What size window or door should I order?

Measure the rough opening, the hole in the wall framing, then order a unit a little smaller so it can be levelled, shimmed and sealed. Here are the common rules with examples.

How do I work out the window size from the rough opening?

A replacement or new-construction window is usually ordered about ½ inch (13 mm) smaller than the rough opening in both width and height. That gap lets the installer level and shim the window and seal it properly. Always check the rough opening size listed by the window maker before you order.

How do I work out the door size from the rough opening?

For a standard interior prehung door, the rough opening is typically about 2 inches (51 mm) wider and 2½ inches (64 mm) taller than the door slab. That leaves room for the jambs and shims. So a 34 × 82½ inch opening suits a 32 × 80 inch door. Exterior doors vary more, so follow the maker's chart.

Worked examples

How do I measure a window opening accurately?

Measure the width at the top, middle and bottom, and the height at the left, middle and right. Use the smallest of each, because walls and frames are rarely perfectly square. When replacing a window without removing the frame, measure inside the existing frame instead and ask your supplier which size to quote.

See the new style first

Roomveil lets you add each window and door to your wall, choose a style, frame colour, glass and hinge side, and see it on a photo of your room. It lists the size to order for each one, in any unit.

These are common rules of thumb and code minimums, given for planning. Always check the product label, the maker's instructions and your local building code before you buy or build.