How much clearance does a window need?

The common rule is 1/2 inch of shim space on each side and 1/2 inch at the head, so the rough opening is the window unit width plus 1 inch and the unit height plus 1/2 inch. Always confirm against the manufacturer's installation sheet, which sometimes calls for more.

Why do doors need more clearance than windows?

Pre-hung doors include a jamb and need room to plumb and shim it, plus clearance at the threshold. A common allowance is the door slab width plus 2 inches and the height plus about 2.5 inches, giving room for the jamb, shims, and floor finish.

Is the unit size the same as the glass size?

No. The unit size is the outside frame dimension of the whole window or door. The glass or panel inside is smaller. Always size the rough opening from the unit dimension, never from the glass or the bare slab.

What is the difference between RO and unit size?

Unit size is what you order. Rough opening (RO) is the framed hole between studs and header that the unit drops into, made deliberately larger so you can shim the unit plumb, level, and square. This tool converts one to the other.

Should I always trust the calculator over the spec sheet?

No. These are industry-standard defaults that suit most residential units, but manufacturers vary. When you have the installation instructions for your specific product, use their stated rough opening — it overrides any general rule.

What is the Window Rough Opening Calculator?

Converts a window or door unit size to the rough opening it needs by adding standard shim clearance. Windows add about 1/2 inch per side and 1/2 inch at the head; doors add roughly 2 inches in width and 2.5 inches in height for the jamb and threshold. Outputs RO width and height for framing layout. It runs free in your browser on Gera Tools, with nothing uploaded.

Window Rough Opening Calculator

Name: Window Rough Opening Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Frame a window opening too tight and the unit will not shim square; too loose and you waste flashing and trim. The rough opening (RO) is deliberately larger than the window unit so it can be shimmed plumb and level. This calculator applies the standard clearances to give you the RO from the unit size.

Rough opening = unit size + shim space

For a typical residential window and a pre-hung door:

Window RO width  = unit width  + 1.0 in   (about 1/2 in shim each side)
Window RO height = unit height + 0.5 in   (shim at the head)
Door RO width    = slab width  + 2.0 in   (jamb + shims)
Door RO height   = slab height + 2.5 in   (jamb + threshold + finish floor)

These are starting defaults. The manufacturer’s installation sheet for your specific product always wins, because some units call for more or less clearance.

Worked examples

Window — standard double-hung, 36 × 48 inch unit size:

RO width  = 36 + 1.0 = 37.0 inches
RO height = 48 + 0.5 = 48.5 inches

Mark the jack studs 37 inches apart (inside face to inside face) and frame the rough sill so the header leaves 48.5 inches of clear height.

Door — pre-hung exterior, 36 × 80 inch (3-0 × 6-8) slab:

RO width  = 36 + 2.0 = 38.0 inches
RO height = 80 + 2.5 = 82.5 inches

A 38-inch RO width allows for the door frame’s two 3/4-inch jambs plus shim room on each side. The 82.5-inch height leaves room for the head jamb, the threshold, and any finish floor height built up under the slab.

Understanding the RO components

Why the door needs so much more clearance than the window

A window unit is factory-finished on all four sides; you only need shim space to plumb and level the frame. A pre-hung door, on the other hand, arrives attached to its jamb, which is usually 3/4 inch thick on each side leg. The RO width allowance of +2 inches covers two jamb legs (1.5 inches total) plus approximately 1/2 inch of shim room. The height adds the head jamb (3/4 inch) plus any floor-level variation and threshold stack.

Unit size vs. glass size vs. rough opening

These three measurements are often confused on job sites:

Measurement	What it is	Use for
Unit size	Outside frame dimension of the complete unit	Ordering, ordering quotes
Glass (DLO) size	Daylight opening — the visible glass area	Estimating light, not framing
Rough opening	Framed hole between jack studs and header	Framing layout and cutting

Always frame the RO; never try to frame to the glass size.

Practical framing notes

King studs run full height alongside the opening, from bottom plate to top plate.
Jack studs (trimmers) sit inside the king studs and support the header; their height sets the top of the rough opening.
Header spans the opening and carries the load from above; it bears on the jack studs, not the king studs.
Rough sill (for windows only) spans between the jack studs at the bottom of the opening; cripple studs support it from below.

Always cross-check the RO with the manufacturer’s rough opening table printed in the installation guide before you cut. Some window lines, particularly European tilt-and-turn systems, require different clearances than the residential defaults used here.

Check the manufacturer’s table before you cut

The ½-inch-per-side convention this calculator applies is the industry default, but the number that governs your build is the one in the window maker’s installation instructions — manufacturers publish rough-opening tables per product line, and some (notably for flanged vinyl and new-construction fiberglass units) specify different clearances or as-small-as-possible openings for structural screws. Cross-check the calculated RO against the spec sheet for the exact unit ordered — e.g. Andersen’s install documentation — before framing. Two code-side notes for US residential work: bedrooms require at least one egress-compliant opening (minimum clear opening area and dimensions per IRC section R310, published in the ICC’s International Residential Code), which constrains how small a bedroom window can be regardless of framing preference; and the header sizing above the opening is a structural calculation, not part of the RO clearance. Finally, remember the RO must be plumb, level and square within the shim space — a half-inch of clearance absorbs construction tolerance only if the opening’s corners are actually where the tape measure says they are.