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Every VELUX window carries a size code — and because VELUX has changed its coding system twice since the 1970s, the same window has up to four different names depending on when it was made. MK06, M06, 306 and GGL 14 are all the identical 780 × 1,178mm window. This page decodes all of them: type a code below, or scroll for the full charts, drawn from VELUX’s official replacement size guide.
Works with every generation: current (MK06), 2001–2013 (M06), pre-2001 (306), and the original single-digit GGL numbers (14).
The current size chart (two-letter codes, 2013–present)
External frame dimensions, width × height. Letters set the width, numbers the height:
| C 550mm | F 660mm | M 780mm | P 942mm | S 1140mm | U 1340mm | |
|---|---|---|---|---|---|---|
| 01 (698mm) | CK01 | SK01 | ||||
| 02 (778mm) | CK02 | |||||
| 04 (978mm) | CK04 | FK04 | MK04 | PK04 | UK04 | |
| 06 (1178mm) | CK06 | FK06 | MK06 | PK06 | SK06 | UK06 |
| 08 (1398mm) | FK08 | MK08 | PK08 | SK08 | UK08 | |
| 10 (1600mm) | MK10 | PK10 | SK10 | UK10 | ||
| 12 (1800mm) | MK12 |
Plus the outliers: BK04 (472 × 978mm, the narrowest), PK25 (942 × 550mm, letterbox format) and MK27 (780 × 624mm). The most common UK loft sizes are MK04, MK06 and MK08 — the 780mm width fits standard rafter spacing with minimal trimming.
Old code conversion chart
Same physical opening, three generations of names — any modern code drops into its predecessor’s aperture, which is what makes like-for-like replacement cheap:
| Size (mm) | Current | 2001–2013 | Pre-2001 | Original GGL no. |
|---|---|---|---|---|
| 550 × 698 | CK01 | C01 | 101 | 9 |
| 550 × 778 | CK02 | C02 | 102 | — |
| 550 × 978 | CK04 | C04 | 104 | 6 |
| 660 × 978 | FK04 | F04 | 204 | — |
| 660 × 1178 | FK06 | F06 | 206 | 5 * |
| 780 × 978 | MK04 | M04 | 304 | 1 |
| 780 × 1178 | MK06 | M06 | 306 | 14 |
| 780 × 1398 | MK08 | M08 | 308 | 2 |
| 780 × 1600 | MK10 | M10 | 310 | 13 |
| 780 × 1800 | MK12 | M12 | 312 | — |
| 942 × 978 | PK04 | P04 | 404 | 31 |
| 942 × 1178 | PK06 | P06 | 406 | — |
| 942 × 1398 | PK08 | P08 | 408 | — |
| 942 × 1600 | PK10 | P10 | 410 | 3 |
| 942 × 550 | PK25 | P25 | 425 | — |
| 1140 × 698 | SK01 | S01 | 601 | — |
| 1140 × 1178 | SK06 | S06 | 606 | 4 |
| 1140 × 1398 | SK08 | S08 | 608 | 10 |
| 1140 × 1600 | SK10 | S10 | 610 | — |
| 1340 × 978 | UK04 | U04 | 804 | 7 |
| 1340 × 1398 | UK08 | U08 | 808 | 8 |
| 1340 × 1600 | UK10 | U10 | 810 | — |
* The original GGL 5 was 700mm wide — 40mm wider than the modern FK06 that replaces it. If you’re replacing one, have the installer check the aperture; it’s the one conversion that isn’t drop-in. (Source: VELUX’s own size guide.)
Reading the full code on your data plate
Open the window and find the metal data plate on the frame edge. A typical marking is GGU MK06:
- The letters before the size are the window type: GGL centre-pivot in pine · GGU centre-pivot in white polyurethane · GPL/GPU top-hung in pine/poly · GBL fixed · GCL conservation · GGLS/GPLS 3-in-1 multi-sash.
- The size code is everything this page decodes.
- Flat-roof windows use six-digit codes instead — width × height in cm (
120090= 120 × 90cm); different product family.
That one plate is all you need to order a replacement window, the right blinds (sold by these same codes), or spares — and it’s the first thing a good installer will ask for.
Choosing a size for a new opening
For replacements, match the code and move on. For new installs, the buyer’s guide covers sizing properly — the short version: glazing area around 15–20% of floor area for a genuinely bright room, go taller rather than wider on shallow pitches, and remember the code drives the price more than the window type does (2026 prices by size). Rival brands use their own incompatible codes, so decide the brand before falling in love with a size.
Frequently asked questions
What size is a VELUX MK06?
780mm wide × 1,178mm high (78 × 118cm, external frame). It's one of the most common UK loft sizes, previously coded M06 (2001–2013), 306 (pre-2001) or GGL 14 (original numbering).
What size is a VELUX 104?
550mm wide × 978mm high (55 × 98cm). The 104 is the pre-2001 code for what's now the CK04 — also written C04 between 2001 and 2013, or GGL 6 in the original single-digit era. A modern CK04 drops into the same opening.
Where do I find my VELUX size code?
Open the window and look at the metal data plate on the frame edge (top right on most models). It shows the window type (e.g. GGL) and size code (e.g. MK06) — everything you need to order replacements, blinds or spares.
What do GGL and GGU mean on a VELUX window?
The type prefix: GGL is the centre-pivot window in pine, GGU the same window in white polyurethane. GPL/GPU are the top-hung versions, GBL is fixed, GCL is the conservation range. The size code that follows tells you the dimensions.
What is the biggest VELUX window?
In the standard range, the UK10 at 1,340 × 1,600mm (134 × 160cm) is the biggest conventional roof window; the MK12 (780 × 1,800mm) is the tallest. Beyond that you're into combi arrangements of multiple windows or the Cabrio balcony (2,520mm tall).
Keep reading
VELUX costs 15–40% more than rival roof windows. Here's what the premium actually buys, when it's worth paying, and when a cheaper brand makes more sense.
Fakro, Keylite, RoofLITE+, Roto and Dakea compared against VELUX on 2026 prices, guarantees and quality — and which alternative fits which job.
Every custom skylight option for UK homes — flat glass, lanterns, walk-on and structural glazing — with 2026 costs, materials advice and the regs.