How to Measure for Pella Windows

Remove the casing from the inside of your windows. Pry the molding away from the wall gently with a flat pry-bar. Remove molding from all four sides so the wall frame behind the window is exposed. If you are going to replace the windows with a similar style, pull the nails through the back of the window casing with locking pliers to prevent holes in the face and set it aside to reuse.

Draw a diagram on paper showing each window that you intend to replace. Mark each wall and window and write the dimensions on the diagram. This will serve as a blueprint for the installation, to indicate which window goes where. Write all rough opening dimensions on this diagram.

Measure the window opening from top to bottom. Make sure you measure from the lumber sill plate underneath the window to the lumber frame header above. Round up to the nearest 1/8 inch to ensure adequate space on all sides of your new windows. Measure this distance in three places. Start with the left edge, then go to the center of the window and finally measure the right edge. The smallest of these three measurements will be the rough opening height for that window.

Measure the window from side to side. Your measurement must be from stud to stud, outside the jamb of the window. Measure across the window at the top. Move down the window about one third of the height and measure across. Move down the window another third of the height and measure across again. Measure the width between studs directly beneath the bottom of the window frame. The smallest of these four dimensions will be the rough opening width.

Take these measurements for each window you need to replace to get the best fit. The casing will need to be removed to expose the frame. Replace it temporarily while awaiting the arrival of your windows, or leave it down as it will need to be removed during installation.