How to measure for replacement windows

Knowing how to measure windows properly is the most important part of ordering made-to-measure frames online. It takes about ten minutes with a tape measure and a pencil. Get it right and your new window will slot straight into the opening.

Every window we make is built to the exact sizes you enter, so a few careful minutes now saves a lot of hassle later.

What you need

• A steel tape measure in millimetres
• A pencil and paper, or your phone
• Someone to hold the other end for wide windows

Always work in millimetres. Centimetres and inches invite rounding errors, and our designer asks for sizes in mm.

How to measure windows: the opening, not the old frame

You are measuring the hole in the wall that the new frame will fill, not the old window. The most reliable way is brick to brick, between the outside reveals.

If the old window is still in, you may need to remove a little internal plaster or trim at the edges to find where the brickwork actually stops. Measuring the old frame alone is risky, because it could have been fitted with large gaps or packed out unevenly.

Measuring the width

Measure the opening from brick to brick in three places: across the top, the middle and the bottom. Write down the smallest figure.

Openings are rarely perfectly square, and the smallest width is the one the frame has to fit.

Measuring the height

Measure from the top of the opening down to the cill (or to where the new cill will sit) on the left, in the centre and on the right. Again, use the smallest figure.

If you are keeping an existing stone or concrete cill, measure down to the top of it at the point where the new frame will sit.

Deduct your fitting gap

Take 10mm off both the width and the height. This leaves 5mm all round for packers, fixings and silicone. The figures you are left with are your frame sizes.

Step	Width example	Height example
Three measurements	1212, 1208, 1210mm	1056, 1060, 1058mm
Smallest figure	1208mm	1056mm
Deduct 10mm	1198mm	1046mm

In this example you would order a frame 1198mm wide by 1046mm high.

Cills

Enter the frame height without the cill. If you add a cill in the designer, it is fitted below the frame and its height is added on top.

Our guide to choosing the right cill depth explains which size suits your wall.

Check the diagonals

Measure corner to corner both ways. If the two diagonals differ by more than about 10mm, the opening is out of square. The frame can still go in, but you should base your sizes on the smallest measurements and expect to pack it carefully.

Common measuring mistakes

• Measuring the old frame or the glass instead of the brick opening.
• Forgetting the 10mm deduction, so the frame is too tight to fit.
• Deducting twice, once on site and again when entering sizes.
• Including the cill in the height when you are also adding a cill in the designer.
• Mixing up width and height. Width always comes first.
• Measuring only once. Three measurements each way catch bowed lintels and uneven brickwork.

Bays, bows and awkward openings

Bays, very old openings and anything out of square can be tricky. Bay windows need angles as well as widths, and each section is measured separately. See our bay and bow windows guide before you start.

If you are fitting the window yourself, our step-by-step guide to fitting a window shows how the fitting gap is packed and sealed.

Not sure?

Call us with your measurements and we will check them before you order. Our team is on 020 3488 9299, Monday to Friday, 8am to 5pm.

Made-to-measure windows are made to your sizes, so they can’t be returned for a change of mind. It is always worth a second check.

Quick answers

Do I measure from inside or outside?

Measure the brick opening, which is usually easiest from outside. Inside measurements can be thrown off by plaster, reveals and window boards.

Why take 10mm off?

It leaves a 5mm gap all round for packers, fixings and silicone. Without it, the frame may not fit or may be forced out of square.

Can I measure for replacement glass the same way?

No. Glass units are measured differently. Use our guide on how to measure glass instead.

Do you fit the windows?

We supply only. If you need a fitter, use our find an installer page.

Design and price your window →