A roof window size code is a letter followed by a number, and it describes the structural opening rather than the glass. The letter sets the width, the number sets the height. Using the common range, C is 550mm wide, F is 660mm, M is 780mm, P is 940mm, S is 1140mm and U is 1340mm. The number gives the height: 02 is 780mm, 04 is 980mm, 06 is 1180mm, 08 is 1400mm, 10 is 1600mm. So an MK06 opening is 780mm by 1180mm. Read the code off the data plate on the top edge of the sash, or measure the trimmed opening if the plate has gone.
That is the short version. The longer version is worth ten minutes, because the code is the single number that decides whether your replacement is a quiet like-for-like swap or a job that touches the rafters. Get it right and a window fitted in 2004 usually matches a current model without opening the structure. Get it wrong and the new frame does not sit in the timber opening you already have.
How the code is built
Three parts sit in the code, and only two of them vary with size.
- The first letter is the width. It is the horizontal dimension of the structural opening, the clear gap between the two trimmed rafters that carry the window. This is the part that matters most, because widening an opening means structural work and narrowing one wastes light.
- The middle letter denotes the size series. Across the common domestic range this is a constant, most often K, which is why so many codes read CK, FK, MK, PK, SK and UK. It is not a size in itself, so read past it to the number.
- The two-digit number is the height. It is the vertical dimension of the opening, measured up the roof slope, not the true vertical. A longer number means a taller window and, on a shallow pitch, a better view out from a seated position.
Because both dimensions are opening sizes, not glass sizes, the code tells a fitter exactly what gap has to exist in the timber. The visible glass is always smaller than the code figure by the frame width, which is why measuring the glass and ordering to it is the classic way to order the wrong window.
The full size-code matrix
The hub page lists the sizes we see most often in Ipswich lofts. This is the complete grid, every width letter crossed with every height number in the common range, with the structural opening in millimetres and the approximate opening area. Find your width along the top and your height down the side, and the cell is your code and its dimensions.
| Height number | C (550mm) | F (660mm) | M (780mm) | P (940mm) | S (1140mm) | U (1340mm) |
|---|---|---|---|---|---|---|
| 02 (780mm) | CK02 550 x 780 |
FK02 660 x 780 |
MK02 780 x 780 |
PK02 940 x 780 |
SK02 1140 x 780 |
UK02 1340 x 780 |
| 04 (980mm) | CK04 550 x 980 |
FK04 660 x 980 |
MK04 780 x 980 |
PK04 940 x 980 |
SK04 1140 x 980 |
UK04 1340 x 980 |
| 06 (1180mm) | CK06 550 x 1180 |
FK06 660 x 1180 |
MK06 780 x 1180 |
PK06 940 x 1180 |
SK06 1140 x 1180 |
UK06 1340 x 1180 |
| 08 (1400mm) | CK08 550 x 1400 |
FK08 660 x 1400 |
MK08 780 x 1400 |
PK08 940 x 1400 |
SK08 1140 x 1400 |
UK08 1340 x 1400 |
| 10 (1600mm) | CK10 550 x 1600 |
FK10 660 x 1600 |
MK10 780 x 1600 |
PK10 940 x 1600 |
SK10 1140 x 1600 |
UK10 1340 x 1600 |
All figures are structural opening dimensions in millimetres, width first. Not every cell is a stocked standard in every model family, and some sizes carry longer lead times, but the code arithmetic holds across the range.
A few of these are worth calling out because they show up again and again in this area:
- MK04 and MK06 are the workhorse loft sizes across Suffolk. If you have a conversion done between 2000 and 2010, the odds are one of these is in the slope.
- CK02 is the small landing or bathroom window in a terrace. Fine for light over a stairwell, too small to light a room on its own.
- PK10 and UK08 are the modern main-light sizes, common on new-build plots at Kesgrave and Martlesham and on generous conversions where the sight line from a chair matters.
- SK06 and UK06 are wide openings that usually mean a rafter was trimmed out, so a like-for-like swap is straightforward but a change of size is not.
Where the data plate lives
The plate sits on the top edge of the sash, hidden behind the frame when the window is shut. To read it, open the window and rotate the sash to the cleaning position, which swings the top edge down into the room. The plate is a printed metal or plastic label carrying the model designation, the size code, a manufacturing date and a serial number. Photograph it in good flat light and you have everything a fitter needs to quote a like-for-like replacement.
Two spots catch people out. On some units the plate is on the top edge but set to one side rather than centred, so run your eye the full width. And on very old windows the label has faded to the point where the code is a ghost, particularly on south-facing slopes where years of direct sun have cooked the print off. If you can make out the date but not the code, tell us the date anyway, because it narrows the generation and helps us read the rest from the frame profile. Our short walk-through on how to find your VELUX size code takes you through the same steps with photos.
Measuring the opening when the plate has gone
On pre-2000 units the plate is often illegible or missing outright. When that happens the code has to be recovered from the timber, and there is a right way and a wrong way to do it.
