A pocket — or retrofit — replacement fits your new window inside the existing frame. Your interior trim, exterior finish, and siding stay untouched, so it's the fastest, least invasive way to replace a window that still has a solid, square frame.
Press a screwdriver into the sill and the bottom few inches of each side jamb. If the wood is soft, flaking, or holds moisture, stop here — a retrofit window inherits every problem of the old frame. A full-frame replacement (brickmould or block frame) is the right call instead.
Record every measurement in inches, to the nearest 1/16″, and always write width before height (W × H). Measure every opening — even the ones that look identical rarely are.
Inside the house, measure the clear distance between the side jambs — the finished frame surfaces the new window will sit between, not the trim and not the stops. Take it at the top, the middle, and the bottom of the opening.
Old frames bow. Measuring in one spot is how windows end up ordered 3/8″ too wide.
Measure from the highest point of the sill (most sills slope toward the outside — hook your tape at the high point, inside) up to the head jamb. Take it at the left, the centre, and the right.
Circle the smallest of your three widths and the smallest of your three heights. Those two numbers — not the average — are your opening size. A window sized to the average gets wedged at the tight spot.
Measure both diagonals, corner to corner. If they're within 1/4″ of each other, the opening is square enough for a standard retrofit.
More than 1/4″ out? Don't guess — send us photos and your numbers, or book a free in-home measure. Out-of-square retrofits are solvable, but they're sized differently.
Lay your level flat on the sill. If the sill slopes toward the exterior — most wood sills do — note it on your sheet. Sloped sills are normal; we account for them with the right sill detail, but we need to know.
Subtract 3/4″ from your smallest width and 3/4″ from your smallest height — that's how every Panes window is sized to its opening. The 3/8″ per side is what lets the new window slide in plumb, get shimmed level, and seal properly. A window ordered at the exact opening size cannot be installed well.
Enter the deducted number as your frame size when ordering. Or use our Quick Quote and choose "Rough opening" when it asks what you measured — it takes the 3/4″ off for you.