VELUX window sizes are set by a short code that pairs a letter with a number: the letter is the width group and the number is the height group. So an MK04 is 78cm wide by 98cm high, and a UK04 is 134cm wide by the same 98cm high. Once you can read the code, you can match or replace any VELUX window without a tape measure in sight.
This guide covers the six standard width letters, the common height numbers, what the type code (GGL, GGU, GPL, GPU) tells you, where to find the code on the window itself, and how to measure if you are fitting from scratch. We have been supplying VELUX across Sussex, Kent and Surrey since 1996, so the questions below are the ones we answer at the trade counter most weeks.
How to read a VELUX size code
A VELUX size code is usually written as a letter, a middle letter, then two digits, for example MK04. It breaks down like this:
- First letter (M) is the width group. Each letter maps to a fixed width in centimetres.
- Middle letter (K) is the size range reference used across the current VELUX generation. Older windows may show the width letter and number without it, but the width and height still read the same way.
- Two digits (04) are the height group, again a fixed measurement in centimetres.
So MK04 is M (78cm wide) and 04 (98cm high). You will often see a further four-digit number after the size, such as GGL MK04 3070. That trailing number is the glazing and finish variant (glass specification, pane type and so on), not the size. For fit and flashing, the size code is the part that matters.
VELUX size chart (UK): width letters and height numbers
VELUX windows come in fixed sizes rather than made-to-measure, so every size is a combination of one width letter and one height number from the tables below. Widths run from 55cm up to 134cm; heights run from 70cm up to 180cm.
| Width letter | Width |
|---|---|
| C | 55cm |
| F | 66cm |
| M | 78cm |
| P | 94cm |
| S | 114cm |
| U | 134cm |
| Height number | Height |
|---|---|
| 01 | 70cm |
| 02 | 78cm |
| 04 | 98cm |
| 06 | 118cm |
| 08 | 140cm |
| 10 | 160cm |
| 12 | 180cm |
Put a letter and a number together and you have the size. Some of the most common on Sussex roofs are:
- CK02 is 55cm x 78cm, a small window for landings and box rooms.
- MK04 is 78cm x 98cm, the single most popular size.
- MK06 is 78cm x 118cm, a taller version of the same width.
- SK06 is 114cm x 118cm, a wide window for larger loft rooms.
- UK04 is 134cm x 98cm, the widest standard size.
Not every letter pairs with every number, so a size you invent on paper will not always exist as a stock product. If you are unsure whether a combination is made, see the full VELUX range or ask us and we will confirm it.
GGL, GGU, GPL and GPU: what the type code means
Before the size, a VELUX roof window carries a type code, and the four you will meet most are GGL, GGU, GPL and GPU. They describe how the window opens and what the frame is made from:
- Second letter is the operation. G is centre-pivot (the sash spins on a central hinge). P is top-hung (the sash swings out from the top for a clearer view and more headroom).
- Third letter is the finish. L is pine, a timber finish that VELUX supply either clear-lacquered or white-painted. U is white polyurethane, a timber core wrapped in tough white plastic that copes better with steam.
So in practice:
- GGL is centre-pivot, pine. The traditional loft-conversion window.
- GGU is centre-pivot, white polyurethane. A good choice for bathrooms and kitchens where there is moisture.
- GPL is top-hung, pine.
- GPU is top-hung, white polyurethane.
The size code (MK04, SK06 and so on) is the same across all four types, so a GGL MK04 and a GGU MK04 need the same opening and the same flashing. That matters when you order, because the flashing kit is priced separately from the window. You can see the wider window options on our roof windows and rooflights page.
How do I find my VELUX size code?
Every VELUX window has a data plate (VELUX call it the type plate) printed on the window itself. To find it, open the window fully by rotating or swinging the sash, then look along the top edge of the sash frame, usually towards the top right. The plate shows the type code and the size code together, for example GGL MK04, along with the manufacture date.
Reading the plate is the reliable way to order a like-for-like replacement, because it removes any guesswork about width and height. If the plate has faded or the window is too high to reach safely, measure the visible glass and the outer frame and bring those figures to the counter, and we can usually work back to the size from there.
How to measure for a VELUX window
VELUX windows are fixed sizes, so you are not measuring the window to order, you are measuring the roof to see which size fits. For a new opening:
- Check your rafter spacing. The gap between rafters sets your realistic width. A 78cm (M width) window suits rafters at roughly 600mm centres with a little trimming; wider windows may need a rafter cut and the opening structurally framed.
- Measure the structural opening, not just the window. VELUX give a required structural opening for each size in the installation instructions, and it is slightly larger than the window’s outer frame to allow for fixing and flashing.
- Allow room top and bottom. Fitting height affects reach and view, and top-hung (GPL/GPU) windows suit a higher position than centre-pivot ones.
If you are replacing an existing window, ignore all of that and simply read the code off the data plate. Same code in, same code out, and the flashing will match your tile or slate. Not sure which flashing you need for your roof covering? Bring a photo or a tile to our Haywards Heath depot and we will match it.
Getting the right VELUX first time
Read the size code, confirm the type, and order the matching flashing for your roof covering, and a VELUX order is straightforward. Get the flashing wrong or the opening size out and it becomes a return and a delay. If you would rather have a second pair of eyes on it, send us the code and your roof type and get a quote, or call the depot and we will price the window, flashing and any extras together.
Frequently asked questions
What sizes do VELUX windows come in?
VELUX windows come in fixed sizes, not made-to-measure. Standard widths run from 55cm (C) up to 134cm (U), with common letters C, F, M, P, S and U. Heights run from 70cm up to 180cm. Each window is one width letter plus one height number, such as MK04 (78cm x 98cm) or SK06 (114cm x 118cm).
What does GGL mean on a VELUX window?
GGL is a centre-pivot roof window with a pine (timber) finish, supplied either clear-lacquered or white-painted. The sash pivots on a central hinge, and the timber interior suits bedrooms and living spaces. Its sister product, GGU, is the same centre-pivot window but finished in white polyurethane, which handles moisture better in bathrooms and kitchens.
How do I find my VELUX size code?
Open the window fully, then look along the top edge of the sash frame, usually the top right, for the data plate (type plate). It prints the type and size together, for example GGL MK04, plus the date of manufacture. Quote that code for a like-for-like replacement so the opening and flashing match exactly.
What is the difference between a skylight and a VELUX window?
Skylight is a general word for any glazing set into a roof. VELUX is a brand, and a VELUX roof window is an openable unit fitted in the slope of a pitched roof. People often say VELUX to mean any roof window, in the same way they say Hoover for a vacuum, but strictly it is one manufacturer.
How do I measure a VELUX window?
For a replacement, do not measure at all, just read the size code off the data plate and order the same. For a new opening, measure your rafter spacing for the width and the structural opening height from the VELUX installation instructions, which is slightly larger than the window frame. Match the closest standard size to that opening.
How much does a VELUX window cost?
As a guide, a common manual double-glazed centre-pivot window (GGL or GGU) in a size like MK04 is often around £250 to £450 for the window alone, with the flashing kit priced separately. Triple glazing, larger sizes and electric or solar INTEGRA models cost more. Ask us for a current price on your exact size and roof type.