the wrong unit.

Is the Navy method as accurate as calipers?

Roughly comparable, and more repeatable, because a tape is easier to use consistently than a skinfold pinch. Both trail DEXA, which measures composition directly.