Body Fat Calculator

The US Navy method

The Navy circumference method estimates body fat from height and circumference measurements: neck and waist for men, plus hips for women.

It was developed for the US Navy in the 1980s as a practical screening tool requiring only a tape measure, and it remains in use for that purpose.

The equations are logarithmic and unintuitive, which is why they are always implemented in a calculator rather than done by hand. What matters for the user is that the inputs are circumferences, and circumferences are where the error comes from.

Accuracy, stated honestly

The Navy method carries a standard error of roughly three to four percentage points against hydrostatic weighing. That means a reading of 22% might reasonably be anything from 18% to 26%.

It performs worst at the extremes: overestimating in very lean individuals and underestimating in very obese ones, which are the groups most likely to be checking.

It also cannot account for individual differences in where fat is stored or in frame size. Someone with a naturally thick neck reads leaner than they are, because neck circumference enters the equation as a proxy for lean mass.

Measuring consistently

Neck is measured below the larynx, with the tape sloping slightly downward at the front. Waist is measured at the navel for men and at the narrowest point for women, depending on the protocol version.

Hips are measured at the widest point. All measurements are taken at the end of a normal exhalation, standing relaxed, with the tape snug but not compressing.

Small errors matter more than people expect. A centimetre of error in waist circumference shifts the estimate by roughly one percentage point. Measuring twice and averaging, at the same time of day, is what makes readings comparable to each other.

The reference ranges

For men, roughly 6% to 13% is athletic, 14% to 17% fit, 18% to 24% acceptable, and 25% and above is classified as obese.

For women the ranges are higher because essential fat is higher: roughly 14% to 20% athletic, 21% to 24% fit, 25% to 31% acceptable, and 32% and above obese.

Essential fat, the minimum required for physiological function, is around 3% for men and 12% for women. Sustained levels near those floors carry real consequences including hormonal disruption, and the very low figures quoted by physique athletes are held for days rather than as a way of living.

The other methods

DEXA is the practical reference standard and costs perhaps £70 to £200 privately. Air displacement and hydrostatic weighing are comparable in accuracy and less available.

Skinfold calipers in experienced hands land within three to four percentage points, similar to the Navy method, and are considerably worse when self-administered.

Bioelectrical impedance, in scales and handheld devices, is convenient and heavily affected by hydration, recent food, recent exercise and skin temperature. Readings can vary several percentage points within a single day, which is why the trend measured under consistent conditions is the only part worth attending to.

One point about interpretation that is easy to lose sight of. Body fat percentage is a ratio, so it moves when either the fat or the lean mass changes. Someone gaining muscle at a stable fat mass sees their percentage fall without losing any fat at all, and someone losing muscle sees it rise without gaining any. Which means the percentage on its own can tell a story the underlying tissue is not telling, and tracking body weight alongside it is what separates the two.