What size window or door should I order?

First decide what you are measuring: the old door itself, the framed hole in the wall, or the inside of an old window frame. Each one gives a different size to order. Here are the three rules with examples.

What should I measure first?

There are three common cases. To replace an interior door in its existing frame, measure the old door itself. To fit a new prehung door or a new-construction window, measure the rough opening, the framed hole from stud to stud. To fit a replacement window inside an old frame, measure inside that frame. Roomveil asks which one you measured.

How do I measure a door I am replacing?

Measure the old door slab itself, edge to edge, across its width and from top to bottom, and order a new slab of that size. Standard US interior doors are 24, 28, 30, 32, 34 or 36 inches wide and 80 inches tall, so a slab that measures 32 by 80 inches is ordered as a 32 × 80 door, often written 2′8″ × 6′8″.

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

The rough opening is the framed hole, measured from stud to stud and from the subfloor to the header. For a standard interior prehung door it is about 2 inches (51 mm) wider and 2½ inches (64 mm) taller than the door, which 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.

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

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

How do I measure a replacement window for an old frame?

A replacement or insert window fits inside the old frame, which stays in the wall. Measure the width inside the frame at the top, middle and bottom, and the height at the left, middle and right. Use the smallest of each, then take off about ¼ inch so the new window still fits where the old frame is out of square.

Worked examples

See the new style first

Roomveil lets you add each window and door to your wall, choose a style, frame color, glass and hinge side, and see it on a photo of your room. It asks whether you measured the old door, the rough opening or the inside of an old frame, then 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.