How Body Fat Percentage Is Calculated

Body fat percentage tells you what proportion of your weight is fat rather than muscle, bone, organs, and water. Unlike weight or BMI, it describes what your body is actually made of. This guide covers how the tape-measure method estimates it, with a worked example you can follow.

Why a tape measure can estimate body fat at all

It seems unlikely that measuring your neck and waist could reveal your body composition. The reason it works is statistical rather than physical: across large populations, where your circumferences sit relative to your height correlates well with how much fat you carry. Fat tends to accumulate in predictable places, so the gap between a fat-storing site (the waist) and a lean-staying site (the neck) carries real information.

That's also the method's built-in limitation. It isn't measuring your fat — it's predicting it from a pattern most bodies follow. Bodies that don't follow the pattern get less accurate estimates, which is covered in how accurate the Navy method is.

The formula

We use the U.S. Navy circumference method, developed by Hodgdon and Beckett at the Naval Health Research Center in 1984. All measurements are in centimetres:

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

Two differences stand out. The women's version includes the hip measurement and uses different coefficients, because female fat distribution differs enough that a single equation for everyone would be meaningfully less accurate. That's why our Body Fat Calculator asks women for a hip measurement and men for only neck and waist.

The log10 terms are what make the relationship fit real bodies — the association between circumference and fat isn't a straight line.

A worked example

Take a man 170 cm tall, with a 38 cm neck and an 85 cm waist.

Step 1 — the circumference difference. Waist minus neck: 85 − 38 = 47 cm. Its logarithm is log10(47) = 1.6721, and for height, log10(170) = 2.2304.

Step 2 — build the denominator:

1.0324 − (0.19077 × 1.6721) + (0.15456 × 2.2304)
= 1.0324 − 0.31900 + 0.34472
= 1.05815

Step 3 — finish the calculation:

495 / 1.05815 = 467.81
467.81 − 450 = 17.8

So the estimate is 17.8% body fat.

For a woman 165 cm tall with a 32 cm neck, 74 cm waist, and 98 cm hip, the sum 74 + 98 − 32 = 140 runs through the women's equation to give about 28.4%. Male and female figures aren't directly comparable — women carry more essential fat as a matter of normal physiology, so the same number means different things.

Working in inches

You don't have to measure in centimetres. Enter feet, inches, and pounds and the calculator converts to metric before applying the formula, so the result is identical either way. If you're calculating by hand, convert first — the coefficients above only work in centimetres.

Why we don't show you a category

Most body fat calculators put your result in a band: "athlete", "fitness", "average", and so on. We deliberately don't.

Those bands vary between organisations, and we haven't identified a citable, authoritative source for them that we're willing to stand behind. Rather than invent plausible-looking cutoffs — or quietly borrow another site's — we show the percentage with its confidence level and leave it at that. If we add bands later, they'll come with a named source.

The bottom line

The Navy method estimates body fat from your height and a few circumferences, using an equation validated against real body-composition data. It takes two minutes and no equipment beyond a tape measure.

Because everything depends on those tape measurements, taking them accurately matters more than most people realise. Calculate yours on the Body Fat Calculator.