How to Measure Double Glazing Units
Accurate measuring is critical when ordering replacement sealed units. This guide walks you through the process in five simple steps.
What You'll Need
Before you start measuring, gather these items:
- Tape measure – must read in millimetres (mm), not inches
- Notepad and pen (or your phone to take notes and photos)
- Step ladder – for measuring high or hard-to-reach windows safely
- Clean cloth – to wipe the glass edges and spacer bar so you can read any printed information
The entire process should take around 15 minutes per window. If you have multiple windows to measure, work through them one at a time and label each set of measurements clearly.
Step-by-Step Measuring Guide
Step 1: Measure the Width (mm)
Measure the width of the sealed unit – not the frame, and not just the visible glass – at three points:
- Top – measure across the top of the glass unit
- Middle – measure across the centre
- Bottom – measure across the bottom of the glass unit
Record all three measurements and use the smallest one. This is your width measurement.
Why three points? Window frames can bow, warp, or settle over time – especially in older properties. If you only measure at one point, you risk ordering a unit that's a millimetre or two too wide to fit. Taking three measurements and using the smallest guarantees the unit will slide into the frame without forcing it.
Step 2: Measure the Height (mm)
Now measure the height of the sealed unit at three points:
- Left side – measure from top to bottom on the left edge
- Centre – measure from top to bottom in the middle
- Right side – measure from top to bottom on the right edge
Again, record all three and use the smallest measurement. This is your height measurement.
The same principle applies as with the width – frames can sag or twist, particularly on larger windows and older timber frames, so the smallest reading ensures a proper fit.
Step 3: Measure the Overall Unit Thickness (mm)
This measurement tells you the glass and cavity configuration of your existing unit. Measure from the outside face of one pane of glass to the outside face of the opposite pane – the full depth of the sealed unit.
Common overall thicknesses and what they mean:
- 20mm – typically a 4-12-4 build (4mm glass, 12mm cavity, 4mm glass)
- 24mm – typically a 4-16-4 build (4mm glass, 16mm cavity, 4mm glass) – the most common UK standard
- 28mm – typically a 4-20-4 build (4mm glass, 20mm cavity, 4mm glass) – common in newer properties
The easiest place to measure the thickness is at the edge of the glass where it meets the frame. If you've removed the beading, you'll have clear access to the edge of the unit. If not, you can usually measure carefully at the edge where the glass is visible.
Getting the thickness right is important – the replacement unit must match the existing frame rebate depth. If you're unsure, contact us and we can advise.
Step 4: Check the Spacer Bar
The spacer bar is the strip of material that runs around the perimeter of the sealed unit between the two panes of glass. Look closely at the spacer bar – many manufacturers print useful information on it, including:
- The gas fill type (e.g., "Argon" or "A" for argon gas)
- The glass type and specification (e.g., "4mm Low-E" or "Planitherm")
- The manufacturer name and manufacturing date
- The EN standard reference (e.g., BS EN 1279)
Use your clean cloth to wipe the edge of the unit if the text is difficult to read. Noting this information helps us match your replacement unit to the correct specification. If nothing is printed, don't worry – we can advise based on your thickness measurement and the age of the property.
Step 5: Note Any Special Requirements
Before you order, check whether your existing unit has any of the following features – these must be specified when ordering the replacement:
- Toughened glass – required by Building Regulations in certain locations (doors, low-level windows, windows near stairs). Look for a small BS EN 12150 stamp or kite mark etched into the corner of the glass. If it's there, your replacement must also be toughened.
- Obscure or patterned glass – commonly used in bathrooms and WCs. Note the pattern name if possible (e.g., Stippolyte, Contora, Arctic). Visit our products page for the full range of obscure patterns we offer.
- Georgian bars – decorative bars that divide the glass into smaller panes. These can be internal (between the panes) or external (stuck on). Note the bar layout and style.
- Low-E coatings – most modern sealed units have a low-emissivity coating on one surface to improve thermal performance. To test for a Low-E coating, hold a lighter flame or candle close to the glass and look at the reflections. You'll see four reflections (two per pane). If one of the reflections appears a different colour (typically purple or blue) compared to the others, that pane has a Low-E coating.
If your windows require toughened glass, we manufacture all toughened sealed units to BS EN 12150 and provide the required safety marking.
Common Measuring Mistakes to Avoid
We see these errors regularly, and they're easy to avoid if you're aware of them:
- Measuring in inches instead of millimetres – all sealed units in the UK are manufactured in millimetres. An error here means your unit will be completely wrong. Double-check your tape measure is set to metric.
- Measuring the frame instead of the glass unit – you need the dimensions of the sealed glass unit itself, not the window frame opening. The frame is always larger than the unit.
- Not accounting for beading – the beading (the trim that holds the glass in the frame) overlaps the edge of the sealed unit by several millimetres on each side. If you measure only the visible glass, your unit will be too small. Remove the beading first, or add the hidden overlap to each edge.
- Measuring at only one point – as explained above, always measure width at three points and height at three points, then use the smallest measurement. A single measurement on a bowed frame can leave you with a unit that won't fit.
Common UK Sealed Unit Sizes
The table below shows some of the most commonly ordered sealed unit sizes in the UK. All sizes are in millimetres (width – height). We supply all standard and bespoke sizes – if your measurements don't match a standard size, we'll manufacture to your exact dimensions.
| Size (W × H mm) | Typical Configuration | Common Application |
|---|---|---|
| 500 × 500 | 4-16-4 (24mm overall) | Small bathroom or WC windows |
| 600 × 900 | 4-16-4 (24mm overall) | Standard casement windows |
| 800 × 1200 | 4-16-4 or 4-20-4 (24–28mm) | Bedroom and living room windows |
| 1000 × 1000 | 4-20-4 (28mm overall) | Large casement or fixed pane |
| 1200 × 1200 | 4-20-4 (28mm overall) | Large fixed panes, feature windows |
Don't see your size? No problem. Every unit we produce is made to order, so non-standard sizes cost exactly the same as standard sizes. See our cost guide for pricing information.
Not Sure? We Offer Free Measuring Surveys
If you'd rather leave the measuring to the professionals, we offer free measuring surveys across the UK. One of our experienced surveyors will visit your property, take precise measurements, check the glass specification, and provide a no-obligation quote – all at no cost to you.
Frequently Asked Questions
Yes, you can measure from inside the house in most cases. You will need to remove the internal beading (the plastic or timber trim that holds the sealed unit in place) to expose the edge of the glass unit. Measure the glass unit itself, not the visible area of glass within the frame. If you are unable to remove the beading, measure the visible glass and add the depth hidden behind the beading on each side – typically 5–10mm per edge.
We supply bespoke sealed units in virtually any shape, including arched, circular, triangular, and trapezoidal. For unusual shapes, provide the widest width, tallest height, and a sketch or photograph showing the exact shape with key dimensions marked. Our team will work from your drawing to produce an accurate template. Contact us for advice if you're unsure how to measure a non-rectangular unit.
Yes. Every sealed unit we manufacture is made to order, so there is no additional charge for non-standard sizes. Whether you need a small 300×300mm unit or a large 3000×2000mm pane, we can produce it to your exact measurements. Bespoke units are typically manufactured and delivered within 7–10 working days. See our products page for the full range.
Measurements should be accurate to within 1–2mm. Sealed units are manufactured to precise tolerances, and a unit that is even a few millimetres too large will not fit into the frame. If your measurements are slightly too small, the unit may still fit but could require additional packers. We always recommend measuring twice and using the smallest measurement when measuring at multiple points.