How often should you actually rescan?

Every measurement has a margin. On a well-maintained DEXA scanner with a technician who positions people consistently, whole-body fat percentage repeats to somewhere around one percentage point, and regional lean mass a little less tightly than that. This is very good — better than any consumer method by a wide margin — but it is not zero.

That margin is the whole reason the interval question matters. If your true body fat has changed by half a percentage point and your measurement error is one point, the second scan can plausibly report a change in either direction. You'd be reading noise and making decisions from it, which is worse than having no second data point at all.

The rule of thumb we use

Wait until the change you expect is comfortably larger than the error. For most people making a real effort — a structured deficit, a training block, a new medication — that lands at three to four months for body composition. Twelve weeks of consistent work produces changes in fat and lean mass that are several times the measurement margin, so the comparison is unambiguous.

Bone is different, and the difference is not small. Bone remodels on a timescale of months to years. Density measurements taken six months apart in someone not on treatment will mostly reflect precision error rather than biology. Twelve months is the shortest interval that usually tells you something, and for many people two years is more appropriate.

What ruins a comparison

Interval isn't the only thing that decides whether two scans can be compared. Four conditions matter about as much, and all of them are under your control.

Hydration. Lean soft tissue is mostly water, and DEXA counts that water as lean mass. Arriving dehydrated can shift your lean reading by a pound or more in either direction. Drink normally, not strategically.

Food. The contents of your digestive tract get measured along with you. A large meal in the hours beforehand adds mass that has to land in some tissue category, and it usually lands in the wrong one.

Recent training. Hard exercise pulls fluid into working muscle for hours afterward. A leg day the morning of your scan can produce an encouraging leg-lean number that has nothing to do with muscle tissue.

Time of day. All of the above vary through the day, so an 8am baseline compared against a 2pm follow-up carries extra noise for no reason. Book the same slot when you can.

None of this is fussiness for its own sake. Control those four things and your follow-up scan is measuring your training. Ignore them and it's measuring your morning.

A note on scanning more often anyway

Some people want to scan monthly, and we understand the appeal — but we'd rather tell you it won't work than take the booking. Monthly scans in a consistent deficit produce a sawtooth line where the real signal is a gentle slope, and the temptation is always to react to the sawtooth. There's also a small radiation dose each time, which is trivial once or twice a year and not worth accumulating for data you can't act on.

If you want more frequent feedback, the bathroom scale and a tape measure are genuinely useful between scans. They're imprecise about composition, but they're free and they trend well. Use them for the week-to-week, and let DEXA answer the question they can't.

When you're ready for a follow-up, book the same time of day as your baseline and we'll chart the two side by side.