How to read a VELUX size code, and order the right window

The size code is a label for the frame. It tells a supplier which frame dimensions the window was made to, and every accessory made for that window is keyed to the same label.

The code is built from two letters followed by two digits. You will see it written as a single run, for example MK04 or CK02. The letters and the digits do different jobs, and the next chapter takes them one at a time. What matters first is what the code leaves out. It says nothing about the glazing, the finish, whether the window is manual or electric, or which way the sash opens. Those sit in separate parts of the product name. Two windows with the same size code can differ in every one of those ways and still fill the same hole in the roof.

That is useful, and it is the reason the code exists. A blind made for MK04 fits any MK04 frame. A flashing kit made for MK04 wraps any MK04 frame. The catalogue is organised the same way, so a size code is the quickest route from the window in front of you to the right box on the shelf.

It is easy to mistake the code for a measurement. It is a label that stands for a pair of measurements, and the measurements belong to the frame rather than to the glass you see from inside. Hold on to that, because it explains most of the confusion people report when they try to match a code to a tape reading.

The two letters broadly describe the frame width and the two digits broadly describe the frame height. Read together, they identify one frame size out of the range.

The letters run through a set of pairs such as CK, MK, SK, UK and PK, with others at the extremes of the range. A pair nearer the start of the alphabet does not simply mean a smaller window, so do not try to guess a width from the letters alone. The reliable way to turn a pair into a width is to look it up in the manufacturer's size chart, or to read it from the plate and then check it against the frame. The letters are a name for a width class, and the chart is the dictionary.

The digits work along the height. The numbers come in even steps, 02, 04, 06, 08 and upward, and a larger number broadly means a taller frame. The steps are chosen so that neighbouring heights can be stacked above one another in a roof with the right gaps, which is what makes the range useful when a designer wants a window over a window.

A worked example helps. MK04 is one of the most familiar codes in the UK range, a mid-width, mid-height frame that appears in a great many lofts. The first letter pair places it in a width class, and the 04 places it in a height class. Ask a supplier for an MK04 blind and you will be handed a blind sized for that frame, whatever year the window was fitted. The smaller CK02 is a narrow, short frame, and our answer on what the VELUX size CK02 means picks that one apart in detail.

The code is printed on a data plate fixed to the window itself. Where the plate sits depends on the generation of the window, so look methodically rather than expecting one spot.

On a roof window the plate is usually on the inside of the frame, somewhere near the top, where a person standing on a chair or a step can reach it with the sash open. Some plates are on the edge of the sash, visible when the sash is swung to its full ventilation position or rotated on its pivot. On older windows the plate can be partly covered by a layer of paint, a trim strip or a blind cassette, so a torch and a dry cloth are useful. We cover the usual positions in the answer on where the data plate is on a VELUX window, and that page is the one to use while you are up there.

Take a photograph of the plate before you climb down. A clear, straight-on picture with your phone is worth more than a scribbled note, because it keeps the whole line of text and lets you read it again in good light at the kitchen table.

1. Open the sash to its normal ventilation position and look along the inside of the top frame.
2. Check the edge of the sash, working from the hinge side across to the handle side.
3. Look behind any blind cassette, trim strip or curtain pole that sits over the frame.
4. Photograph whatever you find, then write the size code down from the photograph.

The size code is one line on the plate, and the lines around it matter as much when you order. A plate is a short record of how that window was built.

The product name is a separate set of letters that names the type of window, its frame material and its finish. The three you will meet most often are covered in our guide to VELUX GGL, GPL and GGU, which sets out the differences. A glazing reference, a short run of characters, says which pane build the window left the factory with. A production date or batch reference tells a supplier roughly when the window was made. Together they let a supplier check what the window was, what it is now and what can be fitted to it.

Not every line is useful to a homeowner. The size code, the product name and the glazing reference are the three worth photographing. The rest is for the supplier, and a clear picture of the whole plate lets them pick out what they need without calling you back.

The layout and wording of plates change over time, and a window fitted some years ago will look different from one fitted last month. Where your plate differs from this description, trust what is printed on it, and use the manufacturer's size chart to decode it.

Once you have the code, four families of product are keyed to it. Each has its own small traps, so it is worth seeing them side by side.

A blind is the simplest. It comes made for a size code, and it clips or fixes to the frame. The model of window still matters, because a few blinds are made for particular window generations, and the colour, the control type and the noise each blind makes differ. We set out the choices in choosing genuine VELUX blinds.

A flashing is more involved, because it is keyed to the size code and also to the roof covering the window sits in. The same frame needs a different flashing in slate than in plain tile or in a profiled tile. The code only settles half of that question, and the roof settles the rest. The various types, with their letter codes, are explained in VELUX flashing types: EDW, EDL and EDZ.

A replacement sash or a replacement pane is keyed to the code plus the plate details. A sash built for an older frame generation does not always fit a newer frame, so the plate's other lines do real work here. Our answer on whether a VELUX sash can be replaced without changing the frame sets out when that is sensible.

The insulation and underfelt collars also follow the code, and they go together with the flashing in a fixed order. Mixing a collar for one size with a flashing for another is an easy mistake to make from a catalogue, which is why the VELUX insulation collar and underfelt collar page is worth reading before you buy anything.

A size code tells you what the window was made to. It cannot tell you how the window was fitted, what the rafters did to the opening or what the roof looks like after years of weather. For that, somebody has to look at the roof.

