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How to measure a window that isn't square
Older windows are rarely square — and a made-to-measure piece is only as good as the number you give. Here's how to measure an out-of-square opening so it still fits.
Measure three, not one
For width, measure at the top, middle and bottom. For height, measure left, centre and right. If they differ (they will), you've found the out-of-square.
Check the diagonals
Measure corner-to-corner both ways. If the two diagonals differ by more than ~1 cm (3/8 in), the opening is meaningfully out of square — plan accordingly.
Which number to use
- Inside mount (blinds/screens in the recess) — use the smallest width and, depending on product, the relevant height, so the piece clears the tightest point.
- Outside mount / curtains — out-of-square hides behind an outside mount or gathered curtains; measure for coverage, not the recess.
- Soft pieces (curtains, some screens) forgive a few millimetres; rigid pieces (blinds) don't — an out-of-square recess often means outside mount.
FAQ
What if the top and bottom widths of my window are different?
That's a normal out-of-square window. For an inside-mount rigid piece use the narrowest width so it clears; for curtains or an outside mount it doesn't matter — measure for coverage instead.
How out of square is too much?
If the two diagonals differ by more than about 3/8 inch, a flush inside-mount rigid blind will show gaps or bind. That's the point to switch to an outside mount, which hides the discrepancy.
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