The formula
How to calculate body fat percentage
The US Navy method estimates body fat from tape measurements alone. It was developed so that fitness standards could be assessed without laboratory equipment, and it remains the most practical home method.
Measure with the tape snug but not compressing the skin, at the end of a normal breath out, and take each measurement three times to average out error. A centimetre at the waist moves the result by roughly one percentage point.
Here is what each field means:
- Height (cm)
- Waist circumference (cm) — at the navel for men, at the narrowest point for women
- Neck circumference (cm)
- Hip circumference (cm) — used by the female equation only
The result updates on every keystroke. The URL updates too, which makes the filled-in version easy to bookmark or send to someone else.
None of the fields above need specialist equipment to fill in — the inputs are the kind of measurements that can be taken at home with a tape measure or scale, which is exactly why this style of calculation is widely used as a first, approximate check rather than a diagnostic one.
Why body fat percentage matters
People usually look up a body fat percentage calculation to put a number on something they can already sense but cannot quite quantify — how their fitness compares with a reference range, how a measurement is trending, or what a change actually means. A calculator turns that vague sense into a concrete figure, which is useful even though the figure itself is only ever an estimate.
This tends to get checked before or after a change in routine — a new training block, a change in diet, a different stage of a pregnancy — as a way of putting a figure on something that would otherwise only be felt rather than measured.
A single number like this is most useful placed in context rather than read in isolation. Reference ranges exist because bodies vary considerably and still count as healthy across a wide spread of values, so a result sitting outside a commonly quoted range is a prompt to look more closely rather than a verdict on its own. The same figure, tracked consistently over time, tends to say more than any single reading ever will.
A calculator like this one is frequently used to prepare a specific number for an upcoming appointment, so that a description like "I think it has changed" can be replaced with an actual figure and a date it was measured on. Clinicians and trainers generally find a concrete number, however approximate, more useful to work from than a general impression.
Worked example
Work through the defaults on this page:
- Height: 175 cm
- Waist circumference: 88 cm
- Neck circumference: 38 cm
- Hip circumference: 98 cm
That gives:
- Body fat, male equation: 19.21 %
- Body fat, female equation: 29.73 %
- Waist to height ratio: 0.5
Those starting figures exist only to show the calculation working, not as a target or a reference value to compare yourself against — replace them with your own measurements in the calculator above and the same formula applies exactly as shown.
Reading the result
Rough categories for men: 6–13% athletic, 14–17% fit, 18–24% average, 25%+ obese. For women, add about 8–10 points to each band — essential fat is higher for physiological reasons.
Where this goes wrong. The method assumes a typical fat distribution and can be several points out for very lean or very muscular people. Track the trend rather than the absolute number, and treat this as general information rather than a clinical assessment.
As with any general-purpose formula, the result here is an estimate derived from population averages, not a measurement specific to you — a reasonable starting point for a conversation with a qualified professional, not a replacement for one. Nothing on this page is medical advice.
The neck acts as a proxy for frame size and for fat that is not stored abdominally. Subtracting it from the waist isolates the abdominal fat the equation is really measuring.
The answer it gives you is body fat, male equation. With 175 cm height, 88 cm waist circumference and 38 cm neck circumference, that comes to 19.21 %. Change any field and the figure moves with it.
It uses a published formula applied exactly as defined, so the arithmetic itself is precise — but the formula is a population-level estimate, and individual results can vary from it for reasons the formula has no way to account for. Treat the output as a reasonable estimate rather than a measured or diagnostic value.
For most people, yes — a single reading is a snapshot, while the same measurement repeated consistently over weeks or months shows a trend, which is generally more informative than any one figure taken in isolation.
Small differences between calculators usually come down to which published formula or reference dataset each one uses — several equations in this area exist in more than one accepted version. This page states the specific formula used in the section above, so it is possible to check directly whether another source is using the same one.