OK. It doesn't really matter how to measure caravan windows, I suppose there is a few different ways it could be done.
Anyway this is how we do it and if you measure in the same way then we will be on a winner yes?
Start off by either peeling the window rubber seals back to see the wood metal or insulation it doesn't matter as long as you have found the actual caravan window opening size (this would be brick to brick in a house window opening) then just draw a line in (pencil so it can be rubbed off) through these points until they meet that's your measuring point. Simple eh.
Now you know how to measure caravan windows get it done come back and send the sizes by using the contact us form. Oh and by the way the short side on slopping caravan windows is measured in the same manner as the long side in my super duper sketch.