Less often than most people book one, and less often than it suits us to sell them.
For most people the useful gap between scans is three to four months. For anyone tracking bone density rather than body composition, it is closer to twelve. Scanning monthly, which plenty of people want to do, mostly produces noise that feels like information.
Here is the reasoning, because you should not take our word for it when we run DEXA scans in Sydney with a scanner sitting in the next room.
Every measurement has an error, including this one
DEXA is the most accurate body composition tool that is practical to use outside a research lab. It is not perfect. Repeat the same scan on the same person on the same day and you will not get an identical number, because of small differences in positioning, in how the software draws the boundary between regions, and in your fluid balance.
That variation is usually reported as a precision error, and for total body fat mass it commonly works out around one to one and a half kilograms in ordinary clinical use. Individual regions are noisier again, because there is less tissue in an arm to average across.
The practical consequence is straightforward. If the difference between two scans is smaller than the machine's own error, you have not measured a change. You have measured the machine.
What that means for scan frequency
Someone in a steady fat loss phase might be losing around half a kilo of fat a week if things are going well, and often less. Over four weeks that is roughly two kilos, which is only just above the error. Over three months it is comfortably clear of it, and the difference between the two scans means something.
So the question is not how often you want data. It is how long it takes for real change to exceed the noise.
Three to four months does that for most people in a deliberate fat loss or muscle gain phase. If change is slower, which it is for anyone maintaining or making small adjustments, six months is more honest.
Bone is slower still. Bone mineral density responds over months to years, and a repeat scan inside twelve months rarely tells you anything a clinician can act on, unless there is a specific reason such as a new fracture or a change in treatment.
The cases where scanning more often is justified
There are a few, and they share a feature: something is changing fast enough to clear the error quickly, and a decision hangs on it.
A fighter in camp, where several kilos are moving over eight to twelve weeks and the weight class decision depends on knowing what came off. We cover the longer term version of that in structured weight loss for female combat athletes.
Someone losing weight rapidly on a medication, where the concern is lean mass rather than total loss, and the trajectory needs checking early enough to change it. That is the subject of how much muscle you lose on a GLP-1.
Return from a serious injury, where one limb is being compared against the other and the regional numbers are the whole point.
Outside those, more frequent scanning tends to produce anxiety rather than better decisions, particularly for athletes with any history of disordered eating, where regular body composition measurement can do real harm. We are careful about that, and we will say no to a scan if we think it is the wrong thing for someone.
What to do between scans
This is the part people skip. The scan does not change anything. What happens in the three months between them does.
Body weight taken under consistent conditions, training load, how sessions feel, sleep, appetite, and for female athletes, whether the menstrual cycle is regular. Those are the daily signals worth tracking, and they will tell you more about whether the plan is working than a monthly body fat percentage will.
If you want the framework for that, the Essentials of Sports Nutrition guide covers what to monitor and why.
Booking sensibly
Book the initial scan when you are about to start something, not three weeks in. Book the follow up when enough time has passed for the change to be real. If you are not sure how long that is for your situation, ask us before you pay for the second one, because we will sometimes tell you to wait.
Our DEXA scan page has current pricing, including a duo pack for people who know they want the comparison. If you are still working out whether DEXA is the right tool at all, DEXA against the InBody at your gym is the place to start, and what a DEXA scan costs in Sydney covers the money side, including one situation where you should get a Medicare rebated scan somewhere else instead.