How to Read an InBody Scan: Only Two Numbers Matter at First

RebelUp Coaching Team August 25, 2026 5min

Short answer

A body composition printout has twenty-plus values. For your first few months, two of them decide everything: skeletal muscle mass and body fat percentage.

An InBody printout gives you twenty-plus values — segmental lean mass, segmental fat, total body water, mineral content, phase angle, visceral fat level.

Two of them matter at the start: skeletal muscle mass and body fat percentage.

Why only two

Body weight alone has no direction. When 2kg comes off you cannot tell whether it was fat, muscle or water. Reading skeletal muscle and body fat together resolves that.

Body fat down, muscle held: this is working
Body fat down, muscle down: you are cutting too fast — raise protein and keep lifting
Body fat flat, muscle up: weight unchanged is still a win
Body fat up, muscle flat: the scale can sit still while the direction is wrong

None of those four cases are distinguishable from body weight alone. The remaining eighteen values do not change the call either.

Every 8-12 weeks, because the error is 1-2%

Readings swing with hydration, meal timing and time of day. Bioelectrical impedance typically moves 1-2% between sessions on the same person, which sets a floor on how small a change you can trust.

Key numbers

Between-reading variationabout 1-2%

Sensible re-test interval8-12 weeks

Left-right gap worth acting on5% or more

Reads 24% body fat → true value roughly 23-25%

Reads 23.5% two weeks later → inside the error band

Measure too often and you are watching error, not change. Around eight weeks, real movement finally clears the noise.

Morning, fasted, after the bathroom
Avoid measuring right after training — dehydration inflates body fat percentage
Use the same machine. Different devices give different numbers

Switching machines breaks the series. If you change gyms, treat that reading as a new baseline rather than comparing it to your old numbers.

The eight segmental fields can wait

Segmental lean mass earns its place when you are chasing a left-right imbalance, and that needs several readings to establish. One scan cannot tell you.

So RebelUp asks for two first. Asking for eight makes most people abandon the form entirely — even with the printout in hand, nobody transcribes twenty values.

Morning, fasted, after the bathroom
Avoid measuring right after training — dehydration inflates body fat percentage
Use the same machine. Different devices give different numbers

Frequently asked

Do home body-fat scales work?

Treat the absolute value as unreliable, but used consistently the same scale still shows direction of change — which is what you need.

When do segmental values matter?

Once left-right difference exceeds about 5%. One reading cannot tell you; you need two or three.

What body fat percentage counts as normal?

Reference ranges differ by organisation and shift with age. Tracking the direction of your own number is more useful than chasing an absolute.

Muscle mass or skeletal muscle mass?

They are different. Muscle mass includes cardiac and smooth muscle; skeletal muscle mass is the part training changes. Track skeletal muscle mass.

What if I drink a lot of water first?

Total body water rises, which pushes skeletal muscle mass up and body fat percentage down. That is exactly why the conditions need to be repeatable.