How to measure for replacement windows
Measure width in three places and height in three places, then order to the smallest of each so the new unit clears an out-of-square opening.
Use the smallest of your three widths and three heights — about 35.50 × 59.40 in — so the new unit fits the tightest point of an out-of-square opening. Then subtract the manufacturer’s replacement deduction per their spec sheet. Measure jamb-to-jamb and sill-to-head, and confirm before you order.
Calculator inputs
Old openings are rarely square. Houses settle, framing twists and decades of paint build up unevenly, so the width at the top of a window is almost never the width at the bottom. The professional habit is simple: measure the width in three places and the height in three places, and order to the smallest of each. Order to the largest and the unit will not fit the tight spot; order to an average and it may bind at one corner. The smallest measurement is the one the window has to pass through.
Measure to the jambs (the inside faces of the frame), not to the trim or the old sashes. Take widths at the top, middle and sill; take heights at the left, center and right, from the high point of the sill to the underside of the head. This tool returns your order size — but treat it as the opening, then apply the manufacturer's replacement deduction before you actually order.
Formula
order width = min(widthtop, widthmiddle, widthbottom)
order height = min(heightleft, heightcenter, heightright)
Then subtract the maker's replacement deduction (often about ¼ inch off each dimension for an insert) per their spec sheet — never guess it.
Worked example
Widths of 35.6, 35.5 and 35.7 in; heights of 59.4, 59.5 and 59.6 in:
order width = min(35.6, 35.5, 35.7) = 35.5 in
order height = min(59.4, 59.5, 59.6) = 59.4 in
So the opening governs at 35.5 × 59.4 in. A half-inch spread across three width readings is normal and exactly why you take three — a single reading at the middle would have missed the tight top and bottom.
Measuring cleanly (and the mistakes that cost a re-order)
A window measured wrong is a custom unit you cannot return. Work through the details:
- Measure to the jambs, not the sashes. The sashes are the moving glass; the frame opening is what the new unit sits in. For an insert, measure the existing frame; for full-frame, measure the rough opening in the studs.
- Three widths, three heights, smallest wins. The tight point can be anywhere — a bowed sill, a sagged head, a leaning jamb. The smallest reading is your working size.
- Check for square and plumb. Compare the two diagonals; if they differ, the opening is racked and you should lean conservative and flag it to your supplier.
- Then apply the deduction. Manufacturers publish a replacement deduction so the unit drops in with room to shim. It is product-specific — read the spec sheet rather than assuming a number.
Once you have the order size, the rough-opening calculator confirms the framed hole for a full-frame job, and united inches gives you the single figure suppliers price by. Measure twice, confirm against the exact product, and allow lead time for custom sizes.
Reference table
| Reading | Take it at | Use |
|---|---|---|
| Width ×3 | Top, middle, sill (jamb to jamb) | Smallest of the three |
| Height ×3 | Left, center, right (sill to head) | Smallest of the three |
| Diagonals ×2 | Corner to corner both ways | Equal = square; unequal = racked |
Then subtract the manufacturer’s replacement deduction per the spec sheet before you order.