The wrong way is to measure the visible frame from inside with a tape and order to that number. The visible frame is not the structural opening. The frame overlaps the timber by roughly 40mm on each side, so a tape reading of the frame is around 80mm narrower and 80mm shorter than the true opening. Order to it and the replacement will not reach the rafters it is supposed to bear on.
The right way is to measure the structural opening, rafter face to rafter face for the width and trimmer to trimmer for the height. That means getting behind the lining, which is a survey job rather than a homeowner job, and it is one of the things we do at survey before a single figure is quoted. We never advise anyone to climb onto a roof to measure from outside. The opening is measured from within the roof space or from the reveal with the lining eased back, in flat working light, and matched to the nearest code in the matrix above.
The 40mm rule. Frame reading plus roughly 40mm each side equals the structural opening. It is a useful sanity check, never an ordering basis. If your frame measures about 700mm wide internally you are probably looking at a 780mm opening, which is an M width, but we confirm that against the trimmed timber before anything is ordered.
Why the code matters beyond the window
The size code does not just order the glass. It drives three other decisions, which is why we ask for it first.
| Downstream item | How the code drives it |
|---|---|
| Flashing kit | The kit is sized to the window, so the code fixes the kit footprint. The covering type then sets which family of kit within that size. |
| Blinds | Blinds are sized to the code, but also to the frame generation. A matching code does not guarantee a matching blind if the profile has changed. |
| Structural work | A same-code replacement needs no structural work. A wider code means trimming a rafter, which changes the job and the price entirely. |
| Building Control route | Like-for-like is a thermal element change under Part L. A larger opening is a structural alteration that is notifiable in its own right. |
This is why a photograph of the plate, sent with an enquiry, lets us give a realistic figure before anyone comes out. The code tells us whether we are quoting a swap or an alteration, and those sit in very different price bands. Our roof window sizes reference goes further into how sizes map to rooms and daylight, and if you are matching a code across generations for a swap, the roof window replacement page covers what changes between old and new frames.
Codes and blinds: the trap in the matching
People reasonably assume that a matching size code means a matching blind. It often does not. Blinds mount to the frame profile, and the profile has changed between window generations even where the code has stayed identical. A blackout blind bought for a 2003 MK06 will not usually fit a current MK06, because the side channels and top brackets locate on a frame that no longer exists. The code matched. The mounting did not. We flag this at survey rather than on fitting day, and the detail is on our blind compatibility page.
Reading a code in practice: three worked examples
- Plate reads MK06. M is 780mm wide, 06 is 1180mm tall. Structural opening 780mm by 1180mm. A current MK06 will match this opening, so a like-for-like swap with no structural work. This is the common Suffolk loft window.
- Plate reads UK04, but the customer wants more height. UK04 is 1340mm by 980mm. Going to a UK08 keeps the same U width, so the same rafters carry it, but adds 420mm of height up the slope. That is a taller opening, which means extending the trimmed opening downward, structural work, and a Building Control notification. Same width letter, very different job.
- No plate, frame measures 620mm wide inside. Add roughly 40mm each side and the opening is near 700mm, which sits between F at 660mm and M at 780mm. This one is not decided from inside. We measure the trimmed opening at survey to settle whether it is an F or an M before ordering, because guessing across that gap wastes a full unit.
Common questions about size codes
What does the K in MK06 mean?
The middle letter is the size series, not a dimension. Across the common domestic range it is usually K, which is why so many codes read CK, FK, MK, PK, SK and UK. Read past it to the width letter and the height number. In MK06, M is the 780mm width and 06 is the 1180mm height, so the opening is 780mm by 1180mm.
Is the code the glass size or the opening size?
The opening size. The code gives the structural opening, the clear gap between the trimmed rafters and trimmers. The glass is smaller than the code figure by the width of the frame all round. This is exactly why ordering to a measurement of the visible glass or frame produces a window that does not fit the timber.
Can I get a bigger window into the same opening?
Only by changing the opening. A taller or wider code means extending or widening the trimmed opening, which is structural work and notifiable to Building Control. Keeping the same width letter but a larger height number, for example UK04 to UK08, keeps the same rafters but lengthens the opening down the slope. We confirm what is possible at survey.
My plate is unreadable. Can you still tell the size?
Yes, by measuring the structural opening at survey rather than reading the plate. We measure rafter face to rafter face and trimmer to trimmer from inside the roof and match to the nearest code. We do not order from a tape reading of the visible frame, because that reads about 80mm smaller than the true opening in each direction.
Get the size confirmed and a fixed price
If you can reach the plate, photograph the size code and send it with a photo of the window from inside and the roof from the garden. That is usually enough for us to tell you whether a like-for-like swap is on and roughly what it costs. If the plate has gone, we measure the opening properly at survey and match it to the matrix above, then give you one written price with the U-value stated on it.
Request a fixed quote for your roof window, or read the fuller size codes explained guide first if you want the background before you enquire. Everything traces back to the main roof window installation page.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.