Step 1. Take 3 horizontal measurements of the window frame for the rough opening width.

• Measure the distance horizontally across your frame at the top, at the middle and at the bottom from each side jamb.
• Circle the smallest measurement of the three figures. The smallest measurement is your rough opening width, the main measurement to reference when purchasing a replacement window.

* Tip: Always list window measurements with width before height.

Step 2. Take 3 vertical measurements of the window frame for the rough opening height.

• Measure the distance vertically across your frame from top to bottom on the left, middle and right side of the window from head jamb to the window sill.
• Circle the smallest measurement of the three figures, this is your rough opening height.

* Tip: If your window sill is sloped, measure from the highest point of the window sill, which is usually right against your window.

Step 3. Measure the depth of the window’s opening.

Finding the correct depth is important because some windows will be too deep to fit into a replacement window opening.
• Run a tape measure from the outside of the frame to the inside and take the measurement from edge to edge of the window frame.
• Do this in several locations around the window frame and rely on the smallest measurement once again as your final dimension.

* Tip: If you can’t open your window, measure the depth on each side of the glass and add them together. Then add the thickness of your window glass pane. A single window glass pane is usually 1/8″ or 3mm in thickness.