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How to Measure for New and Replacement Windows

If you plan to replace the old windows in your home, understand that measuring windows for replacement is much different than measuring for new construction. When measuring for replacement windows, such as vinyl window inserts, the measurements come from inside the window frame. When measuring for new construction, measurements are from the overall opening in the window framing.

Measuring for Replacement Windows

Replacement insert windows are a good option if you don't want to tear apart exterior trim and siding to remove the old window frame. When measuring windows for replacement, first, remove the old windows and expose the window frame. Next, carefully gather the following dimensions:

  • Measure window width: Professionals suggest measuring the top, middle, and bottom of the window jambs to ensure proper measurement. Measure from the surface of the right window jamb to the left window jamb, avoiding any trim or parting strips. After taking the three measurements, use the smallest number--that's where the window fits the tightest in the window opening. This tightest dimension is your critical number for width.
  • Measure window height: Some window sills slope, so measure from the top of the window jamb to the highest point on the bottom jamb. Measure in a few places to make sure the window frame isn't sagging under the weight of the roof line. Again, use the smallest number obtained from all your measurements.

Measuring for New Windows

Measuring for new windows can be simple--measure the width of the rough opening and then the height. A window professional should be able to guide you to the proper window choices based on those two numbers. Windows are sold by width and height--a four-foot by five-foot window is four-feet wide by five-feet tall. When ordering windows, be sure to give the width first, then the height.