Top 5 Common Mistakes When Measuring Glass for Replacement
Top 5 Common Mistakes When Measuring Glass for Replacement
Updated · Expert Tips by Glass Helper
Measuring glass for replacement might seem simple — just height and width, right?
In reality, small measurement errors can lead to poor fitting, draughts, or even cracked glass on installation.
Below are the five most common mistakes people make and how to avoid them for a perfect fit every time.
1) Measuring the opening, not the visible glass
One of the biggest mistakes is measuring the overall frame or aperture instead of the actual glass rebate (the space the glass sits in).
Always remove old beads or measure the visible glass and then add 10–12 mm on each dimension to account for the section hidden by the frame.
Tip: See Glass & Glazing Federation – Domestic Windows Guide (PDF).
2) Forgetting to measure in millimetres
Glass suppliers in the UK typically work in millimetres (mm), not inches.
Using mixed units often causes costly miscommunication.
Measure to the nearest millimetre and double-check conversions before submitting dimensions.
Reference: GOV.UK – Metric and Imperial Measurements.
3) Not accounting for fitting tolerance
Glass expands slightly with temperature, and frames aren’t perfectly square.
Subtract at least 2–3 mm from both width and height measurements to prevent tight fits that cause cracking under pressure.
According to Pilkington Installation Guidance, leaving expansion allowance is essential for longevity and stress reduction.
4) Ignoring glass thickness and type
Many homeowners forget to specify glass thickness or type (toughened, laminated, patterned, etc.).
Thicker glass improves safety and insulation but may not fit existing channels.
For windows below 800 mm from the floor or in doors, safety glass is legally required.
Regulations: Approved Document K – Building Regulations.
You can explore compliant options in our Tempered Glass and
Replacement Window Glass pages.
5) Not checking squareness and diagonals
Frames warp over time. Measuring just width and height misses distortion that can cause one corner to bind.
Measure both diagonals — if they differ by more than 3 mm, your frame isn’t square.
Order glass slightly undersized or use glazing packers for compensation.
See practical advice on frame squareness: FENSA – Homeowner Window Guide.
It’s better to have a slightly smaller glass pane than one that doesn’t fit at all.
Order Replacement Glass Cut to Size
Delivered across the UK – Toughened, Laminated, and Clear Options Available.