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Understanding Interior Bifold Door Sizing

Folding doors have become a go-to feature for homeowners looking to make their interior feel that little more expansive. Internal bifold doors are made up of multiple panels, each of which connects to its neighbours via hinges, allowing the entire thing to collapse and expand in a concertina. This design means that the door doesn’t have to open out anywhere near as far, which makes smaller folding doors a popular choice for storage spaces where there isn’t sufficient room to accommodate a traditional outward-opening door. But this design also allows doors that are large enough to encompass entire walls.

Doors of this sort have become ubiquitous, both in the home and garden. They’re fantastic for bolstering the sense of space you get from a particular room even when they’re closed, as they come equipped with multiple glass panels which allow light to disperse from one room to another (and which allow you to look all the way through the adjacent room). To sweeten the deal still further, they look fantastic, and are sure to form an important talking point the next time you have guests over.

Bifold doors vary tremendously in size, and so they can be installed just about anywhere in the home. With that said, you’ll need to ensure that yours is a precise match for the space you have in mind – which means considering the sizes available and measuring carefully. In this article, let’s take a closer look at interior bifold doors. We’ll see what the sizing options are, and how you can find a door that’ll match with your interior.

What are the Standard Bifold Doors Sizes in the UK?

Like any door, you’ll need to match your internal bi-folding door to the aperture that’s going to house it. Fortunately, this is made relatively straightforward by the fact that doors in the UK tend to follow certain sizing conventions.

The vertical dimension is the easiest to accommodate. While there’s some slight variance from manufacturer to manufacturer, bifold doors tend to be around 6’8”, or 2078mm, tall. A little bit of variance here can spell the difference between a major headache and a straightforward installation, and thus it’s worth measuring up properly before placing your order. Make an assumption and you might come to regret it later on!

The door’s width, on the other hand, can vary in size by several metres, and this variance will depend largely on the number of panels your door will incorporate. Each panel will be between a metre and seventy centimetres wide, and thus you’ll be able to choose between several larger panels and many smaller ones. You might fill a 2800mm-wide aperture, for example, with four 700mm panels.

When making this decision, you’ll want to bear in mind the space to either side of the door; larger panels will need more room to open out into. But, on the other hand, the more panels you have, the more space they’ll take up to either side of the space when they’re folded away.

It’s also worth thinking about the thickness of the panels themselves; like the overwhelming majority of UK internal doors, bi-fold panels tend to be 35mm thick – which tends to strike the appropriate balance between weight and insulation. So, if you have five panels in your door, you’ll have a 175mm stack when the door is folded up.

Different arrangements of panels, of course, will affect how this stack is distributed. A ‘French-fold’ door which opens from both sides will have two equal stacks on either side. An ‘x+1’ door, which pairs a single outward-opening door on one side with a folding door on the other.

Finally, it’s worth reinforcing the distinction between the external dimensions of the frame, and those of the door that sits within. Clearly, the former will be slightly larger than the latter, and it’s vital that the two aren’t confused. Measure your opening without the trim installed.

What Size Bifold Door Do I Need?


Having surveyed the options, it’s time to measure your space and see exactly what’s required. While you’re at it, you’ll be checking that the opening is plumb by taking multiple measurements across the length and height of the aperture. It’s important to get all this right from the very start – as if proper care and attention is absent at this stage, then the result will surely be a sizeable headache later down the line. Hurry things, on the other hand, and you’ll have to spend time, energy, and money correcting mistakes later.

Making the Measurements

Take three vertical measurements: from the left, right and centre. Then take three vertical ones: from the top, middle and bottom. If there’s a discrepancy of more than ten millimetres between them, then you’ve a crooked opening. To be extra sure, whip out a spirit level and place it flush to the edges of the gap. A crooked gap isn’t the end of the world, but it will mean that you need to correct the problem with shims before your door will hang properly.

If the measurements you take are all within acceptable tolerances, then refer to the smallest of each three; it’s better to have a door that’s ever-so-slightly too small than one that’s too big, and it’s much easier to make a space (slightly) smaller than it is to expand it.

Remember, you’re measuring the aperture itself, rather than the trim, which is designed to overhang the aperture and thus disguise the gap between the wall and the door.

What About Fitting Tolerance?

Finally, you’ll want to consider the difference between the edges of the gap and the edges of your door. After all, door frames aren’t built to be pressed right up against the frame they inhabit. This difference is known as the ‘fitting tolerance’.

This is principally because of a phenomenon called warping. Timber doors will change shape over time, as the wood fibres expand and contract in response to seasonal changes in temperature, pressure, and moisture. This effect is far less pronounced in engineered doors than it is in solid-wood ones, since the former are comprised of many different lengths of wood whose grains run in contrary directions, and so the warping effects tend to cancel one another out. But if your fitting tolerance isn’t sufficiently generous, then just a few millimetres might make a big difference. On the other hand, a fitting tolerance that’s too great will result in an overly draughty door, or one that doesn’t fit at all.

A good rule of thumb is to account for a fitting tolerance of around 10-15mm, which strikes a happy medium between these two problems. Be aware, however, that hanging the door improperly will cause problems with catching even if you’ve judged the size of the door perfectly.

Custom Bifold Door Sizes

If you find that the space you have in mind doesn’t come anywhere near matching the available doors, then you might be about ready to despair. If you’re the owner of an older property, built before standard door-sizes became widespread, then you might find yourself in this situation. But fear not: the problem can be addressed in one of two main ways.

The most obvious is to have a door custom-made for the purpose. Custom-made have the considerable advantage of being entirely unique, and thus they’re sure to leave a visual impact on any space they inhabit. You can be reasonably sure, after all, that your door is going to leave a special mark on your property if it’s the only one of its kind. Moreover, you’ll be able to tailor the design of your door precisely to your liking.

What’s more, since custom doors are built specifically to address a particular need, they’ll tend to provide a more accurate fit than their bulk-produced counterparts. On the other hand, since they need to be created for purpose, they also come with accordingly steep price-tags and lead times of several weeks, and thus they’re widely considered a measure of last-resort.

The second option is to alter the size of the aperture to accommodate a standard-sized door, either by installing new brickwork around the edges or by expanding the hole with the aid of a sledgehammer. Though this is often the more practical course of action, it requires considerable skill to be pursued safely and effectively. Drafting in a professional builder to do the work is therefore advisable; after all, attempting to cut corners by doing it yourself might easily result in disaster, and an even costlier bill when you have to bring in a specialist to correct your mistakes!

In Conclusion

A bi-fold door adds considerable function and visual interest to just about any interior – but only if it’s properly fitted. To do the job well, you’ll want to measure up carefully and thoroughly before selecting an option that fits with your existing décor and personal tastes. Once you’re looking at your perfectly-fitted bi-fold doors, you’ll surely consider the effort worthwhile!

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