The short answer
You measure the structural opening, the brick-to-brick gap in the wall, and the bifold is made slightly smaller to leave fitting tolerances of roughly 10 to 20 millimetres around the frame. So a 3 metre structural opening typically takes a bifold frame of around 2,960 to 2,980 millimetres wide. The opening must be square, level and supported by a correctly sized lintel above. Measure width and height at several points and use the smallest reading, since older openings are rarely perfectly square. A surveyor confirms the final dimensions before manufacture, because a made-to-measure door cannot be trimmed on site like a timber one.
Bifolds are sized to the structural opening minus a fitting tolerance, so accurate measurement of the brickwork is what matters, not the old door.
Quick reference
- MeasureStructural opening (brick to brick)
- Fitting tolerance~10–20mm each side
- Take readingsTop, middle, bottom
- UseThe smallest reading
How opening size translates to door size
The bifold frame is made smaller than the structural opening so it can be fitted, packed level and sealed against the weather. The table shows indicative relationships between the brick-to-brick opening and the frame width; the surveyor sets the exact figure for the chosen system, since tolerances vary slightly between manufacturers.
As a rule, the wider the opening the more panels the door uses, keeping each leaf within sensible width limits. The height follows the same principle: the frame is made a little shorter than the opening to allow for fitting and the threshold detail. Always treat these figures as a guide and confirm with a survey before ordering.
| Structural opening | Typical frame width | Common panels |
|---|---|---|
| 1.8m | ~1,760–1,780mm | 2-3 pane |
| 2.4m | ~2,360–2,380mm | 3-4 pane |
| 3.0m | ~2,960–2,980mm | 4 pane |
| 4.0m | ~3,960–3,980mm | 5 pane |
Indicative figures for guidance; always confirm with a survey before ordering.
How to measure an opening
Accurate measurement is essential because a made-to-measure bifold cannot be cut down on site. A mistake on a made-to-measure order usually means a costly remake, so it pays to measure carefully and let the surveyor confirm before anything is manufactured. Good practice is to take several readings and always work to the smallest.
- Measure width at the top, middle and bottom of the opening and use the smallest figure.
- Measure height at the left, centre and right, again using the smallest reading.
- Check for square by comparing the two diagonals; equal diagonals mean a square opening.
- Note the threshold level and floor finish, which affect the frame height and the threshold type.
For a new opening in an extension, the builder forms it to suit the chosen door, but the principle is the same: square, level and correctly supported above.
Structural support above the opening
The wall above a bifold opening must be carried by a lintel or steel beam sized for the span and the load it carries. For a like-for-like replacement into an existing opening, the lintel is usually already there and only needs checking. For a new or widened opening, a structural engineer specifies the beam and building control signs it off as part of the work.
The opening must be fully formed, with the lintel in place and the reveals and threshold prepared, before the bifold is fitted. Getting the structural opening right is as important as the door dimensions themselves, since a sagging lintel will distort the frame and stop the doors folding cleanly. On a wider opening this matters even more, because the beam carries more weight and any movement shows up quickly in how the panels meet and seal.
Planning the opening around the door
When you are creating or widening an opening, plan the door at the same time so the two are coordinated. The panel split, threshold type and floor levels all interact: a flush threshold for level access needs the internal floor set to suit, so the screed and the door specification have to be agreed together rather than left until fitting. Deciding the panel count early also tells the builder how wide the structural opening should be.
Lead time is the other consideration. A made-to-measure bifold takes several weeks to manufacture, so the opening size needs confirming early enough for the door to arrive when the structure is ready. Coordinating the survey, the structural opening and the floor finish avoids the common problems of a door that does not sit level or a threshold that does not match the finished floor, both of which are awkward and costly to fix after the event.
One more point worth stressing is that the survey is not a formality you can skip to save time or money. A made-to-measure bifold is manufactured to the figures the survey confirms, and once the frame is built it cannot be trimmed on site the way a timber door can be planed to fit. That makes the survey the single most important step in getting the opening size right, because it is the moment any out-of-square, unlevel or undersized issue is caught before anything is ordered. A good surveyor checks the diagonals, confirms the lintel is sound, notes the floor finish and threshold, and records the smallest width and height readings. Treating those numbers as the definitive specification, rather than a rough guide, is what avoids the expensive remake that follows a mismeasured order, and it is the main reason most homeowners prefer a supply-and-fit arrangement where the installer carries the measurement risk.
Frequently asked questions
Should I measure the opening or the old door?
Measure the structural opening, the brick-to-brick gap, not the old door. The new bifold is made to suit the opening minus fitting tolerances, so the old door's size is not the right reference.
How much smaller is the door than the opening?
The frame is typically made around 10 to 20 millimetres smaller on each side, leaving room to fit, pack and seal it. The surveyor sets the exact tolerance for the chosen system.
Can I make my opening bigger for a wider bifold?
Yes, but enlarging the opening is structural work needing a new lintel or steel beam, engineer's calculations and building control approval. It adds cost but allows a wider door.
Does the floor level affect the opening size?
Yes. The threshold type and the internal floor finish set the frame height and how the door meets the floor. A flush threshold needs the floor level coordinated with the door, so agree it before the screed is laid.
Who should measure for a bifold?
With supply-and-fit, the installer surveys and carries the risk of any sizing error. If you buy supply-only, you measure, so accuracy matters even more because a mismeasured made-to-measure door means a costly remake.
Sources & further reading
- HomeOwners Alliance — bifold doors guide
- gov.uk — Approved Document A (structure)
- Checkatrade — bifold door cost guide
Figures on this page are typical UK ranges drawn from published sources and depend on your specific opening and material. They are guidance, not a quotation.