The short answer
The size code is printed on the VELUX data plate, a label fixed to the frame of the window. Open the sash and look along the top of the frame or on the inside edge, and you will find a plate with a code that starts with two letters and two digits, such as MK04 or CK02. That code, together with the type code, tells you what to order for a replacement or a matching blind.
Where to look for the data plate
The plate is usually on the head of the frame, the part at the top of the opening, and it is easiest to see with the sash open. On some windows it sits on the inside of the frame towards one side, or on the sash itself. It is a white or silver label, a few centimetres long, and may be partly hidden by a blind box or by paint that went over it during decorating.
If you cannot see it straight away:
- open the window fully and check the frame head, then both sides;
- look behind any blind or its cassette;
- use a torch, since the label is often in shadow;
- take a phone photo and zoom in, which is easier than craning under a sloping ceiling.
A window that has been painted over, or one fitted many years ago, may have a faded plate. In that case the next section helps.
What the codes on the plate mean
Two codes matter. The first is the type, which describes how the window works and what it is made of. VELUX uses three letters:
- GGL
- A centre-pivot window in white-painted pine.
- GGU
- A centre-pivot window in white polyurethane, often chosen for bathrooms and kitchens.
- GPL
- A top-hung window in white-painted pine.
- GPU
- A top-hung window in white polyurethane.
The second code is the size, a letter followed by two digits. The letter gives the width of the window and the digits give the height, so MK04 and CK02 are different shapes, not different versions of the same one. You do not need to convert the code into millimetres to use it, because the code is what suppliers and VELUX itself work from. For a full list of which letter and number means which dimension, see VELUX window sizes and codes explained.
If the plate is missing or unreadable
A plate can peel, fade or end up under a coat of paint. You still have options.
Measure the window frame and compare it with the sizes listed on the VELUX website, at velux.co.uk. Measure the outer frame and not the glass, since the frame is what the size code refers to. Take the width and the height, and measure both a second time. Because VELUX sizes come in a fixed set, a measurement will usually point to one code or to two that are close.
Look at paperwork. An old invoice, a guarantee card or a blind box may record the size. A blind or a flashing kit from an earlier job often carries the code on its packaging or label.
If you are still unsure, send us photographs of the window from inside and outside, a photograph of the frame head and a measurement, and we will tell you what we think it is. We confirm it on site before anything is ordered, because a wrong code means the wrong window or the wrong flashing.
Why the code matters for a replacement
A like-for-like replacement of a VELUX window is easiest when the new window is the same size code as the old one. It then drops into the same opening between the same rafters, and the lining, the plaster and the roof covering around it do not need to change. If you are thinking of a larger or smaller window, the code is the starting point for working out what the rafters will allow.
The flashing is tied to the size too. A kit is made for a given size code and for a type of roof covering. The ED codes show this: EDW suits profiled roofing such as interlocking tiles, EDZ suits flat roofing such as plain tiles or thicker slate, and EDL suits thin slate. A kit for the wrong covering will not seal properly, which is a reason to say what is on your roof as well as the size of the window. There is more in VELUX flashing kits explained.
Reading the plate with confidence
Write the whole code down exactly as printed, including the letters at the front, and photograph the plate before you leave the room. Some plates carry other numbers as well, such as a date or a batch reference. Those are for the manufacturer, and the code you need is the one made of letters and digits that looks like MK04 or CK02. If two windows sit side by side in a loft room, check both plates, since a pair is not always a matching pair, and a landing window may well be a different size from the bedroom ones.
Ordering blinds and accessories
Blinds, awnings and shutters are made for each size code, and most are also specific to a type. If you are buying a blind for an older window, give the full code from the plate, not a guess from the measurements. A wrong size shows up straight away, with gaps down the sides, and cannot be corrected by trimming.
When the code is only part of the answer
The data plate tells you what the window is, not whether it is in good condition. A window can be the right size and still be tired: the seals are failing, the glazing has misted, or the frame has swelled. If the glass is the only problem, only the sealed unit may need replacing, and that is not notifiable work. If the whole window is being replaced, it is a controlled fitting, so it must meet Part L, and the replacement is notifiable to Building Control. We handle that notification.
Also worth knowing is the lining. A window and its lining fit as a pair, so the inside finish may need attention even when the window swaps easily. That is why every Brightview quote comes with a finish schedule that records what happens inside as well as on the roof.
Next steps
Find the plate, write down the full code and take a photo of it, and you have what you need for a quote. Our page on VELUX window installation in Exeter sets out how a replacement is done, and what VELUX GGL means goes further into the type codes. Then use the quote form and attach the photo.