How to Measure a Window for Replacement

What you need

A tape measure, a pencil and paper, and a helper for large windows. Measure the opening, not the old window: the new window is made to fit the hole in the wall, and the old frame may be smaller, warped or packed out with filler that hides the true size.

Step 1 — Measure the width in three places

Measure the width inside the opening at the top, in the middle and at the bottom. Walls are rarely perfectly straight, so the three numbers will often differ. Take the smallest — the window must fit the narrowest point, and a window made to the widest measurement will not go in. Write all three numbers down; the supplier wants the smallest, but the installer wants to know how much the opening varies.

Step 2 — Measure the height in three places

Measure the height at the left, in the middle and at the right, from the sill to the top of the opening. Take the smallest, for the same reason. If the sill is damaged or uneven, measure from the sound brickwork below it and tell the supplier — a new sill may be part of the job.

Step 3 — Check the opening is square

Measure the two diagonals of the opening. If they are equal, the opening is square; if they differ, it is not, and the difference tells the installer how much packing or adjustment the new window will need. An opening that is out of square by more than a few millimetres is not a DIY measurement problem — it is information the installer needs before they order.

Step 4 — Work out the area

The window area calculator gives you the area per window, the total and the glass area. Six windows of 1.2 m × 1.5 m are 1.8 m² each, 10.8 m² in total (116.25 ft²), with a glass area of 9.18 m² after the 15% frame. The glass area matters for ordering: it is the part you see through, and it is what the glazing is priced by.

Step 5 — Order and confirm

Give the smallest width and height to the supplier, with the diagonal check noted. The installer measures again before ordering — their measurement is the one that counts, and a good installer will always re-measure. If your numbers and theirs differ, trust theirs: they measure openings every day.

Worked example

Six windows, each 1.2 m × 1.5 m. The area per window is 1.2 × 1.5 = 1.8 m². The total area is 1.8 × 6 = 10.8 m² (116.25 ft²). The glass area, after the 15% frame, is 10.8 × 0.85 = 9.18 m². Those are exactly the numbers the window area calculator gives you.

Common mistakes

• Measuring once, in one place. One measurement of a wonky opening is a guess. Three places, smallest number, every time.
• Measuring the old window instead of the opening. The old frame is not the hole. Measure the brick-to-brick opening.
• Taking the largest measurement. The window must fit the narrowest point. The largest number is the one that does not fit.

Safety

Measuring is safe work: do it from the ground or from a proper ladder, and never lean out of an open window to reach the outside. Professional installation is needed for high-level windows, large panes and anything above ground-floor height — heavy glass is dangerous to handle, and a dropped pane is a serious injury. Never remove a window yourself if the opening is structural or the glass is large: leave the removal and the fitting to the installer.

Try these calculators

• Window Area Calculator — area per window, total and glass area.
• Window Cost Calculator — window cost by size and type.
• Door Size Calculator — door dimensions and clearances.