Project Guide

How to Measure Windows

1
Check Windows for Square
A person measuring the diagonal of a window.

Measuring for replacement windows begins with determining that the corners of the existing window frame are square. Standard window sizes vary but you can be sure that replacements you order will have 90-degree corners.


To check your windows are in square alignment:

  • From inside the home, hold a tape measure in the top right corner where the horizontal and vertical window trim meet.
  • Extend the tape measure diagonally to the lower left corner where the trim meets and record the measurement.
  • Now make a measurement from the upper left corner to the lower right corner.
  • If the two measurements are within 1/4-inch of each other, most standard window sizes you purchase will securely fit in your window opening. If the window is not within 1/4-inch of square, you’ll need to square up the frame or find an alternate replacement solution.


This quick first step in measuring windows is also useful when learning how to measure for window screens and storm windows.

2
Measure Window Width
A person measures a window width.

Whether you’re learning how to measure for replacement vinyl windows, aluminum windows or casement windows, the dimensions of standard window sizes are presented as Width x Height.


Tip: It’s best practice to accurately measure each window individually, even if two windows in the same room appear to be the same size. 


Measure the window width from the inside jamb on the left and right sides of the window in three places – the bottom, middle and top.


  • Do not include any trim in this measurement.
  • To expose the jamb (the vertical side of the window frame) at the bottom of the window, raise the window sash.
  • Place a tape measure horizontally between the inside jamb on the left and the right.
  • Close the window and make a similar measurement from jamb to jamb near the middle of the window.
  • Measure the distance between the jambs at the top of the window.
  • Record the shortest measurement. This is the window opening width.
3
Measure Window Height
A person measures a window height using a tape measure.

Now find the window height. Again, when learning how to measure windows, remember that trim should not be included in the measurements.


  • Use the tape measure to measure from the sill to the top of the window opening.
  • The sill is where the sash rests when closed. (The sill is not the horizontal trim board that extends from the wall. That piece of trim is the window stool.)
  • Make three vertical measurements – on the left side of the window from the sill to the header, then the center and finally on the right side.
  • Record the smallest of these numbers. This is the window opening height.




Note: Depth is not usually an issue with how to measure windows for replacement and the measurement isn’t considered when placing an order for a new window. However, depth could be a problem if your window opening is too narrow for a new window. Measure to be sure the jamb depth of the existing window is at least 3 1/4-inches, especially if you live in a modular or mobile home.

4
Measure for Storm Windows
A person measures a window width using a tape measure.

The process for how to measure for storm windows is similar to finding measurements for single- or double-hung windows.


To find the width, measure jamb-to-jamb from the inside of the trim on one side of the window to the inside of the trim on the other side.

  • Measure at the bottom, middle and top of the window.
  • Record the shortest measurement as the width.


To find the height, measure from the sill to the inside of the trim at the top of the window.

  • Measure at the left, center and right side of the window.
  • Record the shortest measurement as the height of your storm window.


Using the smallest measurements for the width and height will ensure that the storm window’s flanges will fit inside the exterior trim of the window opening.

5
Measure for Window Screens
A person measures a window width using a tape measure.

Learning how to measure for window screens is straightforward. If a screen is being replaced and the existing damaged screen is available – or another window of the same size has a screen – simply measure the width and the height of the existing screen.


If a screen is being added to a window that does not currently have one:

  • Measure the distance between the screen channels on the window frame. Subtract 1/8-inch from the measurement for the width dimension.
  • To determine the height of the new screen, raise the window sash and measure up from the lip (a ridge that holds the screen in place) along the screen channel to the sash. Add 1/8-inch to the measurement for the height dimension.