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.

Window Rough Opening Calculator

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.

How it works

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.

Example and notes

A 36 by 48 inch window unit needs a rough opening of 37 by 48.5 inches. A 36 inch pre-hung exterior door needs roughly a 38 inch wide by 82.5 inch tall RO for a standard 80 inch slab. Lay out your king studs to the outside of the jack studs, keep the header bearing on the jacks, and check the spec sheet before cutting if you have it.