"Pocket windows" ?

It is hard to tell from your post what you are asking but they said measure the frame. You seem to be talking about the window part that moves. That is a called the sash. Don't measure the sashes. Measure the whole frame. Check to see if there is a Pella store near you. Lowes sells their low end stuff. They may be able to special order their better stuff.

Imagine the window removed to the point you will install the new one. Measure that dimention. If you're not up to the point of visualizing that, you need to do a little more research by reading more installation guides.

when i ordered my windows from 84 lumber,they came out and verified my measurements. i got a quote from another company who also came out for free. lowes wanted $50 (refunded on purchase). try the smaller home improvement stores,and they might do it for ya. good luck.

Hello - For all - These Pella "pocket windows" are designed to work inside existing frames. No need to remove any trim or anything.

To Boothbay - They will make the sashes (The upper part and lower part) the same size I suspect. They want your top to bottom measurement and you side to side measurement. They will not need to know that your current sashes are different UNLESS you are trying to match up something existing.

All you ever wanted to know is here

I see. My problem with that idea is that you never fix or renew the flashing. Every window replacement should include doing. If you don't and the window is ruined by moisture they will not help you warranty-wise.