What are the standard VELUX sizes?

The code is two parts. The letter at the front tells you the width of the window in a set of standard steps, and the two-digit number after it tells you the height. Once you know the two lookups, any VELUX code turns straight into millimetres in your head.

Width is stepped like this:

• C is 55 cm wide
• F is 66 cm wide
• M is 78 cm wide
• P is 94 cm wide
• S is 114 cm wide
• U is 134 cm wide

Height runs on the number:

• 01 is 70 cm tall
• 02 is 78 cm tall
• 04 is 98 cm tall
• 06 is 118 cm tall
• 08 is 140 cm tall
• 10 is 160 cm tall

So MK04 is 78 cm across by 98 cm high. PK10 is a big one, 94 by 160 cm. The middle letter you often see written in the code, the K in CK02 or the middle character in some listings, is part of VELUX’s own reference and does not change the size, so you can read past it and go straight to the outer letter and the number. These are the outside frame measurements of the window itself, not the hole in the roof. The structural opening between the rafters needs to be a little larger, and that gap is what the flashing kit is built to cover.

VELUX does not make every letter and number in combination. The catalogue is a grid of sensible pairings, and a handful of them account for most homes. Here are the sizes you are most likely to be quoted, with the code, the window size and where each one tends to suit.

The reason MK04 turns up so often is the roof itself. A great many British roofs, especially the 1930s semis and the post-war estates across south Essex, were built with rafters at roughly 600 mm centres. A 78 cm wide window drops neatly into a single rafter bay with one trimmer at the top and one at the bottom, so the roof structure barely has to change. Go wider than that, to a P, S or U, and a rafter usually has to be cut and the load carried on trimmers to either side, which is more work in the loft and worth knowing before you fall in love with a size.

If you are replacing a VELUX that has reached the end of its life, you do not measure it, you read it. Every window carries a data plate, a small silver or grey label fixed to the frame. On most models it sits at the top of the window, and you see it by opening the window fully and looking at the top edge of the sash or the head of the frame.

The plate gives you two useful things. The type code, such as GGL or GGU, tells you the model and how it opens. GGL is the centre-pivot with a painted timber finish, GGU is the same shape in a white polyurethane finish that suits a bathroom. After that comes the size code, the letter and number pair from above. Quote both to a supplier and you will get an exact match, which matters because a replacement of the same size drops into the existing flashing and timber without opening the roof up any wider than it already is.

Older windows from before roughly 2000 used a different numbering system, so a very old plate may read as a code like 304 or 606 rather than MK04. If yours does, the safest route is to measure the outside of the frame in millimetres, width first then height, and match that against the modern grid. A window measuring around 780 by 980 mm on the frame is an MK04 in today’s sizes.

A bigger window is not simply more daylight. It is more glass for the sun to pour through, and on a south-facing roof that is the point where sizing and glass specification stop being separate questions. Double the glazed area and you double the solar energy landing in the room on a July afternoon, which is why the size you pick and the glass you put in it have to be decided together rather than one after the other.

This is the whole basis of what we do. Southend faces south across the Thames Estuary, so the back of a huge number of houses here catches sun from mid-morning until it swings west, with reflected light off the water adding to the total. Fit a large PK08 or a pair of MK06 windows into a south-facing loft with plain clear glazing and you have built a warm room that gets warmer every summer. The fix is not a smaller window, because you still want the light. It is the right glass in the size you want: solar-control glazing with a G-value around 0.3 or below, which blocks a good share of the summer heat while keeping the room bright. On a north-facing roof the same size window can take plainer glass and you will be glad of the free winter warmth. If overheating is the worry, our energy-efficient skylight installation page sets out how orientation, size and glass specification fit together.

For a brand-new window rather than a swap, the size is a balance between the daylight you want and the roof you have. A rough guide used for loft conversions is that the glazed area should be at least a tenth of the floor area of the room to feel properly lit, and rooms with sloping ceilings often want more than that. Taller windows, the 06 and 08 heights, bring the bottom of the glass down so a standing adult sees out rather than up at the sky, which matters more in a bedroom than a bathroom.

Before you settle on a code, check three things. Measure the rafter spacing in the loft, because it decides whether you can stay within a single bay or need to trim a rafter. Note which way the roof slope faces, because that sets the glass, not the window. And look at where the window will sit relative to the ridge and the eaves, since a new rooflight is generally permitted development where it projects no more than 150 mm beyond the roof plane and sits below the ridge, though conservation areas and flats are exceptions and it is always worth checking with the local authority. Any new or enlarged rooflight is also notifiable under Building Regulations for its thermal performance and its glazing safety.

If you would rather hand the sizing over, tell us the room, the rough opening you have in mind and which way the roof faces, and we will come back with a size code, a glass specification and a price. You can see how we handle the whole job on our VELUX window installation page, or ring 01702 898232 and request a quote, and we will look at the roof and the light in the room, not just the hole you want to fill.