Body Fat Percentage

Body Fat Percentage: what is it?

Body fat percentage is the number clients most want and coaches should be most careful with. Nothing available in a gym measures it directly. Skinfolds measure the thickness of a fold of skin and fat and push that through a regression equation. Bioimpedance measures electrical resistance and infers body water. Even DEXA infers fat from how two X-ray energies are attenuated. Each of these produces a plausible number, and the numbers frequently disagree with each other by several percentage points on the same person on the same day.

Commonly used reference ranges

The ranges below are the classification bands most widely used in the fitness industry. They are orientation, not diagnosis, and they shift with age. Essential fat is the floor required for normal physiological function, and women carry more of it for reproductive and hormonal reasons.

How much error to expect

The honest position is that a field estimate should be read with a margin of several percentage points around it, not as a precise value. Skinfolds done by a trained, consistent tester on the same client are reasonably repeatable, but the equation converting fold thickness to percentage was built on a population that may not resemble your client. Bioimpedance moves with hydration and can shift meaningfully within one day. DEXA is more consistent but differs between machines and software versions.

The practical consequence is simple. Never compare a client's number to a number produced by a different device, a different tester or a different method. And never present the estimate to a decimal place: it implies a precision none of these tools have.

Using it well as a coach

Body fat percentage earns its place when it is one input among several. Read it next to scale weight, waist circumference and photos, and the story usually becomes unambiguous even when any single measure is noisy. Weight flat, waist down, estimate down, photos clearly different: that is a recomposition, and you do not need the estimate to be accurate to know it.

It also has a real clinical use as a sanity check on BMI. A muscular client can sit in the overweight BMI band with a body fat estimate in the athletic range, which is exactly the misclassification BMI is known for.

• Round to the nearest whole number, or report a two-point range.
• Same method, same tester, same conditions, every single time.
• Never set a client goal in body fat percentage alone.
• Treat very low targets in general population clients as a conversation, not a program.

When not to measure it

Skip it with clients whose relationship with their body makes a percentage counterproductive, and with anyone showing signs of disordered eating. Girths and performance markers give you the same programming information without handing the client a number to fixate on.

Skip it too when you cannot standardise. An estimate taken after a heavy meal, a hard session or a night of poor sleep tells you about that morning, not about the training block.