[header] How to install replacement windows [title] Take the width measurement from inside-to-inside of the window frame. [title] Take the height measurement from the bottom of the window frame to the top of the window frame. [substeps] Some people take the height measurement from the top of the sill. 
This leaves room to attach a " header expander " to the top of the new window. Others, though, dislike using header expanders because they decrease effective window size, and complicate the trim process.