Enter window size, choose unit, then set shim and flashing allowances fast. Get width and height rough openings, area, and printable exports for jobsites.
The rough opening is the framed space that accepts the window unit plus working clearance.
| Window size (in) | Side each (in) | Top (in) | Bottom (in) | Rough opening (W×H in) |
|---|---|---|---|---|
| 36 × 48 | 0.25 | 0.25 | 0.50 | 36.5 × 48.75 |
| 48 × 60 | 0.25 | 0.25 | 0.75 | 48.5 × 61 |
| 60 × 48 | 0.125 | 0.125 | 0.25 | 60.25 × 48.375 |
Example values are illustrative—verify your window’s specification sheet.
A rough opening is the framed hole that receives the window unit. If it is undersized, the unit binds, bows, or won’t square. If it is oversized, you risk weak fastening, excess foam, and water issues. Crews target a controlled clearance that still allows shimming and plumbing.
Many installers start with about 1/4 inch per jamb (1/2 inch total width allowance) and 1/2 inch at the head. Sill allowance varies by detail: some use 1/4–3/8 inch when setting on a level sill, while others leave more room when a sloped sill pan or backdam is planned. Always follow the manufacturer’s rough opening callout when provided.
Use the unit’s frame width and height, not the glass size. For flanged windows, the frame dimension is usually the “unit size,” while “net frame” or “daylight opening” can be smaller. For block-frame or retrofit units, confirm whether the dimension refers to the overall frame or the insert-only portion.
Field notes can mix millimeters, centimeters, and inches. Conversions change rounding, so decide on a working unit early. A practical rule is to keep framing dimensions in inches and record the allowance separately for quick spot-checks.
The clearance is not “extra space”; it’s working room for shims. Typical shim stacks at hinge points can be 1/8–3/16 inch, and you may need more at one side to correct out-of-plumb studs. Leave enough room to square the unit without crushing the frame.
Rough openings must coordinate with stud spacing, king/jack studs, headers, and sill cripples. Small changes—like adding 1/2 inch to width—can affect how many cripples fit and whether a fastener lands in solid framing. The calculator helps lock a target opening before you cut and nail off.
Water management needs room for a sill pan, self-adhered flashing, and backer rod or low-expansion foam. Too tight an opening can tear flashing or force the unit out of plane. Too loose can create thick foam beads that shrink or crack. Balanced tolerances reduce callbacks.
Verify unit size and handing, confirm the allowance values, check sill level, and measure diagonals to confirm squareness. After framing, dry-fit the unit, then adjust with shims before fastening. Record final dimensions for inspection.
A common starting point is 1/4 inch per side for width and 1/2 inch at the head. Sill allowance depends on your pan and leveling method. Use the window’s specification sheet whenever it lists a required rough opening.
Use the overall frame (unit) width and height. Glass or visible opening sizes are smaller and can lead to an undersized opening. If you have a retrofit or block-frame unit, confirm which dimension the manufacturer calls “unit size.”
Allowances are often different because shimming and flashing needs vary by edge. Separate inputs let you match real jobsite details, such as extra head room for leveling or a sill pan, while keeping tight jamb control.
Enter your window size in millimeters or centimeters, choose the same unit in the calculator, and it converts consistently. For framing, many crews still mark inches on studs, so consider printing the result in inches for layout.
Too tight can rack the frame, tear flashing, and prevent proper shimming. A reliable air seal comes from correct flashing, backer rod, and low-expansion foam—not from forcing the unit into a tight hole.
Most rough opening callouts are for the framed opening that the unit fits into; the flange overlaps the sheathing, not the hole. Always confirm the manufacturer’s definition because some specialty systems specify differently.
Measure width at top, middle, and bottom; height at left and right; and both diagonals for square. Confirm sill level. Dry-fit the unit and confirm shim space before fastening so adjustments stay easy.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.