Most gyms in Sydney have an InBody machine near the front desk now, and a scan is usually free with a membership. A DEXA scan costs money and needs a booking. So the question we get asked more than any other is whether the paid one is worth it.
The answer depends entirely on what you are trying to find out, and for a decent number of people the free one is fine.
What each machine is actually doing
An InBody, along with every other bioelectrical impedance device, sends a small current through your body and measures the resistance it meets. Fat, muscle and water conduct electricity differently, so the machine estimates your body composition from that resistance using an equation built from a reference population. It measures resistance. Everything else on the printout is inferred from it.
A DEXA scanner passes two low energy X-ray beams through you and measures how much of each is absorbed by different tissues. Bone, fat and lean soft tissue absorb the two beams differently, so the scanner separates them directly rather than estimating them, and reports each region on its own. You get left arm, right arm, left leg, right leg, trunk. You also get bone mineral density, which no impedance device can produce at all.
Why hydration is the catch with impedance
Because impedance is measuring water as much as anything else, anything that shifts your fluid balance shifts the reading. Training that morning, a salty dinner, where you are in your menstrual cycle, how much you drank on the way in, a heavy carbohydrate meal the night before. All of it moves the number.
In practice this means a fortnight of good work can show up as no change at all, or as a two kilo fat loss that never happened. For a fighter checking in during fight camp, when fluid is being manipulated deliberately, impedance readings become close to unusable. We have written separately about how acute weight cutting works for female combat athletes and why fluid shifts make that period so hard to measure.
DEXA is not immune to this. A scan taken straight after a hard session or a big meal will read differently to one taken fasted and rested, which is why we ask people to come in under the same conditions each time. The effect is smaller, though, and it is predictable enough to control for.
Where InBody genuinely earns its keep
We would rather say this than have you find out after paying us. If you are training consistently, not competing in a weight class, and you want to know roughly whether you are heading in the right direction, a weekly InBody under standardised conditions will tell you that. Same machine, same time of day, fasted, before training, every time. Watch the trend over six weeks and ignore any single reading.
That is a real answer to a real question and it costs nothing. Plenty of people do not need more than it.
Where the difference starts to matter
The gap opens up when a decision hangs on the number.
Weight class selection is the clearest case. A fighter wanting to know whether a division is realistic needs to know how much of their current weight is fat that can actually come off, and how much is lean tissue that will not. Getting that wrong by three kilos costs a camp. The regional breakdown matters here too, because fat distribution is not even and total body percentage hides a lot.
Bone density is the second case, and impedance simply cannot do it. For any athlete with a history of stress fractures, a long period of low energy availability, or a menstrual cycle that has gone quiet, bone is the thing you want measured. That is the same conversation we have with female athletes managing RED-S, and it is not a conversation a gym scale can start.
The third is anyone losing weight quickly, whether through a fight camp or a weight loss medication, where the question is not how much you have lost but what you have lost.
What we do with the result
A scan on its own is a number, and numbers on their own tend to make people anxious rather than better fed. At our Alexandria clinic the DEXA and RMR testing is read by an Accredited Sports Dietitian in the same appointment, so you leave with the interpretation rather than a PDF you have to work out yourself. That is the part most Sydney providers do not include, and it is the reason we bothered putting a scanner in a dietetics clinic rather than sending people to radiology.
So which should you book
If you want a rough direction of travel and you will be consistent about conditions, use the free InBody at your gym and check it weekly.
If a decision depends on the answer, or you need bone density, or you are in a weight class, book the DEXA. Our guide to when a DEXA scan is most useful covers the specific situations in more detail, and if you are wondering how often to repeat one, we have been deliberately blunt about that in how often you should actually get a DEXA scan.
If you want the underlying fuelling principles before you measure anything, the Essentials of Sports Nutrition guide covers what the numbers are supposed to be informing.