How to measure windows for replacement: width, height, and each window type
Measure each window's width at the center, trim to trim, then its height, trim to trim, and write both down with the window's type (Pella). On a bay or bow window, measure each panel's width on its own, since the center one can differ. Those numbers price an estimate; the installer confirms the exact size before the windows are built.
Anthony Hicks30+ years in construction. Updated November 5, 2026.
Reading as
The three steps
Pella’s steps for a first measurement, the one that gets a window priced:
- Width: across the window at its center, from trim to trim.
- Height: up the window, from trim to trim.
- Write both down, with the window’s type and the room it’s in.
By window type
| Window | Width | Height |
|---|---|---|
| Casement or awning | At the center, trim to trim | Trim to trim |
| Double-hung or single-hung | In the middle, trim to trim | Trim to trim |
| Sliding | At the center, trim to trim | In the middle, trim to trim |
| Bay or bow | Each panel at its center, the center panel noted on its own | At the center, trim to trim |
The tape measure guide covers reading the fractions off the tape.
Before the order
These measurements set the budget, not the build. The window and frame size an installer orders can differ slightly from them, and Pella’s own professionals confirm the measurements and inspect the openings before the windows are made.
A list for the estimate
For each window, the estimate needs:
- The room and which wall it’s on.
- The type: double-hung, casement, sliding, picture, bay or bow.
- Width and height, as measured above.
- Anything unusual: rot at the sill, a broken seal, a grid pattern to match.
The window replacement cost guide prices a typical window by state. On an iPhone Pro, BluMatrix’s room scan records each window’s width and height with the walls, so the list comes with the floor plan.
Common questions
Do you measure the inside or outside of a window for replacement?
For a first estimate, Pella’s steps measure from trim to trim. The installer takes the exact measurements for the order.
Where do you measure the width of a window?
At the center, from trim to trim; on a bay or bow window, each panel at its own center.
Do I need exact measurements to get a quote?
Not for a first one. Pella’s steps call the first measurement a starting point for a cost estimate, and its professionals confirm the exact measurements before the windows are built.