How to Measure for Replacement Glass Units: A Step-by-Step Guide
Measuring a replacement glass unit is simple once you know the three numbers to take. This step-by-step guide shows exactly how, so you order right first time.
The only thing that stops most people ordering glass online is the worry of measuring it wrong. It is simpler than it looks. You need three numbers, and this guide walks through exactly how to take each one so your made-to-measure unit fits first time.
What you need
- A steel tape measure (not fabric)
- A pen and paper
- Five minutes per window
The three measurements
Width and height are the two obvious ones. The third, and the one people forget, is the overall thickness of the unit. Getting all three right is what makes a replacement drop straight into the frame.
Measuring width and height
Measure the visible pane of glass, then add roughly 10 to 12mm to each dimension to account for the glass that sits hidden behind the frame rebate. The unit needs that extra so it beds properly into the frame rather than sitting loose in the opening. Measure in millimetres, measure twice, and if the opening is slightly out of square take the smallest of the two width readings and the smallest of the two heights.
Measuring the thickness
Open the window if you can and measure across the edge of the existing unit, or read it off the spacer bar, which is often stamped with the cavity size. Most homes have 24mm or 28mm units. A 28mm unit is typically 6mm glass, a 16mm cavity, and 6mm glass. Matching the overall thickness matters because the new unit has to fit the same rebate depth.
Common mistakes to avoid
- Measuring the frame instead of the glass
- Forgetting the rebate allowance, leaving the unit too small
- Rounding to the nearest centimetre - always work in millimetres
- Assuming every window in the house is the same size - measure each one
From measurement to order
Once you have your width, height and thickness, choose your spec and enter the sizes straight on the product page. The price updates live, including VAT, and the unit is cut to those exact measurements. If you are replacing a failed unit, our guide on misted double glazing explains how to pick the right replacement spec.
Got your sizes? Enter them on any SmartaView glass unit to see the price and order cut to measure.
Author at SmartaView, writing about aluminium glazing, home improvements and the buying process.