Body Fat Calculator (US Navy method)

How to measure so the number means something

Consistency matters far more than accuracy here. A slightly wrong tape position used identically every time still tracks change; a correct position used inconsistently does not.

• Morning, before eating, after the toilet.
• Tape snug, not compressing the skin.
• Waist at the navel, horizontal all the way round — check in a mirror.
• Stand relaxed, normal breath out. Do not suck in.
• Take two readings and average them.

How accurate is it, really

The Navy formula is a regression fitted to population data. It works reasonably for average body shapes and less well at the extremes — very muscular people with thick necks tend to be under-estimated, and people who carry fat centrally tend to be over-estimated.

For context on alternatives: DEXA carries ±1–3%, skinfold callipers ±3–5% with a trained tester, and bioimpedance scales ±4–8% with heavy dependence on hydration. None of them is a direct measurement — every method infers fat from something else.

Which is why the practical advice is to stop trying to know your body-fat percentage and instead track whether it is moving: waist circumference, photographs, and your logbook.