How to Measure for Replacement Windows

Take the right measures for the right fit.

You know you need a replacement window — but how do you know what size to buy? Proper window measurement is critical to finding just the right fit. Our guide can help.

You just need a few minutes, a tape measure, and a pen and paper.

Note: This guide applies to measuring with the real wood window frames in place.

Measure the Width: Measure between the window jambs (the vertical sides/pieces that form the frame of the window) at three different points: the top, the middle and the bottom of the opening. If your existing wood window has range jamb liners, remove those to measure the inside width of the frame.

Make sure to measure from the surface of the jambs, not from the trim strips or parting stops in the old window opening.

Use the smallest of the three measurements as your width when ordering the window.

Measure the Height: Start at the high point of the windowsill (the ledge forming the bottom part of your window), which is the surface closest to the inside of the window, and measure to the top of the window opening. If your existing wood window has range jamb liners, remove those to measure the inside height of the frame.

From here, take three measurements: one at the left side, one at the center and one at the right side of the window.

Use the smallest of these three height measurements when ordering your window.

Measure the Depth: Measure between the inside window trim and the outside blind stop strip (a rectangular molding nailed between a window frame’s outside trim and sash.) There must be at least 3-1/4 inches of pocket depth between these two points in order to accept the frame of the replacement window.

NOTE: Ignore all pulleys and parting strips that may exist when measuring for depth dimensions, as they’ll be removed when installing the replacement window.

When you order your new replacement windows, provide the opening size only. YKK AP will cut heights and widths to the nearest 1/8-inch increment after a 1/4-inch deduction.