Replacement Window Fit and Clearance Calculator
Enter your rough opening dimensions to calculate the correct replacement window size with proper clearance on all sides.
Formulas Used
Window Width = Rough Opening Width − (2 × Side Clearance)
Window Height = Rough Opening Height − Top Clearance − Bottom Clearance
Shim Space (each side) ≈ Side Clearance ÷ 2
The remaining clearance gap (after shimming) is filled with low-expansion spray foam for insulation and air sealing.
Assumptions & References
- Standard industry practice recommends ¼" (0.25") clearance on each side, top, and bottom for replacement windows (Andersen, Pella, JELD-WEN installation guides).
- Minimum clearance of 1/16" is required to allow for shimming and leveling.
- Maximum recommended clearance is 1"; larger gaps require additional framing or blocking.
- Shim space is estimated as approximately half the clearance gap; actual shimming depends on out-of-square conditions.
- Remaining gap after shimming should be filled with low-expansion polyurethane foam (e.g., Great Stuff Window & Door) per AAMA 2400 installation standards.
- Rough opening dimensions should be measured at multiple points (top, middle, bottom for width; left, center, right for height); use the smallest measurement.
- These calculations apply to standard insert/pocket replacement windows. Full-frame replacements follow different sizing rules.
- Always verify local building codes and manufacturer specifications before ordering.