How to Measure for Replacement Windows
Measure the width and height in three places each, order to the smallest of each, and check depth and squareness — the two-minute method that keeps a custom-ordered window from arriving too big to fit or too small to seal.
Replacement windows are cut to order, so the single most expensive mistake you can make happens before anything is bought: mismeasuring the opening. Order a hair too big and the unit will not go in; order too small and you are left filling gaps with foam and trim that never look right and never seal. The method professionals use removes almost all of that risk, and it takes about two minutes per window.
The smallest-of-three method
Old openings are rarely square. A house settles, framing twists, and paint builds up in the corners, so the width at the top of a window is almost never the width at the bottom. If you measure once and order to that number, you are gambling that you happened to hit the tightest point. Instead, measure the width in three places — near the top, across the middle, and near the bottom — and the height in three places — at the left, the center, and the right. Then order to the smallest width and the smallest height. A window sized to the tightest point will always fit; a window sized to the widest point may not.
Measure to the surfaces the new unit actually bears on. For an insert (pocket) replacement you measure the existing frame, jamb to jamb and sill to head, because the new window drops into that frame. For a full-frame replacement, where the old frame is torn out down to the studs, you measure the rough opening in the framing. The two numbers are not the same, which is exactly why you decide insert versus full-frame before you measure, not after. The full-frame vs insert guide walks through that choice.
A worked example
Say a double-hung opening reads 35.6, 35.5, and 35.7 inches wide at the three points, and 59.4, 59.5, and 59.6 inches tall. The smallest width is 35.5 inches and the smallest height is 59.4 inches, so you order to 35.5 × 59.4. Notice the differences are small — a tenth or two of an inch — but that tenth is the whole game. The how-to-measure calculator does this comparison for you: enter all six readings and it returns the two order dimensions.
One more step before you order: most manufacturers apply a replacement deduction to the size you give them — they build the unit slightly smaller than your measured opening so it slides in with room to shim. Read the spec sheet and confirm whether the size you enter is the opening size or the unit size, because getting that backward doubles the deduction and leaves you a window an inch small.
Check depth and squareness too
Width and height decide whether the window fits the hole; two more checks decide whether it fits well. First, depth: an insert needs enough pocket depth between the interior stop and the exterior blind stop for the new frame to seat. Measure it and compare to the manufacturer's minimum before you assume an insert will work. Second, squareness: measure the two diagonals of the opening, corner to corner. If they match, the opening is square; if they differ by more than about a quarter inch, the opening is racked, and you will need to shim the new unit plumb and level rather than simply following the old frame.
United inches and the rough opening
Two numbers fall straight out of your measurements and are worth writing down. United inches — width plus height — is how suppliers quote and price a window, so a 35.5 × 59.4 unit is about 95 united inches; see united inches explained and the united-inches calculator. The rough opening is the framed hole a full-frame window needs, and it runs larger than the unit — roughly a half inch of shim space per side. The rough-opening calculator and the standard rough-opening gaps table give the usual allowances, but always confirm the exact rough opening on the product's own spec sheet, because it varies by brand.
Common mistakes
- Measuring once. The whole point of three readings is to catch the out-of-square opening. Skipping it defeats the method.
- Measuring the sash instead of the opening. Measure the frame or rough opening, not the glass or the moving sash.
- Ignoring the deduction. Ordering the raw opening size without the manufacturer's replacement deduction gives a unit that binds.
- Forgetting depth. A shallow pocket quietly rules out an insert; check it early.
- Rounding to a stock size. If your opening is 33.25 × 57.5, it is a custom order; forcing it to a nearby stock size leaves gaps. The standard vs custom size reference tells you which you have.
None of this replaces an installer's site visit. Measuring is a screen you can do to plan a budget and sanity-check a quote — get itemized written quotes from licensed, insured installers, and let them confirm the final order sizes against the product they carry.