How is body fat actually measured, and how accurate is each method?
Every available method is an estimate built on assumptions, not a direct measurement — and knowing which assumption each one makes tells you when it will be wrong.
The methods, roughly in order of accuracy:
Four-compartment models, combining several measurements to estimate fat, water, protein and mineral separately. The research reference standard, and rarely available.
DEXA. A low-dose X-ray scan distinguishing bone, lean tissue and fat, with regional breakdown. Good precision, but hydration status shifts results, and different machines and software versions are not interchangeable — so never compare a scan on one machine with a scan on another.
Air displacement (Bod Pod) and hydrostatic weighing, both estimating body density then converting it to fat percentage using an assumed density for lean tissue. That assumption varies with age, sex and ethnicity, which is where error enters.
Bioelectrical impedance — the method in bathroom scales and handheld devices. It passes a small current and infers composition from resistance, which depends heavily on body water. Readings shift with hydration, food, exercise, alcohol and time of day. Cheap, very repeatable, and potentially quite inaccurate in absolute terms.
Skinfold calipers. Accuracy depends almost entirely on the person taking them; a skilled, consistent tester tracks change well.
Tape measurements — waist circumference in particular. Crude for body fat percentage and genuinely useful for health risk, because abdominal fat is what most strongly predicts metabolic outcomes.
The practical conclusions:
Trends beat absolute numbers. Use one method, one device, same conditions, same time of day, and watch the direction over months.
Absolute percentages carry an error margin of several points, so precision to a decimal place is false.
Do not compare across methods. A different method is not a change in your body.
Weight plus waist plus photographs plus performance together give a better picture than any single number.
General information, not medical advice.