Body fat calculator
An estimate of body fat percentage from tape measurements using the US Navy method — with an honest account of how approximate that is.
Estimate your body fat
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| Body fat | — |
|---|---|
| Category | — |
| Fat mass | — |
| Lean mass | — |
| Realistic range | — |
How to measure so the number means something
The formula is only as good as the tape. Measure first thing in the morning, before eating, with the tape snug but not compressing the skin, and keep it horizontal.
Neck: just below the larynx, sloping slightly downward at the front. Waist: at the navel for men; at the narrowest point for women. Hips: at the widest point, women only. Take each measurement twice and use the average.
What this method can and cannot do
The US Navy method estimates body fat from circumferences and height. It was developed for screening large populations cheaply, not for precision. Against hydrostatic weighing — the reference standard — it typically sits within about 3–4 percentage points, which means a reading of 18% honestly means "somewhere around 14–22%".
That sounds like a criticism and is not. Part of the error is consistent for the same person measured the same way, so repeated measurements can still show a trend. But a single tape measurement can easily move by a centimetre, and that alone shifts the estimate by a percentage point or more — so a change of one or two points between two readings may be nothing at all. Take several readings under the same conditions (same time of day, same tape, same spot) and look at the trend over weeks rather than any one pair of numbers.
Why body fat rather than BMI
BMI cannot distinguish muscle from fat, which is why it misclassifies muscular people as overweight and can miss high body fat in people of normal weight. A circumference method is not precise, but it is at least measuring something closer to the thing you care about. Waist circumference alone is also a useful standalone marker: above 94cm for men and 80cm for women is associated with raised health risk regardless of what any percentage says.
Common questions
How accurate is the US Navy body fat method?
It sits within roughly 3–4 percentage points of hydrostatic weighing for most people. That is fine for tracking change over time with consistent technique, and not precise enough to treat a single reading as an exact figure.
What is a healthy body fat percentage?
Commonly cited ranges are 8–19% for men and 21–32% for women, with athletic ranges lower. These are population conventions rather than medical thresholds, and healthy ranges vary with age.
Why does my result differ from a smart scale?
Bioelectrical impedance scales estimate from how easily a current passes through you, which is strongly affected by hydration, food and time of day. Neither method is precise; both are more useful for trend than for absolute value.
Is body fat percentage better than BMI?
It measures something closer to what matters, since BMI cannot tell muscle from fat. But a tape-based estimate has a wide error bar, so the most reliable use of either is watching the direction of travel rather than one reading.
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