Bi-fold doors are a great way of injecting some character and charm into your home. Though they’re similar in build to an ordinary door, in practice they act more like a removable wall – they can be extremely wide, which means that they’re able to fit a variety of gaps. If you’re commencing a new build, or you’re looking to make extensive modifications to an existing one, then you’ll have the luxury of shaping the surrounding wall to fit your door. If you’re not, however, then you’ll need to carefully select a door that’ll match with the existing makeup of the building.

In this article, we’ll attempt to demystify the process, and see how we might select a bi-folding door of precisely the right size for the room you have in mind.

How are bi-fold doors sized?

Most suppliers will provide a few different options for their folding door configurations.   We at Aspire provide our bi-fold doors in eighteen different sizes in total, though your options will be slightly limited by your choice of door.  The total size of the door will be determined by the size and number of the panels which comprise it, so you’ll also need to consider whether you’d like a large number of smaller panels or a small number of larger ones.

Internal bi-fold doors are designed to fit gaps that are larger than a 1.8 metres, and the largest of them can be nearly five metres wide.  Within these sizes, doors can be configured in a host of different ways.  These configurations are typically described in terms like ‘5+1’ or ‘4+0’, with each number in a pair describing the number of panels attached to one side of a frame.  A door where both numbers are equal would open from the centre.

What are the standard sizes?

The standard sizes for folding doors are typically described in millimetres.  This is so because of the precision required to ensure that there aren’t any bothersome gaps around the edges of the door.  These gaps can present a heat-efficiency and security concern in exterior doors, but they can also cause aesthetic problems in interior ones – which makes proper measuring for the ideal size essential.

The standard height of an opening for a door is 2095mm.  For the width, however, there are several standard sizes, and a few non-standard ones in between.  These include:

  • 1905
  • 2130
  • 2370
  • 2820

You can look at more options using the filters in our bi-folding doors section.  Before you do so, however, you’ll want an idea of what size door you require – and in order to get such an idea, you’ll need to measure your opening.

How do I measure the size of door I need?

While measuring the space that your folding door is destined to occupy might seem a straightforward task, there are a few steps which, if not performed properly, will result in inaccurate measurements – which in turn can lead to you ordering a door which doesn’t fit properly.  Mistakes here can waste time, energy and money, and so it’s best to ensure that the task is performed correctly from the start.  For this reason, many prefer to call in professional help to perform the measurement on their behalf.  If you’d prefer to take care of the measuring yourself, however, then it’s easy to do so.  Let’s examine how.

Measure the gap

The first step is largely self-explanatory:  whip out a tape measure and record both the height and width of the aperture.  Note that while doing this, you’ll need to account for a slight gap between the edges of the gap and edges of your door.  This gap is known as the ‘fitting tolerance’, and will allow you some leeway if the edges of the gap aren’t quite parallel to those of the door.  When we’re dealing in millimetres, such unevenness might not be obvious at a glance – and it might not become so until the door begins to rub against the frame where it shouldn’t.  A fitting tolerance of between ten and fifteen millimetres is typical.

Measure the angles

You’ll want to measure the angles before proceeding any further. Do this using a large spirit level – place it flush to the edges of the gap in order to ensure it’s as square as possible.  It’s also best to take multiple measurements of both the width and height of the gap, in order to account for any unevenness in the frame of the door.  Use the smallest measurement when considering the size of the gap – a door that’s slightly too small will cause far fewer problems than one that’s too big.

Decide on a door

Now that you know the precise dimensions of the door you’re looking for, you’ll be able to start composing a list of suitable options.  You’ll need to consider whether you’d like the rail to be on the floor, ceiling, or both, and whether you’d like the rail to be flush or raised, as this will have an impact on the size of door you can fit into the gap.  Raised rails tend to protrude 45mm from the floor or ceiling they’re protruding from.


When you’re choosing interior doors, size matters perhaps more than any other consideration. In order to be sure that your door matches the gap you intend to place it in, be sure that you take the necessary steps to measure your door before you place your order – that way you’ll restrict the likelihood of a costly error to a bare minimum!

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