Summarize the following:
Use a tape measure or extendable slide rule 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. A slide rule with extension may result in more accurate interior measurements. If you are using a tape measure, make sure you account for the width of the tape measure itself, which is usually printed on its label. Your window may be surrounded by a plastic or aluminum 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, use 1/2" (1.25 cm) as an approximation. From the windowsill 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 result as the height. If your windowsill is sloped, measure from the highest point, which is usually right against your 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 weight and height. If you cannot open your window, measure the depth on each side and add them together. Alternatively, approximate by measuring the depth on the interior side and multiplying by two. To the result from either of these methods, add the thickness of your glass pane if you know it. A single pane of glass varies in thickness, but 1/8" (3mm) is a reasonable estimate. There is usually a minimum depth required for installing a specific replacement window or storm window, but as long as the depth is larger than that minimum, you do not need an exact measurement. 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 should let the manufacturer of the new windows or shutters know that the opening is "racked" and give them these measurements. 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.
Measure the width at three points and use the smallest measurement. Account for the lining around the window if present. Measure the height at three points. Measure the depth of the window if you are installing replacement windows. Check whether the window opening is rectangular. Be clear about where you measured when ordering windows or shutters.