The answer
Open the window and look at the frame for a data plate, a small label printed with a code such as MK04. The first letter gives the width and the final digits the height, so MK04 means 78 by 98 cm. It is usually visible only when the sash is open, which is why people miss it. If the plate has gone or been painted over, measure the outside of the frame from the roof or ask us to, and we match that to the nearest standard size. Do not measure the glass or the lining inside the room.
Where the data plate is
The plate is a sticker or a printed label on the window frame. It is placed so that it is hidden when the window is shut, which is good for appearance and awkward when you are hunting for it. Open the sash fully, and look at the frame beside or below the opening, around where the hinge or pivot is. A torch helps, because the roof void and the lining are often dark, particularly in a loft room on a winter afternoon.
The plate carries several groups of letters and numbers. The size code is the one with a letter followed by another letter and two digits, such as MK04, SK06 or CK02. Other groups describe the product series, the glazing and the date it was made. If you read the whole line out or photograph it, we can work with all of it.
Wiping the label hard can smear it away, and peeling it will tear it. Handle it gently, because it is the quickest route to a right answer, and a replacement label is not easy to come by. If it is faint, a photo taken at an angle with the flash off often shows what the eye cannot. Our fuller guide to VELUX size codes explains how every part of the code is built.
Reading the size from the code
Reading the code takes a few seconds. Width comes from the opening letter, height from the closing digits, and the letter between them can be skipped when you only want dimensions.
So in MK04 the M means 78 cm wide and the 04 means 98 cm high. In SK06 the S means 114 cm wide and the 06 means 118 cm high. Other widths follow the same pattern: C for the narrowest at 55 cm, then F, P and U, up to 134 cm. Heights run from 02 at 78 cm through 04 at 98 cm and 06 at 118 cm to 08 at 140 cm.
These are the outer dimensions of the frame, not the glass and not the opening seen from inside. That is why two windows with the same code can look slightly different from the room, if one was lined with a deep splayed reveal and the other with a plain boxed one.
If you want a replacement of the same size, the code is what we start from. It tells us the opening we should expect to find, and the survey confirms it.
If the plate is missing or unreadable
Plates do go missing. They are painted over during redecoration, peeled off by accident or rubbed away on an old frame. When that happens, the window can still be identified, but it takes a measurement.
The figure that matters is the outer size of the frame, which means getting to the outside of the roof. That is a job for the survey, not for you on a ladder. We measure the frame width and height, note the roof covering and pitch, and match the result to the nearest standard code. Where an older window sits between two standard sizes, we note both and check the opening before anything is ordered.
Measuring from inside gives a different number, and it is the wrong one. The lining reduces the visible opening, and a deep reveal changes it further. If you only have an inside measurement, send it anyway, with a photo of the whole window, and we can tell you whether it points to one code or another.
A window from another maker will not carry a VELUX code at all. Other brands label by plain width and height or their own system, and we look at those on the survey.
A quick check that the number makes sense
Once you have a code, hold it against what you can see. A window that looks tall and narrow from the room should have a small width letter and a larger height number. One that looks wide and shallow should be the other way round. If the code says a wide window and yours is clearly narrow, the plate may belong to a different window, perhaps one replaced in an earlier repair, so look at the others in the house too.
It also pays to write the whole code down with the date you found it. Houses change hands, lofts are converted in stages, and a note in your records saves the same hunt next time.
What to send us, and what happens next
You do not have to know your size to get a quote. The survey carries no fee, and a price comes only after the roof has been seen. Even so, a few photos let us arrive prepared:
- the data plate with the sash open, or the best view of where it should be
- the whole window from inside, lining included
- the roof from the garden or street, to show the covering
Post them through the quote form, or WhatsApp them to 07880 284133. The size you have is only the first step. Whether a new window can go in the same opening, and whether the flashing can stay, is covered in our pages on VELUX flashings for tiles and slates and on VELUX window installation and replacement.
A fixed written quote follows the visit. Building Control is dealt with by us where the job is notifiable, and the finished work comes with a ten-year workmanship guarantee in writing.