Window & Door Rough Opening Calculator
Work out the rough (framed) opening a window or door needs, allowing a shim gap all round, plus the header/lintel length including its bearing each end. Handles both windows (gap top and bottom) and doors (open at the floor).
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the unit width and height of the window or door in millimetres.
- Set the shim/clearance gap per side (10 mm is typical) — it is added to both sides of the width.
- Put 1 in the door box for a door (no sill) and add a threshold allowance if needed; read the rough opening width, height and the header/lintel length.
How it works
The rough opening is the unit size plus a shim gap on each edge so the unit can be levelled and packed. Width = unit width + 2 × gap. A window gets a gap top and bottom, so height = unit height + 2 × gap. A door is open at the floor, so it gets one gap at the head plus any threshold allowance: height = unit height + gap + sill. The header (lintel) spans the opening and bears on the trimmer studs, so its length = rough opening width + 2 × bearing (default 100 mm each end).
Worked example
Worked example. A 900 × 1200 mm window with a 10 mm shim gap needs a rough opening of 900 + 20 = 920 mm wide by 1200 + 20 = 1220 mm high. With a 100 mm bearing each end, the header is 920 + 200 = 1120 mm long.
Common mistakes
- Adding the gap only once instead of to both sides of the width.
- Treating a door like a window and adding a gap at the floor where there is no sill.
- Sizing the header from this length alone — the bearing length and lintel section must come from the structural design for the span and load.
Frequently asked questions
How much bigger should a rough opening be than the window?
Usually about 10 mm per side, so roughly 20 mm on the overall width and height, to allow packing and squaring. Always check the window or door manufacturer's stated rough-opening size, as it varies by product and fixing method.
Why is a door rough opening handled differently from a window?
A door has no sill or frame across the bottom — it opens to the floor — so you only add a shim gap at the head, not at the base. A window is packed on all four sides, so it gets a gap top and bottom as well as each side. Add a threshold allowance for the door if a sill or floor finish sits under it.
Does the header length include the bearing?
Yes. The header/lintel length here is the rough opening width plus a bearing at each end (default 100 mm) so it rests on the trimmer studs. The actual bearing length and the lintel size must be taken from the structural design for the span and the loads above.
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