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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.
| Difference between diagonals | Opening is | What to order |
|---|---|---|
| Up to ¼ in | Square | A pocket (insert) replacement, sized as below |
| ¼ to ½ in | Slightly out | A pocket replacement; shim and caulk the gap |
| More than ½ in | Out of square | A 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.
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, centre 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.
Width jamb to jamb at the top, middle and bottom; height sill to head at the left, centre and right; both diagonals. Use the smallest width and height, less ¼ inch, as the order size.
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.
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.
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.