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How to Measure a Sealed Unit for Replacement (No Guesswork)
Accurate measurements and clear photos help your glazier supply the right replacement double-glazed unit first time.
Replacing a misted or broken double-glazed unit is usually quicker and more cost-effective than replacing the whole window. However, a sealed unit must be made to the correct size and specification, as being even a few millimetres out can lead to delays, a poor fit or another site visit.
The key is to measure the existing unit rather than guessing from the frame opening. This guide explains which dimensions matter, what photographs can help a glazier, and the common mistakes to avoid when ordering replacement double glazing.
Understand the three measurements
Overall sealed unit size is the complete width and height of the double-glazed panel, measured from edge to edge. This includes both panes of glass, the spacer bar and the sealed perimeter. These are normally the dimensions your glazier needs when manufacturing a replacement unit.
Visible size is the area of glass you can see from inside the room or outside the building. It is smaller than the overall sealed unit size because the frame, beads and seals cover the edges. Glass size can refer to an individual pane or the visible glass area, so always clarify which measurement is being requested before placing an order.
Measuring the visible glass or the open frame and then ordering a unit that is “roughly the same” often causes problems. The replacement may not fit behind the beads, may leave an excessive gap or may not allow enough room for packers and seals.
Step-by-step measuring guide
Measure in millimetres and write down exactly what you find. If the unit is still in the frame, carefully remove the internal glazing bead only if you are confident doing so. Otherwise, measure as accurately as possible and send photographs so a professional can confirm the dimensions.
Measure the width. Measure the sealed unit from left to right at the top, middle and bottom. Record the smallest measurement, unless a glazier advises otherwise.
Measure the height. Take measurements at the left, centre and right. Again, note the smallest figure and check whether the unit is square.
Measure the thickness. Measure the complete sealed unit, not just one pane. Common sizes include 20mm, 24mm and 28mm, but many other thicknesses are used.
Check the details. Note the spacer bar colour, any markings, Georgian bars, patterned glass, obscure glass or special coatings.
Record the location. State whether it is a window, door, side panel or low-level pane, as safety glass may be required.
If a unit has been removed, place it on a flat surface and measure edge to edge with a steel tape. Do not rely on the frame opening, as manufacturing tolerances and the space needed for packers mean the sealed unit is normally slightly smaller than the aperture.
Designer suggestion: include a simple labelled diagram showing width measured horizontally, height vertically and overall unit thickness through the edge.
Details that affect the replacement
The spacer bar sits between the panes and helps maintain the sealed cavity. Its colour, finish and markings can affect the appearance of the replacement, particularly where the unit is visible from the outside. A gas-filled unit may have an argon-filled cavity, but this cannot always be confirmed by sight alone, so mention any information printed on the spacer or paperwork.
Georgian bars can be fitted inside the sealed unit, applied to the glass surface or arranged in a particular pattern. Photograph the layout and count the sections where possible. Also note toughened or laminated markings, acid-etched designs, bevelled glass and any tinted or solar-control appearance.
Measure width and height at three points, recording the smallest figures.
Check the complete unit thickness rather than assuming it is 24mm.
Look for spacer bar markings, safety stamps and Georgian bar details.
Photograph the frame, beads, drainage holes and the full pane in context.
Photographs that help a glazier
Good photographs can confirm details that are difficult to measure at home. Take a clear close-up of the spacer bar through the edge of the unit, especially if it has printed markings, as well as a full image showing where the panel sits in the frame.
Include photographs of the internal glazing beads, frame profile and any drainage holes along the bottom edge. A picture of the handles, hinges and surrounding window or door can also help identify the system and establish whether the panel is in a door or another location requiring safety glazing.
Designer suggestion: show a photo checklist with callouts for spacer markings, internal beads, frame profile and drainage slots.
Common measuring mistakes
One of the most frequent errors is measuring the opening instead of the existing sealed unit. Another is failing to account for packers, which support the glass and help prevent movement or stress within the frame. Never assume every double-glazed panel is 24mm, as older and newer systems use different cavity sizes.
Safety requirements must also be considered. Glass in doors, door side panels and many low-level areas may need to be toughened or laminated, even if the original pane does not have an obvious marking. A glazier can assess the position and recommend the appropriate specification.
Extra planning for flats and managed buildings
If you live in a flat or managed development, check access arrangements before booking work. The building may have restricted working hours, parking rules, lift limitations or a requirement to protect communal areas.
Ask whether the replacement must match the existing glass appearance, spacer colour or Georgian pattern. A managing agent or freeholder may also need photographs, product details or approval before work starts, particularly where the building has a consistent external appearance.
Get your measurements checked
Accurate information makes replacement double glazing much easier to quote and manufacture, but you do not have to identify every specification alone. Send your measurements and clear photographs to Hunters Glass Ltd for confirmation before ordering.
For advice, to arrange a measurement or to book double glazing repairs, call Hunters Glass Ltd on 03301331655. A professional check can prevent an incorrect unit, reduce disruption and help ensure your replacement sealed unit fits and performs as it should.
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