Measuring Guide for Replacement Windows: Get the Perfect Fit
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- Issue Time
- May 9,2025
Summary
Windows don’t last forever—years of weather exposure, UV rays, temperature fluctuations, condensation, and general wear can cause frames and glass units to degrade. Over time, they become less energy-efficient, less attractive, and may even compromise your home's comfort.
Before you start, gather:
A tape measure (preferably metal for accuracy)
A notepad and pen (or smartphone for notes)
A level (to check for squareness)
A step ladder (for high windows)
Take three measurements (top, middle, bottom) of the window opening.
Here's how to collect the measurements for all three places with your tape measure:
Top: Start inside the left jamb and stretch your tape measure across the window to the inside of the right jamb. Record this measurement.
Middle: Repeat the process at the center, going from the inside of the left jamb to the right jamb, and record the measurements.
Bottom: Repeat the process at the bottom, going from the inside of the left jamb to the right jamb, and record the measurements
After collecting the measurements, circle the smallest one, as this is the measurement to start shopping with.
Similar to measuring width, you’ll also measure three places for window height: the left side, center, and right side.
It will work fairly the same way as the previous step, but follow the instructions below for proper measurement collecting,
For insert replacements, measure from the sill to the head jamb.
For full-frame replacements, measure from the bottom to the top of the rough opening.
Left side: Position your tape measure at the top of your sill (the part right underneath the bottom head), and pull your tape measure upward to stop at the top of the head. Record this measurement.
Middle: Repeat the process at the center of the window and record the measurement.
Right side: Repeat the process at the right side of the window and record the measurement.
Again, use the smallest measurement.
Measure the diagonals of the window opening (top left to bottom right, and top right to bottom left).
If the measurements differ by more than ¼ inch, the frame may be out of square, requiring adjustments during installation.
Depth of the frame – Ensure the new window fits within the existing space.
Obstructions – Look for trim, handles, or exterior features that may affect fit.
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