Before you start calling window installers, you can estimate your replacement window costs by using a tape measure, a level, and a calculator. Measuring for a replacement window is easy, but it's also easy to make a critical mistake if you don't follow a few expert suggestions.

If you want your window installer to leave your frames intact, measuring for a replacement window should take place inside your home. Measure the width between the inside jambs, a few inches below the top of your windows. Repeat the process at the middle of the window and at the bottom, so you have three measurements.

Next, measure from the bottom of your window's head jamb to the top of the sill. Again, take three measurements: one at the center, and one near each of the sides. While your window installer will use only the smallest measurement from each set, measuring for a replacement window this way accounts for any shifting of your home's foundation or frame.

Give Your Replacement Windows Breathing Room in a New Frame
A visit from your window installer is also a great opportunity to replace your window frames. If you are measuring for a replacement window with a new frame, you measure on the outside your home. Like the previous example, measure at three locations for both the width and the height. This time, measure to the ends of your current window frame and add an inch and a half to both length and width. Measuring for a replacement window this way makes it easier for your window installer to provide quick phone estimates of your replacement window costs.

Sources
HGTV
Lowe's