How to Use This Tool
Take the measurements, pick the formula, and read the range. The single number in the middle is the formula's output; the range around it is what your measurement error does to it.
Where to put the tape
- Neck — just below the larynx, tape sloping slightly downward at the front. Do not flare the shoulders or tense the neck.
- Waist — for the male formula, at the navel. For the female formula, at the narrowest point, which is usually higher. This difference matters and is frequently ignored.
- Hip — female formula only, at the widest point of the buttocks with feet together.
- Height — without shoes.
Stand relaxed and breathe out normally. Do not pull the tape tight, and do not hold your stomach in. Take each measurement twice; if the two differ by more than half a centimetre, take a third.
Why the answer is a range
Measured for this page: moving the waist tape by 2 cm changes the male estimate by about 1.4 percentage points and the female estimate by about 0.9. Two centimetres is not a careless error — it is the difference between the navel and slightly below it, or between a relaxed and a slightly firm pull.
So an answer of "13.7%" is arithmetic, not measurement. The honest reading is "somewhere around 13 to 15", and the useful thing is not today's number but the direction it moves over months when you measure the same way each time.
Where the formula breaks down
It was fitted to a general population, so it is least reliable at the edges. For very lean people, particularly women, it can return values below the essential fat level — a physiological impossibility. At waist 70 cm, hip 92, neck 31 and height 168 it returns 0.5%, which is not a number about a human being. When the output falls below essential fat this tool says so instead of printing it.
It also tends to overestimate for very muscular people with thick necks, since a large neck measurement is read as leanness, and it says nothing at all about where fat is stored. That last gap is why the waist-to-height ratio is shown alongside: it uses one of the same measurements and is a better predictor of metabolic risk than any body fat percentage.
How this compares to other methods
Against DEXA, generally regarded as the practical reference, the Navy method typically sits within about three to four percentage points for most people — and further out at the extremes. Bioimpedance scales are in a similar band and are strongly affected by hydration, which is why they can move two points between morning and evening. Calipers in trained hands do better than either; in untrained hands, worse.
None of this makes the number useless. It makes it a trend line rather than a fact. Measure the same way, at the same time of day, every few weeks, and the direction of travel is meaningful even though any single reading is not.