Before we agree a replacement, we read the plate and then measure the frame and the opening on site, and the two have to agree. If they do, the replacement is a straight swap for the same frame size. If they do not, something has changed since the window was fitted, and that is the moment to find out why. A previous owner may have reframed the opening. An earlier fitter may have used a frame one size down and packed the gap. A plate can be fixed to a sash that was swapped from another window. These situations are uncommon, but they explain why the plate is a first clue and never the last word.

The rafter spacing sets a second limit. A roof window frame needs to sit between two rafters with a margin on each side, or the rafters have to be trimmed and the load carried by a new header. Some codes suit the usual rafter centres in a 1950s to 1970s Crawley semi with no structural work. Others need a trimmer, and that is better known at the quote stage than on the day. The same size label can be straightforward in one roof and awkward in the next.

Plan the quiet while you are at it. The frame size tells a supplier which glazing options exist for that opening, and the glazing build is where a loft bedroom gets its calm. If you are thinking about aircraft on the Gatwick routes, or rain on the roof at night, read VELUX glazing options for a quieter loft before you settle on a pane. The size code rarely rules a quieter pane out, and the plate's glazing line tells you what you start from. Every quote from us comes with a written quiet spec for that room, so the choice is written down.

Choosing a code for a window that does not exist yet is a different job from reading one that does. It starts from the room and the roof, and the code comes last.

Work from the room first. Where does the bed, the desk or the bath sit, and where should the daylight fall? The height of a sloping ceiling decides how far up the slope a window can go, and the pitch decides how much of the glass is usable from a standing position. Our page on which VELUX size suits a loft bedroom works through that for a typical sleeping room.

Next comes the roof. A frame must fit between rafters or be framed in with new ones. The covering has to be cut back and dressed into a flashing without forcing a tile course. Roof windows can be set side by side or one above another, and the size labels are chosen so that adjacent frames line up with fixed gaps, which is why the catalogue publishes recommended spacing for a pair. The answer on fitting VELUX windows side by side explains how that works in practice.

Last, the code. With the room, the pitch and the rafters known, you choose the frame class whose width and height suit all three. Because the letters are keyed to width and the digits to height, you can move one without moving the other, and a slightly narrower or shorter choice often sidesteps a rafter problem. It is the usual way a survey turns into a size.

A like-for-like replacement is the easiest case, and the code carries most of the weight. Even so, a few checks are worth doing.

Start with the plate, and copy the size code and the product name. Then compare the plate against the frame with a tape, measuring the outside of the frame across the width and down the height. The numbers will not match the size label in any neat way, since the label is a name and not a dimension. What you are checking is that the frame matches the size chart's published figure for that code. If it does, the plate is telling the truth.

Next, look at the flashing and the lining. A replacement window sits in the existing flashing in some cases and needs a new one in others, depending on the age and the condition of the existing kit. Our answer on whether a new VELUX will fit the old opening goes into the question of the surround in more detail.

Finally, ask whether the same model is still the right model. A window that was the sensible choice for a landing years ago may not suit a bedroom now. If the room is changing use, choose a fresh glazing build, and treat the swap as a chance to renew the whole unit, blind and flashing included. On the pages for VELUX window installation in Crawley we set out how we approach each of those steps.

A plate can be painted over, scraped off or simply worn away. That is a common enough situation that it has its own page, and there is a sound way to work around it.

Without a plate, the size has to come from the frame. You measure the outer width and height of the frame with the sash closed, and you compare the measurements to the manufacturer's published size chart to find the code that matches. The measuring needs care, because the frame edge is hard to see beneath trim or plaster, and the answer is only as good as the tape reading. Our answer on how to find your VELUX size if the data plate is missing goes through where to measure, what to write down and how to check the result against the chart.

If you are unsure at any stage, stop before ordering. A wrong flashing or blind is an easy mistake to make and an irritating one to return. Sending us photographs of the window, inside and out, along with a tape reading is usually enough for us to confirm a size before anything is bought. Use the quote form on this page, or message us on WhatsApp, and include the photographs.

Most errors with size codes come from a handful of habits. Knowing them saves a return trip to the loft.

• Reading the code from the old box, the old receipt or another window, when it belongs on the plate of the window in front of you.
• Treating the letters as a size order, so that a pair earlier in the alphabet is assumed to be smaller.
• Expecting the digits to be centimetres. They are a height class and not a measurement.
• Ordering a flashing from the code without checking the roof covering.
• Assuming two windows side by side in one roof must carry the same code.

The last one is worth a sentence. A bathroom window may well be a different size from the bedroom window next to it, because the two rooms had different needs on the day they were fitted. Read each plate separately.

These are the things that come up once the plate has been found and the code is written down.

Does the size code tell me the exact glass size?

No. It names the frame size, and the visible glass is smaller, because the frame and the sash take a share. If the visible glass matters to you, measure it directly, and use the code only to find the frame and its accessories.

Can I use a code from a brochure or a website instead of the plate?

Use the plate for the window you already have. A brochure code describes a window that may have been made differently from yours, and an old one may not match the current range. The plate on the frame is the record of what is actually in your roof.

Will the same code always mean the same frame?

The label is stable, and accessories made for a code are made to its frame size. The product name and the window generation still matter, since sashes and some fittings differ between generations. The plate gives you both pieces, so copy the whole of it.

Who should measure the opening for a low-pitch window?

Anyone can read a plate, but the opening and the roof around it need a proper look, particularly where the pitch is shallow. Our page on the best VELUX for a low-pitched roof covers what to check, and we can confirm the size at a survey.

If you have a code in hand and want it checked against your roof, send us the photograph through the quote form and we will confirm the frame, the flashing and the glazing before anything is ordered.