How to size a window rough opening
Use the window's frame size, frame to frame, and the minimum rough opening its maker lists. Andersen's A-Series picture window tables, for example, pair a 1 ft 3-1/4 in window with a 1 ft 4 in minimum opening, about 3/4 in larger each way, and note that the minimum may need to grow for some installations[1]. Other lines use other allowances, so read the product guide for the exact window.
Sill height and cripples
The rough sill height is the distance from the subfloor to the bottom of the opening. Under the sill sit cripple studs on layout, standing on the bottom plate under a single or double 2x sill; above the header, cripples carry the top plates. The calculator gives both lengths from your wall height and header size, and the wall layout comes from the stud calculator.
Headers for windows
Window headers in bearing walls are sized from IRC Tables R602.7(1) and (2), and the king studs at each end from Table R602.7.5 in exterior walls[2]. Nonbearing walls can use a flat 2x4 over openings up to 8 ft. For the doors in the same wall use the door rough opening calculator, and the framing calculator for the full wall.
Common window opening mistakes
- Using the glass or sash size. Rough openings come from the frame size.
- No room for flashing. Keep the full allowance; the gap is where shims, flashing and sealant go.
- Uneven sill. Level the sill before setting the window; a twisted opening racks the frame.
Frequently asked questions
How much bigger is a rough opening than a window?
Usually about 1/2 to 3/4 in in each direction, as the window maker specifies; Andersen A-Series picture windows list 3/4 in.
How do I calculate window sill height?
Measure from the subfloor to where the bottom of the window should sit, then frame the rough sill at that height.
What size header do I need for a window?
Read IRC Table R602.7(1) or (2) for the span, building width and load; the header length is the opening plus 3 in.