Body fat percentage

How to take the measurements

Tape placement decides your result more than the equation does, so it is worth doing the same way every time. Use a flexible tape, pull it snug without compressing the skin, and keep it level all the way round.

• Neck — just below the larynx, tape sloping slightly down at the front.
• Waist — men, at the navel. Women, at the narrowest point.
• Hip — women, at the widest point around the buttocks.
• Measure first thing, after the loo, before eating, and breathe out normally — do not suck in.

A centimetre of difference in where you put the tape moves the answer by around a percentage point, which is why the same person gets different numbers on different days and concludes the calculator is broken. It is not; the measurement is.

What the Navy method can and cannot see

The equations were published by Hodgdon and Beckett in 1984, fitted on US Navy personnel, and they work by inferring fat from the difference between a circumference that grows with fat and one that mostly does not. That is a clever proxy, and it has two consequences worth knowing.

First, it is a proxy — it cannot distinguish fat around your organs from fat under your skin, and it is the first kind that carries the metabolic risk. Second, it was fitted on a young, fit, mostly male population, so it drifts on bodies unlike that sample. Against DXA, published comparisons show it reading roughly two and a half points low in men and one to two points high in women.

Healthy ranges, and where they came from

The bands this page shows are the American Council on Exercise descriptive categories, which are widely used and are guidelines rather than a clinical standard. The age-banded healthy range is from Gallagher and colleagues (2000), and even that was derived by working back from BMI cut-offs rather than from health outcomes directly. There is no official clinical threshold for body fat percentage the way there is for BMI.

Essential fat is not a target to beat

Somewhere around 2–5% in men and 10–13% in women is structural — in the nervous system, the bone marrow, the organs. Getting near it is not an achievement, and for women the consequences of trying show up early: menstrual disruption, falling bone density, and the cluster now called relative energy deficiency in sport. If your number is in the athletic band and you are still cutting, that is worth a conversation with someone qualified rather than another month of deficit.

Tape, scales, calipers or DXA

A tape costs nothing and is repeatable, which makes it good at trends and poor at absolutes. Bioimpedance scales respond to how hydrated you are, so they move several points with a glass of water. Calipers are good in trained hands and poor in untrained ones. DXA is the practical reference standard and still carries its own error of a point or two.

The useful habit with any of them is to measure the same way, at the same time of day, and to read the trend. For turning the number into a plan, thelean body mass calculator takes a body fat figure directly, and the BMR calculator will use it to run Katch–McArdle.

This is a screening estimate for adults, not a diagnosis, and it is not valid during pregnancy or for under-18s.