How to Measure for Replacement Glass

Accurate measurements are the foundation of every successful glazing job. An incorrect measurement means wasted glass, wasted time, and an unhappy customer. Here's how to get it right every time.

Step 1 — Measure the Rebate The rebate (or rabbet) is the groove in the frame that holds the glass. Measure the full width and height of the rebate opening, not the visible glass area. Use a steel tape measure for accuracy.

Step 2 — Deduct for Clearance Standard practice is to deduct 3mm from both the height and width measurements. This gives 1.5mm clearance on each side, allowing for frame irregularities and thermal expansion. For larger panels (over 2m in any dimension), increase the deduction to 4–5mm.

Step 3 — Measure Diagonals Measure both diagonals of the opening to check that the frame is square. If the diagonals differ by more than 3mm, the frame is out of square and you may need to allow extra clearance or discuss frame rectification with the customer.

Step 4 — Check the Depth Measure the rebate depth to ensure your chosen glass thickness (plus any glazing tape or sealant) will fit. For double glazed units, the rebate typically needs to be at least 20mm deep.

Step 5 — Record Everything Write down each measurement with the panel location (e.g. "Kitchen W1 — 1520 × 980mm"). Include the glass type, thickness, and any special requirements such as obscure glass or tinting.

Pro Tip: Always measure at least twice. If the two measurements differ by more than 1mm, measure a third time and use the most consistent reading.