How to Measure for a Block Frame (Straight-Through) Replacement
A block frame window has no nailing fin and no attached brickmould — a clean square frame that slides straight through the opening. It's the full tear-out option: everything comes out back to the rough or masonry opening, and trim is finished after the install.
When to choose this method
You're gutting the opening — old frame, stops, and trim all coming out
You're renovating and will re-trim the interior anyway
The opening is masonry or you're replacing basement units set directly in the wall
Tools
Steel tape measure (25 ft)
Pencil and paper — or your phone
Small level or your phone's level app
A helper for tall openings
You're measuring the bare opening
The block frame is sized to the opening that's left after everything is removed: wood-framed walls measure stud-to-stud and rough sill to header; masonry walls measure the clear masonry opening. If the old frame is still in, measure behind it where you can and confirm after tear-out.
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.
Measure the opening width at three heights
Clear width of the rough or masonry opening at the top, middle, and bottom. Framing lumber twists and masonry tapers — the three-point rule is not optional here.
Measure the opening height at three points
From the rough sill (or masonry) up to the header or lintel, at the left, centre, and right.
The smallest number wins
Smallest width × smallest height is your opening size. Write it down with the location of the tight spot — your installer will thank you.
Check the diagonals
Both diagonals within 1/4″ means a straightforward install. Beyond that, the window is sized to the largest square rectangle that fits inside the opening — send us the diagonals along with your numbers and we'll size it correctly.
Take the ordering deduction
Subtract 3/4″ from the smallest width and 3/4″ from the smallest height — the standard Panes fitting allowance. That's 3/8″ per side for shimming the unit plumb, square, and level — then insulation and trim close the gap.
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.
Measure your jamb depth
Because the block frame sits inside the wall, the wall's depth decides whether you need extension jambs to meet your drywall. The frame body is 3-1/4″ deep; extensions make up the rest. One more minute with the tape — guide below.
Sliding Patio Doors
Measure the rough opening width at three heights and the height from the subfloor (not the finished floor) to the header at three points.
Take the same 3/4″ deduction both ways — it's the fitting allowance our sliding doors are built around, and a deducted size within an inch of a standard (59-1/2″, 71-1/2″, 95-1/2″ wide) still prices as a standard door.
Tell us your finished floor height if flooring isn't installed yet — the door threshold is sized against the finished floor.
Note the operating panel side, viewed from outside.
Entry Doors
Entry door rough openings measure the same way: width at three heights, height from subfloor to header at three points.
Entry doors take a different deduction than windows: we size the pre-hung frame at 1″ under your rough opening width and 1/2″ under the height — send us the opening numbers and we handle it.
Sizing from the slab instead? A single-door frame is the slab width plus 1-1/2″, and the slab height plus 3-3/8″ (inswing) or 2-1/2″ (outswing).
Note swing direction and hinge side, viewed from outside.