Body Fat Calculator (US Navy Method)

This body fat calculator estimates body-fat percentage using the US Navy circumference method. Body-fat percentage tells you more than weight alone, because it separates fat from muscle. This calculator uses the US Navy circumference method — height, neck, waist and hip — to estimate it with nothing but a tape measure, then converts it into fat mass and lean mass.

The formula

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 measurements are in centimetres; imperial inputs are converted for you.

Reading the result

The ACE categories give the number context — athlete, fitness, average or high — with different ranges for men and women. Pair body fat with your weight to see lean mass, which is the figure to protect when you lose weight.

Worked example

A man, 180 cm tall, 80 kg, neck 38 cm, waist 92 cm:

waist − neck = 54
log10(54)    ≈ 1.7324 → × 0.19077 = 0.3305
log10(180)   ≈ 2.2553 → × 0.15456 = 0.3486
denominator  = 1.0324 − 0.3305 + 0.3486 = 1.0505
BF%          = 495 ÷ 1.0505 − 450 ≈ 21.2%

That lands in the ACE Average band for men, which runs from 17 to 24%, and at 80 kg it corresponds to roughly 17 kg of fat mass and 63 kg of lean mass. The lean figure is the one to watch across a diet: if your weight falls and lean mass falls with it, you are losing muscle rather than only fat.

Measuring correctly matters more than the formula

The Navy method is a regression fitted to circumference measurements, so its accuracy is bounded entirely by how carefully you take them. A centimetre of error on the waist moves the result by a noticeable fraction of a percent, and inconsistent technique between two sessions will look like a change in body composition when nothing has changed at all.

What keeps it repeatable: measure bare skin rather than over clothing; keep the tape snug but not compressing; breathe out normally before reading the waist; take the neck just below the larynx at its narrowest; take the waist at the navel for men and at the narrowest point for women; take the hip at its widest. Measure at the same time of day, and use the same tape every time.

Notice what the formula actually consumes — waist minus neck for men, and waist plus hip minus neck for women. It is measuring how far your trunk exceeds a baseline set by your frame, which is why a thick neck partly offsets a large waist.

How good is the number, really?

Reasonable for tracking, not diagnostic. Against a DEXA scan the circumference method typically lands within a few percentage points for an average build and drifts further at the extremes: it tends to overestimate very lean people and underestimate those carrying a great deal of weight, because the regression was fitted to a population containing few of either.

That limitation does not make it useless. What matters for most people is the direction of travel, and a tape measure used consistently shows that plainly. Compare yourself against your own previous reading rather than against a category boundary, and read the ACE band as context instead of a verdict. BMI Calculator makes the same trade from a different direction — simpler, and blind to muscle — while Ideal Weight Calculator puts the two side by side.

Private and precise

This calculator produces estimates from published formulas and is not medical advice. For anything about your health, talk to a doctor or a registered dietitian.