Window Sizes and How to Measure

Accurate measurements decide whether a replacement window fits. Modernize's measuring guide describes taking width and height at three points and ordering to the smallest of each. For bedrooms and basements you also check the code minimums, since UpCodes publishes IRC R310, which requires a net clear opening of at least 5.7 square feet. Order to the smallest reading so the unit clears the tightest point in the opening.

Measure the width at three points

Openings are rarely perfectly square, so width is measured in several places rather than once. Modernize instructs measuring horizontally across the frame at the top, middle, and bottom, from each side jamb, and writing down each figure.

Once you have three width readings, you circle the smallest of the three. Modernize identifies that smallest figure as your rough opening width, because a window built to the widest reading would not pass through the narrowest point of the frame.

Measure the height and depth

Height follows the same three-point logic as width. Modernize instructs measuring vertically from the head jamb to the window sill on the left, middle, and right, then using the smallest of the three figures as the rough opening height.

Depth determines whether a replacement unit physically fits the wall. Modernize describes running the tape from the outside of the frame to the inside in several locations, and again using the smallest measurement as the depth.

How the ordered window size relates to the opening

The window you order is not the same as the opening you measured. Modernize notes that a replacement window is sized slightly smaller than the measured space to allow for fit and installation clearances.

Modernize gives an approximate rule that the replacement window should be about two inches narrower and two inches shorter than the measured space. Manufacturers vary in how they apply clearances, so you confirm the ordering method with your supplier before you buy.

Size minimums for bedroom and basement windows

When the window serves a sleeping room or basement, measurement is not only about fit but also about code. UpCodes publishes IRC R310, which requires an emergency escape and rescue opening with a minimum net clear opening of 5.7 square feet.

The same section sets a minimum opening height of 24 inches and a minimum width of 20 inches. Because a smaller replacement can void egress compliance, you check that the new window still delivers the required clear opening before you order it.