How to measure for replacement windows

What size goes on the order?

You give us the frame size, which is the brick opening less a fitting gap. Made-to-measure windows are built to that size, so a measuring mistake becomes a window that does not fit.

How do you measure the opening?

With the old frame still in, measure the brickwork from reveal to reveal at the top, middle and bottom, and from the head to the cill at the left, middle and right. If the old frame is out, measure the same points on the bare opening. Take the smallest width and the smallest height.

How big should the fitting gap be?

A common allowance is 5mm each side, so 10mm off the overall width and height, but check the gap your installation method and sealant need. Bigger gaps are needed where the opening is badly out of square.

What about the cill?

Decide whether the frame sits on a new cill or on the existing stone or brick cill. If it sits on a new cill, the frame height is measured to the top of that cill. Tell us the cill depth and length separately.

What else goes on the schedule?

For each window: width by height in millimetres (width first), opener layout viewed from outside, glass type, finish inside and out, trickle vents, and any escape hinge or safety glass requirement. Number each window and mark the same number on your survey sheet.

This guide is general information for trade customers. The person carrying out the work remains responsible for meeting the building regulations and any planning requirements for the specific job.