Width and height, three points each.
For width, measure across the opening at the top, the middle, and the bottom, from the inside face of the frame on one side to the inside face on the other. Write down all three and use the smallest. For height, measure on the left, the center, and the right, from the sill to the head, and again use the smallest. Old openings settle out of square, and the smallest measurement is the one the window has to clear.
Measure to the inside of the existing frame, not to the glass and not to the outer trim. If you are replacing the whole unit including the frame, note whether you are measuring the rough opening or the existing frame, and tell your supplier which, because it changes the size they build.
The rule that prevents a misfit.
When in doubt, smaller is safer than larger. A window built slightly under the opening can be shimmed and sealed to fit. A window built larger than the opening does not go in at all. This is why three measurements and the smallest-wins rule exist.
If the numbers vary a lot between points, the opening is significantly out of square, which is worth telling your supplier so the unit and the install allow for it. For anything you are unsure about, send us the rough numbers and a photo, and we will confirm the build size before anything is made.
Common questions.
- How do you measure a window for replacement?
- Measure the opening width at the top, middle, and bottom, and the height at the left, center, and right, measuring to the inside of the existing frame. Use the smallest width and the smallest height, because openings are rarely perfectly square and the window has to clear the tightest point.
- Should I measure to the frame or the glass?
- Measure to the inside face of the existing frame, not to the glass and not to the outer trim. The frame-to-frame opening is what the replacement window has to fit into.
- What if my measurements are different at each point?
- That means the opening is out of square, which is normal in older homes. Use the smallest measurement so the window clears the tightest point, and tell your supplier the opening is out of square so the install can allow for it.
Project in motion
Not sure on a measurement?
Send your rough numbers and a photo of the opening. We confirm the build size in writing before anything is made.