You have the scan. What does it actually say?

A DEXA report or a smart scale gives you body fat and lean mass, but not the number that matters while you are losing weight: of everything you have lost, how much was muscle? Two readings is all it takes to work that out exactly — no model, no estimate.

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Your readings

This does not change the arithmetic. It changes how much confidence we put on the answer, and we will show you why.

First readingwhen you started
Second readingnow

Calculated on your device. Nothing is sent anywhere.

Your split will appear here.

You need two readings taken at different times. One reading tells you where you are; only two can tell you what changed.

The whole calculation

There is no model here and nothing to trust us about. Lean mass is whatever is not fat, and the share is one division:

Lean mass  =  weight × (1 − body fat % ÷ 100)
Lean lost  =  lean at start − lean now
Weight lost  =  weight at start − weight now
Lean share  =  lean lost ÷ weight lost × 100

"Lean" here includes bone and body water as well as muscle, which is what a DEXA report means by the word too — so the two are comparable.

Why a scale and a scan are not the same evidence

A smart scale estimates body fat by passing a small current through you and measuring resistance. Water conducts, so hydration moves the reading — the same body can measure several percentage points apart between a Tuesday and a Thursday, or before and after a large drink.

That error does not cancel out here. The share is a ratio of two differences, so both readings' error feeds into it, and the smaller your weight change the more it distorts the answer. That is why the band we show is wide for scale readings and gets wider still when little weight has been lost.

If you use a scale, weigh under the same conditions every time — first thing, after the toilet, before drinking, same device. That will not make it a DEXA, but it removes most of the noise you control.

If you only have one reading

Then this page cannot help you yet, and it will say so rather than guess. A share describes a change, and a change needs two points in time. What you can do meanwhile is use the muscle check, which estimates the same figure from your weight, protein, training and age — and then come back here when you have a second scan to check the estimate against.

That comparison is worth doing. If our estimate and your scan disagree badly, your scan is right and we would genuinely like to know — tell us.

Before you read too much into it

A lean loss of roughly a quarter to a third of total weight lost is what the GLP-1 trials typically found. Being in that range is ordinary, not a failure. Lean mass includes water. A large early drop often reflects glycogen and the water bound to it rather than muscle, so the first few weeks read worse than they are. This is not a diagnosis and not medical advice. If a scan worries you, take it to the clinician who prescribed your medication — that is exactly the conversation it is useful for.