Getting the sizes wrong on bifold doors is not a small error you can tidy up later. A few millimetres can affect frame fit, packers, threshold detail, plaster lines and, in some openings, whether the doors will work at all. If you are looking up how to measure bifold doors, the key is to measure the structural opening properly rather than guessing from old frames or finished trims.
For homeowners, that usually means getting clear enough dimensions for an early quote while knowing when final survey sizes should be left to a specialist. For builders and experienced renovators, it means recording the opening in a way that reflects real site conditions – not the ideal version on the drawing.
How to measure bifold doors without costly mistakes
The first distinction to make is whether you are measuring for budget pricing or for manufacture. If you only need an estimate, you can measure the aperture fairly simply. If the doors are being ordered, the opening needs to be checked more carefully for level, plumb, square, threshold build-up and finished floor height.
Bifold doors are made to fit a structural opening with a fitting tolerance. They are not measured in the same way as internal wardrobe doors or off-the-shelf room dividers. Systems such as Smarts Visofold 1000 Bifold Doors, Schuco ASFD75 Bifold doors, Cortizo Bifold Plus and Origin OB49 Bifold Doors are fabricated to precise sizes, and the survey must reflect the actual construction opening they are going into.
Start with the structural opening, not the old frame
If you are replacing existing patio or French doors, do not measure the visible glass, the sash size or the outer face of the current frame and assume that is your new bifold size. Old frames often sit inside trims, plaster returns or sealant lines that hide the true opening.
The right place to measure is the structural aperture – the masonry, timber or steel-framed opening the new outer frame will sit within. If the old doors are still in place, your measurements can only ever be provisional unless you know exactly how the opening has been formed.
Measure width in three places
Take the opening width at the top, middle and bottom. Use the smallest figure as your working width.
That matters because many openings are not perfectly parallel. Brickwork can bell out, plaster can narrow the visible gap, and steel support details can reduce the clear opening in one area. If the bottom is wider than the top, the frame still has to fit the tightest point.
Measure height in three places
Now measure the opening height on the left, centre and right. Again, use the smallest figure.
Be clear about where you are measuring from and to. The bottom point should be the actual load-bearing or finished threshold position, not loose flooring, old tiles you plan to remove or insulation build-up that has not yet been installed. At the top, measure to the underside of the lintel or structural support, not to trim or plasterboard unless those finishes are fixed and staying.
Check level, plumb and square before you trust the numbers
A width and height alone do not tell the full story. Bifold doors need an opening that is reasonably true, especially on wider runs where multiple panels must align, lock and roll smoothly.
Check the sill or threshold area with a long spirit level. If the floor drops significantly across the opening, a low threshold can become more complicated than it first appears. You may need to allow for packing, local floor correction or a revised threshold detail.
Then check both jambs for plumb. If one side leans, the smallest width may not be the only concern. The frame can be installed and packed to suit, but the amount of tolerance available is limited.
Finally, measure the diagonals from corner to corner. If both diagonal measurements are close, the opening is roughly square. If they differ noticeably, the aperture may need preparation before final installation. A slightly out-of-square opening is common. A badly formed one is where problems start.
Understand fitting tolerance
Manufacturers do not usually make the frame to the exact structural opening size. A deduction is normally applied to allow for fitting tolerance, packers and perimeter sealing. The exact amount varies depending on the system, installer preference and site conditions.
That is why a quote size and an order size are not always identical. On a supply-only project, this point matters even more. If you are ordering your own Schuco, Cortizo, Smarts or Origin bifolds, you need to know what deduction is being assumed and whether the threshold, cill and floor finish have been accounted for.
Thresholds, cills and floor finishes change the measurement
This is where many otherwise careful measurements go wrong. The visible opening might suggest one overall height, but the final door height depends on the threshold design and the finished floor levels inside and out.
A fully weathered threshold above external paving is not measured in the same way as a low threshold aiming for flush internal-external transition. If your patio level is still to be built, the finished paving height is part of the survey. If you guess it, the threshold can end up too high, too low or poorly weathered.
Some projects also require a cill. Others do not. A projecting cill can alter the way the frame is sized and supported, especially where the door sits proud of the outer skin or interfaces with render, cladding or cavity details.
New openings need more than brick-to-brick sizes
On extensions and self-builds, the drawing might show a clean 3000mm or 4000mm opening, but real construction often lands slightly off that figure. Before manufacture, check the built opening rather than ordering from the architectural drawing alone.
It is also worth confirming the finished floor build-up, external ground level, cavity closure details and any steel deflection allowance. Large aluminium systems are precise products. They perform best when the surrounding structure is equally well considered.
Panel configuration affects what is practical
When measuring bifold doors, size is only one part of the decision. The width you have available influences the number of panels, panel widths, traffic door options and how the stack will sit when open.
For example, a three-panel arrangement behaves differently from a four-panel or five-panel layout. Wider individual leaves can look impressive, but there are practical limits depending on system design, glass weight and ease of use. Slimmer systems may offer different panel size ranges compared with more traditional platforms such as Smarts Visofold 6000 or higher-spec options like ASFD90.Hi Bifold Doors.
This is one reason measured openings should be reviewed alongside the desired configuration, not in isolation. An opening may physically take a bifold, but a sliding door could be the better answer if uninterrupted glass and cleaner sightlines matter more than full aperture access.
Measuring existing openings for quote purposes
If you want a fast quote, measure the width in three places, the height in three places, note the smallest figures and mention whether it is a replacement opening or new build aperture. Add as much context as you can about threshold preference, outer leaf finish and whether the old frame remains in place.
A supplier can usually give a sensible budget figure from that. What they should not do is treat those rough dimensions as a final order size without checking site details.
For supply-and-install projects, a final survey should confirm everything before manufacture. For supply-only, you carry more responsibility for accuracy, so it is worth being cautious if the opening is uneven, still under construction or dependent on future floor and paving levels.
Common measuring mistakes
The most common mistake is measuring the existing frame instead of the structural opening. After that, it is usually forgetting floor finishes, ignoring out-of-level thresholds or assuming every bifold system works to the same deductions and panel limits.
Another regular issue is failing to record whether the doors open in or out, and which end or centre section is intended as the main access leaf. That may not change the opening size itself, but it affects how the configuration is specified and whether the final setup suits the room.
On renovation projects, be wary of plaster that flares inwards, tiled floors that will be removed, or external surfaces that are due to rise. All three can make a perfectly reasonable measurement look accurate when it is not.
When to measure it yourself and when to hand it over
If you are in the early stages and comparing products, measuring it yourself is absolutely sensible. It helps you narrow down options, compare systems and understand whether a particular arrangement is realistic for the space.
If the product is about to be ordered, that is the point to slow down. On straightforward apertures, experienced trade buyers may be comfortable taking final dimensions. On domestic projects, especially where low thresholds, renovated openings or premium systems are involved, a professional survey is usually the safer route. That is particularly true if you want the doors to sit neatly with flooring, plaster lines and external finishes rather than simply fit the hole.
At Bifolding Door Factory, that distinction matters because a good bifold door is not just a product – it is a measured and installed system. The smartest way to approach it is to use your own dimensions to get the project moving, then let final sizing be driven by the opening as built, not by assumption.
Take the tape measure seriously, but do not let guesswork make the final decision. The best bifold installations start with accurate dimensions and finish with doors that look right, operate smoothly and suit the opening they were designed for.