Free OnlineCalculators & Converters
Window Opening Calculator
Calculate rough opening dimensions and header size for windows and doors. Add multiple openings of different sizes.
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Width (in)Height (in)QtyRough Opening
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Key Features — Window Opening Calculator
Rough opening width and height
Structural header size recommendation
Multiple window support
Total glass area
Standard framing practices
Why Use This Tool — Window Opening Calculator
Free Online Window calculatorRough opening calculator — for EveryoneFast & Easy Window rough opening sizeHeader size calculator — 100% FreeFree Online Door rough opening calculator
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Frequently Asked Questions — Window Opening Calculator
What is a rough opening?
The framed opening before the window is installed. Typically window size + 2" wide and + 2.5" tall for shimming.
How to size a window header?
Span ≤36": doubled 2×4; ≤48": doubled 2×6; ≤60": doubled 2×8; ≤72": doubled 2×10. Consult engineer for load-bearing.
How to measure for replacement windows?
Measure width and height at 3 points each; use the smallest. Add 2" to width and 2.5" to height for rough opening.