Hi guys, getting a reverb but how do I mesure for it? Top of seat tube to saddle rails?

Depends on the the size of frame it’s going on. You can measure the exposed length of your existing post height.

I bought a 125mm drop 380mm length and it just about fits on my bike which is a medium frame. The collar is about 1″ above the seat binder so there’s a lot of post in the frame.

I’ve just bought one and had to go 100mm.
The 125mm needs aprox 195mm from top of clamp to seat rails, 100mm, 25 less
I needed 175 max.

It’s going onto a 21″ orange crush, 30.9 post size.

So if my exposed post is for example 150mm should I take 25mm off and get the 125mm for example?

Just measured, my post is 190mm clamp to seat rails.