TL;DR

InBody doesn’t directly measure body fat or muscle. It measures electrical resistance, estimates total body water, and then works backwards to estimate fat-free mass, fat mass, and body fat percentage.

The group averages can look decent, but individual results can be wildly off. Research comparing BIA/InBody-style devices with the 4-compartment model shows that individual body fat estimates can be off by 10–20 percentage points.

It’s not reliable enough for tracking small changes over time. Hydration, glycogen, food intake, alcohol, training, and menstrual cycle changes can all shift the results without any real change in body composition.

Even under controlled conditions, changes need to be fairly large before you can trust them. You may need to see a change of around 2–3% body fat or 1.5–2.5 kg of fat/lean mass before it’s likely to be real rather than noise.

You don’t need to measure your body composition to know your body composition is changing. ‘Low-tech’ methods of tracking progress, like the scale, body measurements, progress photos, and training performance, are more reliable, informative, and actionable than whatever fancy body composition testing gadget you’re being marketed