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Calculate window sizes from opening measurements or vice versa
Ordering the wrong size window is an expensive mistake. Too small and there's an ugly gap. Too large and it won't fit at all. Window sizing involves understanding the difference between the rough opening in your wall and the actual window unit size.
The rough opening is the framed hole in your wall—it's always slightly larger than the window unit to allow for shimming, leveling, and insulation. Standard windows fit standard rough openings, but your openings might not be standard, especially in older homes.
Our window size calculator helps you determine the right window unit size for your rough opening, or the rough opening you'll need for a window you want. Either way, you'll get the measurements right the first time.
Window sizing involves two key measurements:
Windows that fit correctly are easier to install, seal better, and look professional.
Custom windows often can't be returned. Even standard windows involve hassle and restocking fees if wrong.
Gaps around poorly fitted windows leak air, costing you money in heating and cooling.
Correct sizing means the window goes in smoothly without forcing or excessive shimming.
Mistake: Measuring the old window instead of the opening
Solution: The existing window includes frame thickness. Measure the actual opening to get accurate dimensions for a new window.
Mistake: Assuming openings are square
Solution: Walls settle and shift. An opening might be 36 inches at top but 36.5 at bottom. Measure multiple points.
Mistake: Forgetting about sill depth
Solution: If you're getting a different window style (like going from a picture window to a casement), check that your sill depth accommodates it.
Mistake: Not accounting for installation method
Solution: New construction and replacement windows size differently. Make sure you're ordering the right type for your project.
Professional measurement is worth it when:
The rough opening is the framed hole in your wall. The window unit is smaller—typically 1/2 to 1 inch narrower and shorter than the opening. This gap is needed for shimming, leveling, and foam insulation.
For replacement (insert) windows, measure inside the existing frame at top, middle, and bottom for width; at left, center, and right for height. Use the smallest measurements and check that the frame is in good condition.
Nominal size is a standard designation, not the exact measurement. A 3040 window (30×40) isn't exactly 30 inches wide—it's designed to fit a rough opening of about 30.5×40.5 inches. Always check actual unit dimensions.
Yes, but it requires removing framing and may need a structural header if you're widening significantly. Making it smaller is easier—just add framing lumber. Either change affects the wall and needs proper support.