So think of it this way.

If eating a mere 300 calories over a true TDEE of 2335 say led to more than 1 lb slow gain in 12 days of time, or 2.6 lbs in 4.5 weeks - then it wasn't fat weight because of eating over a true TDEE.

Water weight fluctuations is likely it. So you probably don't want to do the math on that 4-5 weeks or it would cause you to probably estimate a TDEE lower than then 2320.

TDEE is an estimate, especially by any calculator.

If you slowly increased eating goal and each weight measurement was best for minimizing known water weight fluctuations (morning after rest day eating normal sodium levels), and you went more than the 30 days your hormones cause an actual change in BMR burn - then you may indeed have found your true TDEE. As it stands right now.

Throw in inaccuracy in food labels even with the best logging possible - and that 2320 could actually be higher in reality, though it appears that on paper.

At least as long as you keep that on paper, it should work.

And it may be more than 2320, but 2635 was too big a jump.

See how this 2535 on paper looks after a bit - that may be it.

Then again if spring approaches for you too - your TDEE may just rise as most do being more active.

But very true, continuing to gain fat isn't in itself useful. Water weight fine, which may be your variance and loss on weeks you don't exercise.

What has been the rate of gain eating at 2500?

Is it 5 lbs in the last month, 10 to 15 over starting?
(and true TDEE would increase with weight, but not that much, especially if not that active moving the extra weight)

Because a stressed out body (from lack of sleep, disease, over-training, under-recovery, too much diet, stress about diet, ect) can easily gain 20 lbs of water weight from cortisol being elevated.

So in theory, 2500 could actually be still enough below TDEE to be a stress, along with a combo of things. I'll bet unlikely though.

May think of that stress point where the body does negative things as a line - and all your stresses stack up to it, go over the line - negative things. That line is genetic. Diet may not be a part of it, it may be a small part of it, may be a major part of it.

You sound like you are eating high enough (unless your logging is inaccurate the opposite normal direction and eating way less) that I wouldn't expect diet to be it.

But you have any other stressors in life?

Now you could be eating slightly over your TDEE and gaining stress weight - let's do the math on 1 month with 5 lb gain, pretending it was all fat gain.

5lbs x 3500 / 30 days = 583 cal eating over true TDEE daily, if 5 lbs only fat gain.

You think you are that incorrect on your logging to truly be eating that much over your true TDEE? Or are you having any binge times where logging is out the window, or lots of eating out where it's very inaccurate?

But if you are gaining water weight from stress, your amount over TDEE could be much less.

Which also means if the stress remains - you'd gain weight eating at true TDEE anyway.
Could gain water weight eating under TDEE too, in which case scale would stay the same.


What's a great way to tell if this is fat or water weight?

Hope you've been measuring several body sites - since scale weight alone is a bad indicator of body composition and weight change reasons.

If fat weight is changing - you should likely have an idea of what sites show that first.
Whereas cortisol water weight is spread all over.