How to measure a sash window (and the one mistake everyone makes)

Measuring for made-to-measure windows is the part everyone dreads, so let's take the fear out of it: ten minutes, a tape measure, and three rules.

Rule one: measure the hole, not the window. You're measuring the brick opening — the structural hole in the wall — not your old frame. Old frames are often packed, shimmed and settled; the opening is the truth.

Rule two: three measurements, use the smallest. Openings are never perfectly square. Measure the width at the top, middle and bottom; measure the height at the left, centre and right; write all six down. We work from the smallest of each. Then run the tape corner-to-corner both ways — if the diagonals differ by more than about 10mm, just tell us in the order notes.

Rule three: every opening gets measured. Two windows that look like twins are routinely 10–20mm different. Measure both. If two openings genuinely must match, tell us which — that's a design decision, and we'll flag it on your drawings.

The one mistake everyone makes. Wide openings — anything past about 1600mm — are usually built as two sashes sharing one frame, with a timber divider between them. The mistake is doing the arithmetic yourself: splitting the opening into "two windows" and guessing the divider. Two 820mm windows plus a divider does not neatly equal an 1800mm opening, and we've watched exactly that sum go wrong. Give us the full opening size and let us do the division — it's what the drawings are for.

And here's the promise that makes it safe. Every order comes back to you as a production drawing with every final dimension on it, and nothing is made until you approve it. If you'd rather not measure at all, book a measuring visit (£149) and we guarantee the fit. And if you self-measure and something's still wrong, you don't pay full price twice — re-makes are 70% of list. Read the measuring guide