How to Measure Body Composition

The argument running through this whole site is that the scale measures the wrong thing — that what matters is how much of your loss is fat and how much is lean tissue, which is the case made in muscle is the whole game.

Which raises the obvious question: measure it with what?

The options, honestly ranked

DXA, and its limitations

Dual-energy X-ray absorptiometry is the most useful option generally available. It separates fat, lean soft tissue and bone, and it does it by region — so you can see whether the loss is coming off your legs or your abdomen.

Two things worth knowing before you treat it as truth.

“Lean mass” is not “muscle.” It includes water, glycogen, organs and connective tissue. Glycogen binds water at roughly 3 grams of water per gram of glycogen, so a low-carbohydrate week reduces lean mass on a scan without you having lost any muscle. This is also why creatine shows up as lean mass gain — it draws water into muscle cells, which is real and is not new contractile tissue. That quirk is covered in the creatine rankings.

Conditions matter more than people expect. Same machine where possible, similar time of day, similar hydration, and not immediately after a heavy training session or a very salty weekend.

A baseline before or early in treatment is worth more than any single later scan, because it converts an argument into a comparison. Every 3–6 months after that is plenty — more often is measuring noise, and it involves radiation exposure, small as it is.

Bioimpedance: read the trend, ignore the number

Consumer scales and handheld devices pass a small current through the body and estimate composition from how it is resisted. Since fat and lean tissue conduct differently, the physics is sound. The problem is that hydration dominates the signal, and hydration varies with fluid intake, sodium, exercise, alcohol, the menstrual cycle and time of day.

So a single reading of “24.3% body fat” is not a fact about you. A series of readings taken at the same time of day, before eating or drinking, before exercise, on the same device does contain information about direction.

That is the correct way to use one: watch the trend, ignore the decimal places, and never compare a reading from one device to a reading from another.

If you want a scale anyway, the body composition scale rankings cover which ones are worth it and how to use one without being misled.

The two free measurements that matter most

Waist circumference. It predicts cardiometabolic risk better than weight or BMI, because it approximates visceral fat — the metabolically active kind. It also frequently keeps falling during a scale plateau, which is precisely when people most need evidence that something is happening, as why you stopped losing weight explains.

Technique matters more than people think:

1. Measure directly against skin, not over clothing
2. At the midpoint between the lowest rib and the top of the hip bone — or consistently at the navel, but pick one and stay with it
3. At the end of a normal breath out, not sucked in
4. Tape snug but not compressing the skin
5. Same time of day, ideally morning before eating

Monthly is enough.

A strength log. If your working weights are holding or rising while your total weight falls, you are losing the right tissue. It costs nothing, you record it anyway if you train, and it measures function rather than a proxy for it. Grip strength and chair-stand time are the versions used clinically, particularly in older adults — taking a GLP-1 after 65.

What to do with the scale

Do not abandon it, and demote it.

• Weekly, not daily
• Same morning, same conditions, after the toilet and before eating
• Read the 4-week trend, not the individual number
• Expect 1–2 kg of upward movement in the first fortnight if you start creatine, which is water and not fat

Weight is a real signal with a poor signal-to-noise ratio over short periods, and on this drug it is the number most likely to make somebody stop a treatment that is working.

A measurement schedule that works

The point of measuring at all

Not reassurance. A decision.

The reason to know your lean mass trajectory is that if it is going badly, there is something to do about it — more protein, more load, a slower rate of loss. Measuring without being prepared to act on the result is just a more expensive way of worrying.

And if you cannot access any of this, the honest version is that protein at 1.2–1.6 g/kg and two resistance sessions a week is the right answer regardless of what a scan would have shown. The measurement changes your confidence. It rarely changes the plan.