Most bifold doors have custom-made manufacturing sizes according to requirements, so if you are wondering how to measure for bifold doors, start by taking three horizontal width measurements across the opening, three vertical height measurements, and two diagonal measurements to check the aperture is square, then use the smallest width and height and subtract 10mm for the door size.
As people from different walks of life have diverse needs, and every house is different, the specifications for bifold doors also vary, which is why inaccurate measurements at the start can ruin a project and create unnecessary headaches for homeowners, DIY installers, manufacturers and fitters.
Due to the flexible nature of a bi-folding door, the utmost care must be taken from the smallest measurements to ensure no mistakes are made. We’ve put together this guide to help anyone planning to install bifold doors measure the aperture accurately, covering width, height and diagonal checks, how to compile the final measurements for ordering, and the questions a manufacturer may ask before your bifold doors are made.
How Do You Measure the Aperture for Bifold Doors?
Measure the structural opening in three places for width (top, middle and bottom) and three places for height (left, centre and right), then check both diagonals. Use the smallest width and height and deduct around 10mm from each to allow a fitting tolerance. The opening needs a level floor, plumb sides and a sound lintel before any aluminium bifold doors can be fitted.
The aperture is the opening in which your bifold doors will fit. It should include a level, flat floor, two vertical sides at 90-degree angles to the bottom, and a top running parallel to the floor. Use a large spirit level to check your vertical and horizontal walls to ensure they're accurate and square, while allowing for any wall irregularities in your measurements.
If your top and floor run parallel and level to each other and your sides are at right angles, your aperture will be perfectly square.
Taking measurements for the aperture of bifolds hardly differs from how you take measurements for patio doors, but there are a few vital distinctions.
With a bifold door, you're required to pay special attention to the tracking hardware above and below the door and the height measurements of the threshold. Besides that, measuring for bifold doors is no different from measuring for sliding doors.
You can call in an expert to carry out the measuring and other necessities needed for fitting your bifold doors, or you can do it yourself. Either way, we hope our guide will provide you with greater insight into the process and the need for accuracy.
Step 1: Measure the Width in Three Places
Like the door frame for any external doors, the outer frame of bifold doors must be installed to be perfectly square and level. This will ensure that your doors will fit suitably and function as they should.
The first measurements you'll need are the outside ones, beginning with the aperture's width. For this guide, we'll assume that the opening is prepared and ready for the installation of the door.
You should take three measurements horizontally across the opening, and for greater accuracy you can also check the opening at roughly 500mm intervals across the width. Your measurements should be from wall to wall, ideally including a side-to-side top section, middle section and bottom section width. Write down each measurement you make, as these details will matter for ordering or fitting, but it'll be the smallest width you'll use as the foundation for the door installation.
Step 2: Measure the Height in Three Places
As you did with the width, measure the outside height from the finished floor to the underside of the header by making three height measurements in the same way and write them down. For example, make special notes of the smallest measurements again.
It's also necessary to note the height of your current threshold and to contemplate what future entry threshold you'll require, with the threshold height based on the final floor level, especially if the finished floor build-up may change. Particular bifold doors come with flush or lower entry thresholds to provide easier access for baby carriages and wheelchairs.
Step 3: Check the Diagonals to Confirm the Opening Is Square
Take diagonal measurements from the left bottom corner to the right top corner and vice versa. These measurements should ideally be the same, or at least very close to each other. Please note these, too, as the bifold door manufacturer may require them.
Don't worry if your aperture doesn't seem to be exactly square after measuring the diagonals, as this can be easily rectified during the door fitting.
Why Do Accurate Measurements Matter?
It's essential to measure as accurately as you can for the following reasons:-
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It'll be much easier to achieve the floor level transition from outside to the interior if precise measurements are taken.
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You might need to alter the step-over height for doors that open outwards slightly.
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You'll need to make sure that doors that inward-opening won't scrape against and damage the inside flooring.
Step 4: Deduct 10mm From the Smallest Width and Height
Once you've taken all the necessary measurements, please note the shortest width and height figures and then deduct 10 millimetres from them. You're doing this as your new bifold doors will have slightly smaller manufacturing sizes than those you've measured to ensure clearance, a small gap for adjustment during installation, and easy fitment.
If a fitment team is coming out, they will also check the condition of your walls to ensure they are strong enough to support the door frames and suitable for installing the outer frame.
What Will the Manufacturer Ask When You Order Bifold Doors?
Expect to confirm the final width and height, the number of panels, whether the doors open inwards or outwards, which side they stack to, whether you want a traffic (everyday) door, and your frame colour, glazing and threshold choices. Having these decided before you order speeds up manufacture and avoids costly changes. You can work through every option in our online door designer.
When you place your order for your bifold doors, the manufacturers will likely need specific information before making or supplying your bifolding door. Be prepared with answers to the following questions, and confirm your preferred panel configuration before ordering:-
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What colour will your doors be?
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How many door panels would you like to include?
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Will your doors fold inwards or outwards?
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Should the doors fold to the left, right, or both ways from the centre?
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Will you need a traffic door?
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Should we allow space for patio slabs, carpeting or floor tiles?
The more information you've considered and have answers for, the easier the bifold door fitting process will be. Visit our website to find out how to fit bifold doors and further tips on aluminium doors.
Bifold Door Measuring FAQs
How can I find out the size of a bifold door to buy?
Measure the height on the left and right sides and through the centre. Take the smallest measurement and subtract the fitting tolerance of 10mm to ascertain the door size you'll need to buy, then confirm the final size against the supplier's installation guide before ordering. Make sure you know whether the doors should open to the inside or outside, and check if there's enough space for the chosen size to install with the required tolerance.
How do I measure to fit interior bi-folds?
Remove existing doors before measuring. Measure the opening's top, middle and bottom horizontally and the left, right and centre vertically, and write down the smallest figures for both. Check the clearance distance your door will require. Check with an expert about how much space you'll need for hardware and tracks.
Is there a trend that's replacing bifold doors?
Not really, although a sliding door is the best alternative to bifold doors when upgrading old patio doors. The problem with sliding doors is they don't give you full doorways like bifold doors do when open.
What is the standard size of a bifold door in the UK?
There is no single standard size because most bifold doors are made to measure, but common UK widths run from around 1800mm up to 4800mm or more, with heights typically around 2100mm. Individual panels are usually between 700mm and 1000mm wide. See our bifold door cost guide for how size affects price.
Do you measure bifold doors from the inside or outside?
Measure the structural opening, which is usually easiest from outside once any old frame has been removed, as the new frame sits within the brickwork reveal. Measuring from inside over plaster or trims can give a misleadingly small figure.
How many panels do I need for my opening?
Divide the opening width by roughly 700mm to 900mm per panel. As a guide, an opening of about 2400mm suits a 3 panel bifold door, while 3000mm to 3600mm openings usually suit 4 panel or 5 panel bifold doors.
What should I do after measuring?
Once you have your order sizes, check the opening is ready for installation and follow our step-by-step guide to fitting bifold doors. If the doors drop or bind after fitting, our bifold door hinge adjustment guide explains how to correct them.
