Getting the measurements right before ordering a replacement window pane can save you time, money, and frustration. Whether you're replacing a cracked single pane or upgrading to double glazing, accurate sizing is essential — glass is cut to order and typically can't be returned once cut. Here's a straightforward guide to measuring up like a pro.
Tools You'll Need
Before you start, gather the following:
- Steel tape measure (not a fabric one — these can stretch and give inaccurate readings)
- Pencil and notepad (or your phone for notes)
- Safety gloves if any broken glass is still in the frame
- A straight edge or spirit level to check if the frame is square
Understanding the Rebate
The rebate (sometimes called the rabbet) is the stepped groove inside the window frame that the glass sits in. You don't measure the full visible opening — you measure the glass that fits into the rebate. This is a common source of confusion for first-timers.
The glass pane needs to sit inside the rebate with a small gap on all sides (typically 2–3 mm) to allow for thermal expansion and slight frame movement. Your glazier will account for this, but it's worth understanding when you take your measurements.
Step-by-Step: How to Measure
Step 1 — Remove any remaining glass safely
If broken glass is still in the frame, carefully remove it wearing heavy gloves. Dispose of it safely — wrap it in newspaper and place it in a cardboard box before putting it in the bin. Check WorkSafe NZ for guidance on safe handling of broken glass.
Step 2 — Measure the rebate opening (width)
Measure the width of the rebate opening from the inside edge of the rebate on the left to the inside edge on the right. Take this measurement at three points: top, middle, and bottom. Record all three.
Step 3 — Measure the rebate opening (height)
Measure the height of the rebate opening from the inside edge of the rebate at the top to the inside edge at the bottom. Again, take three measurements: left side, centre, and right side. Record all three.
Step 4 — Use the smallest measurement
If your three width measurements differ, use the smallest figure. Do the same for height. This ensures the glass will fit even if the frame is slightly out of square — which is common in older NZ homes, particularly those built on timber piles that have shifted over time.
Step 5 — Check for square
Measure diagonally from corner to corner in both directions. If the two diagonal measurements are equal, the frame is square. If they differ by more than about 5 mm, mention this to your glazier — they may need to cut the glass slightly out of square to match the frame.
Step 6 — Note the glass thickness
If any original glass remains in the frame, measure its thickness with a tape measure or vernier calipers. Standard single glazing in NZ is commonly 3 mm or 4 mm thick. Double-glazed units (IGUs — insulated glass units) are thicker and require a different rebate depth. Check the existing rebate depth to confirm what will fit.
A Simple Measurement Table
| Measurement | Location 1 | Location 2 | Location 3 | Use this value |
|---|---|---|---|---|
| Width (mm) | Top | Middle | Bottom | Smallest |
| Height (mm) | Left | Centre | Right | Smallest |
| Diagonal 1 (mm) | Top-left to bottom-right | — | — | For reference |
| Diagonal 2 (mm) | Top-right to bottom-left | — | — | For reference |
Ordering Your Replacement Glass
Once you have your rebate measurements, your glazier will typically deduct 3–4 mm from each dimension to give the correct clearance gap. If you're ordering glass yourself from a supplier, ask them to confirm the deduction they apply — don't assume.
For homes built after 1996, the NZ Building Code (in particular clause H1 for energy efficiency) may influence what type of glass is appropriate, especially if you're upgrading. If your home is a rental, the Healthy Homes Standards require adequate glazing and insulation — double glazing may be worth considering at the same time.
For technical guidance on glass performance and weathertightness, BRANZ publishes useful research and best-practice guides.
Tips to Avoid Common Mistakes
- Measure twice, order once — glass is cut to your specification and can't be refunded.
- Don't measure the old broken pane if it has already been removed — measure the frame rebate directly.
- If the frame is timber and has been painted many times, check that paint build-up isn't reducing the rebate opening.
- For double-glazed units (IGUs), always have a qualified glazier measure and install — these are more complex and the seal integrity is critical.
When to Call a Professional
While measuring a window is a DIY-friendly task, cutting and installing glass is best left to a qualified glazier in most situations. Glass cutting requires specialist tools and experience, and incorrect installation can lead to cracking, water ingress, or — in the case of safety glazing — a serious injury risk.
In NZ, glazing in certain locations (such as near doors, low-level panels, and bathrooms) must use safety glass (toughened or laminated) to comply with NZS 4223 and the Building Code. A licensed glazier will know exactly where safety glass is required.
If you're replacing a double-glazed unit, dealing with a large pane, or unsure about the frame condition, it's well worth getting a professional in. You can find qualified local glaziers on tradedirectory.co.nz.
Window & Glass Association
