How to Measure for a Replacement Window Glass (Step-by-Step UK Guide)

Step-by-Step Guide · 2025

Read time: ~7 mins · Updated: 21 Sep 2025

Ordering the wrong size replacement window glass can cost time, money, and stress.
With just a tape measure and careful checks, you can measure correctly the first time and make sure your glass fits perfectly.
This UK homeowner’s guide will take you step-by-step through measuring width, height, and squareness for new glass panes.

Tools you need

• Steel tape measure (avoid cloth or flexible tapes)
• Notepad & pen (record each measurement)
• Helper (optional but useful for large windows)

Step 1: Measure the width

Measure the opening’s internal width at the top, middle, and bottom. Write down all three measurements and use the smallest as your final width.
This ensures the new glass won’t be too wide for tighter sections of the frame.

Step 2: Measure the height

Measure the internal height on the left, centre, and right. Again, take the smallest measurement as your final height.
Frames often aren’t perfectly square—this method ensures a safe fit.

Step 3: Check squareness

Measure diagonally from top left to bottom right, and top right to bottom left.
If the difference between the diagonals is more than 5 mm, your frame is out of square.
This is important to tell your supplier so they can help you adjust or recommend the best fitting method.

Step 4: Finalise your order

• Deduct a small fitting tolerance (usually 2-3 mm) from both width and height.
• Specify the type of glass: toughened, laminated, patterned, or double-glazed unit depending on your needs.
• Request polished edges for safety if edges will be exposed.
• Confirm compliance with UK safety standards (BS EN 12150 / BS EN 14449).

Frequently asked questions

Do I measure the old glass or the frame?
Always measure the frame opening, not the broken glass. Glass can shrink or chip, so relying on it risks ordering the wrong size.

Should I deduct size myself?
Yes, usually deduct 2-3 mm from both width and height to allow fitting tolerance. If unsure, confirm with your supplier.

What if my frame is not square?
Note down the diagonal difference and inform your supplier. In some cases, a slightly smaller pane or special fitting is advised.

Is toughened glass required?
Yes, if the glazing is within 800 mm of the floor or in a door/side panel. For guidance, see FENSA.

Can I install it myself?
Small single panes can be DIY, but for large or double-glazed units, professional fitting is recommended for safety and compliance.