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How to Measure for Replacement Windows

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By Srinivas Gaddam, integration architect · Reviewed by OneCalculatorHub editors, figures checked against the cited standards · Updated 29 August 2026

Measure jamb to jamb in three places for width, sill to head in three for height, and both diagonals. The smallest width and height, less ¼ inch, is the size to order; the diagonals tell you whether the opening is square enough for a pocket replacement.

Your opening

What the diagonals tell you

Difference between diagonalsOpening isWhat to order
Up to ¼ inSquareA pocket (insert) replacement, sized as below
¼ to ½ inSlightly outA pocket replacement; shim and caulk the gap
More than ½ inOut of squareA full-frame replacement, or have the opening corrected first

Measure the diagonals from the same corners the widths and heights came from — the inside of the frame.

How this was worked out

Width is measured from the inside face of one side jamb to the other, at the top, middle and bottom. Old frames bow; the window has to fit the narrowest point, so the smallest of the three is used.

Height is from the sill (the flat part the sash rests on, not the sloped stool) up to the head jamb, at the left, center and right; the smallest is used.

Order size is the smallest width and the smallest height each less ¼ inch, which leaves room to level and shim. Most manufacturers ask for the opening size and take the deduction themselves — check the order form; the calculator shows both numbers.

Square check. If the two diagonals are within ¼ inch the opening is square and a pocket replacement drops in. Beyond ½ inch the sash will bind or leak, and a full-frame replacement is the right job.

Sources. There is no published standard for how to measure a pocket replacement window — the method here is the manufacturer convention common to US makers’ measuring instructions, and the deduction differs between them, so this page gives the common range rather than one figure. The three-point width and height check, the ¼ in deduction and the diagonal square check are those instructions. Confirm the deduction on the order form of the maker you use; their sheet governs the order, not this page. Window and door product standards are maintained by the Window and Door Manufacturers Association. Last reviewed 29 August 2026.

Common questions

How do you measure a window for a replacement?

Width jamb to jamb at the top, middle and bottom; height sill to head at the left, center and right; both diagonals. Use the smallest width and height, less ¼ inch, as the order size.

Do I subtract for replacement windows?

Usually ¼ inch from the smallest width and height, so the new unit can be shimmed level. Some makers take the deduction for you — read the order form and send the opening size if it says so.

What is the difference between a pocket and a full-frame replacement?

A pocket replacement fits inside the existing frame and keeps the trim; it needs a square, sound frame. A full-frame replacement removes everything to the rough opening and is the fix for rot or an out-of-square opening.

Where is the sill on a window?

The flat surface the bottom sash sits on, inside the frame. The sloped piece below it on the inside is the stool; the outside slope is the sill nose. Measure from the flat sill.

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