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Body Fat Calculator

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Estimate body fat percentage two ways: the US Navy tape method, which uses your neck, waist and — for women — hip measurements, or the Deurenberg equation, which needs only height, weight, age and sex. Both are population regressions rather than measurements, so the result is shown as a band, and the other method's answer sits alongside it for comparison.

Method

Uses circumference measurements. The better of the two for most people, because a tape around the waist responds to the fat that is actually there, where a weight-based formula cannot tell muscle from fat.

Sex
Units
years
cm
kg
cm

Just below the larynx.

cm

At the navel.

Taking the measurements

The tape method is only as good as the tape work. Measure on bare skin, keep the tape horizontal and snug without compressing, and take each measurement twice — if the two disagree by more than half a centimetre, take a third.

Neck
Just below the larynx, with the tape sloping slightly down at the front. Do not flare the shoulders or tense the neck.
Waist (men)
Horizontally at the navel, at the end of a normal breath out. Not pulled in, and not at the narrowest point.
Waist (women)
Horizontally at the narrowest point of the torso, usually just above the navel, at the end of a normal breath out.
Hip (women)
Horizontally around the widest part of the hips and buttocks, with feet together.

How to use the body fat calculator

  1. 1Choose a method. The tape method is more informative, because a tape responds to where fat actually is; the BMI method needs no tape at all.
  2. 2Set your sex and units, then enter age, height and weight. The equations were fitted on adults, so they are refused below 18.
  3. 3For the tape method, measure your neck just below the larynx and your waist horizontally — at the navel for men, at the narrowest point for women. Women also need the hip measurement, taken at the widest point.
  4. 4Read the estimate as the band it is shown as, not as the single figure. Compare it against the other method's answer underneath.
  5. 5Repeat in a few weeks with the same method and the same tape. The trend is far more reliable than any one reading.

Examples

Man, tape method

Input
180 cm, 80 kg, waist 90 cm, neck 40 cm
Result
18.4% — average band, 14.7 kg fat, 65.3 kg lean

Roughly 14–22% once the uncertainty of the method is allowed for.

Woman, tape method

Input
165 cm, 62 kg, waist 75 cm, hip 95 cm, neck 32 cm
Result
27.4% — average band

Women's bands sit higher than men's throughout; essential fat alone is roughly twice as high.

The same person without a tape

Input
180 cm, 78 kg, age 30, male, BMI method
Result
19.6%

Close here, because the build is unremarkable. On a heavily muscled person the two diverge sharply.

A measurement in the wrong unit

Input
Height entered as 1.8 rather than 180
Result
An explanation that the figure looks like metres, rather than a nonsense percentage

About the body fat calculator

Why circumferences predict fat at all

The US Navy equations, published by Hodgdon and Beckett in 1984, exploit a simple observation: fat accumulates preferentially around the abdomen, while the neck stays comparatively lean. The difference between a waist and a neck therefore tracks body fat better than either measurement alone, and adding height accounts for the fact that the same girth means different things on different frames.

The result is a regression — an equation fitted so that its output matches, on average, what hydrostatic weighing found for the people in the study. It predicts a group mean. An individual whose fat sits mostly on the hips and thighs rather than the abdomen will read lower than they are, and the equation has no way to know.

What the BMI route inherits

Deurenberg's 1991 equation converts BMI, age and sex into a fat percentage. It needs nothing but a set of scales, which is its whole appeal, and it carries every weakness BMI has: it cannot distinguish muscle from fat, so an athlete reads as overweight and a light, untrained person reads leaner than they are.

It is included here because it is genuinely useful as a cross-check rather than as a primary method. When it agrees with the tape method, the tape result is more believable. When it disagrees sharply, that gap is telling you something about your build that neither number alone would.

Use the trend, not the reading

Every method here carries a measurement error that is larger than the change you would expect in a fortnight. Read a single result as a position — around a fifth, around a quarter — rather than a precise value, and put the effort into consistency instead of precision: same method, same tape, same time of day, same point in the week.

Tracked that way the errors largely cancel, because they repeat in the same direction each time. A waist that has come down two centimetres over two months is a real change even if the absolute percentage attached to it is uncertain. That is the reading worth acting on.

What none of this measures

Body fat percentage says nothing about where the fat is, and location matters more to health than quantity. Visceral fat around the organs behaves very differently from subcutaneous fat, and two people at the same percentage can carry very different risk.

Waist circumference on its own — which you have already measured for the tape method — is a better predictor of metabolic risk than body fat percentage in much of the research, and it needs no equation at all. If you take one number from this page to a doctor, that is a good candidate.

Frequently asked questions

How accurate is this?
Treat it as ±3 to 4 percentage points. The tape equations were validated against hydrostatic weighing and typically land within about that range of it for most people, with larger errors at the extremes — very lean and very heavy individuals both read poorly. The reference methods disagree with each other by a couple of points too, so a figure quoted to two decimal places anywhere is false precision.
Why do the two methods give me different answers?
Because they are looking at different things. The circumference equations respond to where your girth actually is, so they notice a heavy waist. The BMI equation only knows your weight relative to your height, so it cannot tell muscle from fat, and it reads high for a muscular person and low for a light sedentary one. A large gap between the two is itself useful information: it usually means your build is unusual for your weight in one direction or the other.
What is a healthy body fat percentage?
There is no single number, and the bands shown here are descriptive categories from population data rather than health thresholds. Broadly, adult men sit in the high teens to mid twenties and adult women roughly seven points higher, because women carry more essential fat for reproductive function. Very low levels carry real risks of their own — hormonal disruption and reduced bone density among them — so lower is not automatically better.
Where exactly do I put the tape?
Neck just below the larynx with the tape sloping slightly down at the front, and do not flare your shoulders. Waist horizontally at the navel if you are male, at the narrowest point of the torso if you are female, in both cases at the end of a normal breath out with no pulling in. Hips at the widest point with your feet together. Measure on bare skin, twice each, and take a third reading if the two differ by more than half a centimetre.
Can I use this for a child or a teenager?
No, and the tool refuses ages below 18 rather than returning something plausible. Body composition changes rapidly through growth, so young people are assessed against age- and sex-specific percentile charts instead of against adult equations. That comparison is a job for a paediatrician or a school health service.
Is this a medical assessment?
It is not. Nothing here diagnoses a condition, and no result should be used to decide on treatment, a restrictive diet or a training programme on its own. If a figure concerns you, or if you are managing a health condition, take it to a doctor, who has access to methods this page cannot approximate and to the rest of your history.

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