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Note: It is important to understand which measurements to take. Read the following instructions and have a tape measure handy to learn how to measure a window for any purpose. Remember your measurements should always read width by height. 

• Measure the width at three points. 

Use a tape measure to measure the width of the window opening at the base, middle, and top. Write down the smallest of these measurements as the width. Be sure to measure from the surface of the jambs or frame, not from the short extensions that hold the window in place. 

• Account for the lining around the window if present. 

Your window may be surrounded by a plastic jamb liner that slots around its vertical sides. This will be removed before the replacement window is installed, so measure its width and add it to the width of the window opening. If you cannot access the liner’s width to measure, make note to discuss this when you visit your local window dealer. 

• Measure the height at three points. 

From the window sill surface closest to the window itself, measure the height to the top of the window opening. Do this at the left, center, and right edges, and write down the smallest measurement as the height. 

• Measure the depth of the window. 

Measure the depth between the two stops that extend in front of the window frame. Try to measure at the shortest point, but it is not vital that you measure in three places as you did the width and height. 

• Check whether the window opening is rectangular. 

Measure the window opening from the top left corner to the bottom right corner and write it down. Measure the distance from the opposite corners (top right to bottom left) and compare with the first measurement. If these lengths are not equal, your window opening is not square or rectangular. In this scenario, you or your contractor may need to use shims to level up the window properly. 

• Be clear about where you measured when ordering windows. 

Some manufacturers of replacement windows, storm windows, or exterior shutters may request that you order items slightly narrower than the size of your opening to make sure they fit. You may follow that manufacturer’s instructions to reduce the size by the appropriate amount or give them the exact dimensions of your window opening. The important point to remember is to be absolutely clear which method you are using, since if both you and the manufacturer narrow the window size, it may not fit at all.