How to Measure Glass, Double Glazed and Triple Glazed Units.

How to Measure Your Replacement Sealed Units

Getting accurate dimensions is critical for your new glass to fit perfectly. There are two main ways to measure your double glazing.

Method 1: Removing the Unit (Recommended for Accuracy)

For 100% accuracy, we highly recommend removing the glass unit from the frame before measuring.

Step 1: Remove the Unit
Taking the glass out is simpler than it sounds. Watch our video guide here: How to remove a double glazed unit.

Step 2: Measure Height & Width
Once the unit is out, simply measure the exact height and width of the glass pane from edge to edge.

Step 3: Measure the Thickness
Measure the total depth of the unit (Inner Glass + Spacer Bar + Outer Glass).
Tip: If you are unsure about the makeup of your unit, you can purchase our specialized Glass Thickness & Cavity Measuring Tool.

Method 2: Measuring In-Situ (Leaving Glass in Frame)

If you prefer not to remove the glass, you can measure it while it is still in the frame, but you must account for the glass hidden inside the rebate.

Step 1: Measure the Visible Glass
Measure the height and width of the visible glass area (this is known as the "sight size").

Step 2: Check the Rebate Depth
Remove just one glazing bead to see how far the glass sits inside the frame (usually between 10mm and 15mm). Multiply this number by two and add it to your visible measurements.
Tip: If you are unsure about the makeup of your unit, you can purchase our specialized Glass Thickness & Cavity Measuring Tool.

Step 3: Measure the Thickness
Open the window sash so you can access both sides. Using "reach around" calipers (or a careful tape measure check), measure the total thickness of the unit from the outside pane to the inside pane.

Important Note: Precision is key. We recommend measuring in millimeters (mm) to ensure your replacement unit fits perfectly.

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Guide to Measuring Triple Glazed Units

Precision is essential when ordering heavier triple-pane glass. We outline two effective techniques to ensure your dimensions are spot-on for a seamless installation.

Method 1: Removal for Precision (Best Practice)

Due to the weight and complexity of triple glazing, removing the unit allows for the most reliable verification of size and total depth.

Step 1: Extract the Glass
Carefully de-glaze the window. Although heavier, the process mimics standard units. See our visual tutorial: Removing a sealed unit safely.

Step 2: Record Dimensions
With the unit free, take exact edge-to-edge readings for both height and width in millimeters.

Step 3: Confirm Overall Depth
Measure the full thickness (spanning all three panes and both spacer bars).
Tip: Triple units are thick (often 36mm-44mm). To identify the specific internal makeup (e.g., glass thickness vs cavity size), use our Optical Glass Gauge.

Method 2: In-Frame Measurement (Alternative)

If removal isn't feasible, you can calculate the size while the glass remains fitted, provided you carefully assess the frame rebate.

Step 1: Sightline Assessment
Measure the visible area of glass (tight to the beads and gasket) to establish the 'tight size'.

Step 2: Determine Coverage
Unclip a single bead to reveal how deep the glass sits within the profile (the rebate). Add this concealed depth (x2) to your tight size for the final manufacturing dimension.
Tip: Avoid guesswork. Verify your rebate depth accurately using our Measuring Tools.

Step 3: Total Thickness Check
Open the sash to access the edge profile. Using caliper tools, clamp across the full width of the unit (exterior pane to interior pane) to get the total depth measurement.

Critical Weight Note: Triple glazing is significantly heavier than standard glass. Always ensure you measure in millimeters (mm) and verify that your hinges can support the unit depth/weight.

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How to Measure Single Pane Glass

Whether for a greenhouse, shed, or internal door, getting the correct dimensions for single glazing is straightforward if you follow these steps.

Method 1: Measuring Loose Glass (Replacements)

If you have the old piece of broken glass or a template, this is the easiest way to ensure an exact match.

Step 1: Use the Template
If the original pane is intact (or you can piece it together safely), lay it on a flat surface.

Step 2: Record Dimensions
Measure the exact height and width from edge to edge. If the shape is irregular, it is best to provide a hard template.

Step 3: Check the Thickness
Single glass usually comes in 4mm or 6mm. Use a tape measure or calipers to check the edge thickness.
Tip: For safety glass or specific shelving, ensuring the correct thickness is vital for structural strength.

Method 2: Measuring the Frame (Tight Size)

If the glass is missing entirely, you will need to measure the opening where the glass will sit.

Step 1: Measure the Tight Opening
Measure the full width and height of the opening where the glass sits (the rebate), right up to the edges of the frame material.

Step 2: Apply a Tolerance (Deduction)
Glass needs room to expand and fit. Deduct approximately 2mm - 3mm from both your total height and width measurements. This ensures the pane fits easily without being forced.

Step 3: Determine Thickness Required
Check the depth of the rebate (the groove). Ensure the glass thickness you order (e.g., 4mm) is thin enough to fit into the groove with room for putty, beading or clips.

Safety Note: Always measure in millimeters (mm). When handling single glass, especially broken shards, we strongly recommend wearing protective safety gloves.

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