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How to Measure Openings for Folding Doors (Bi-Fold Systems): A Complete Step-by-Step Guide
How to Measure Openings for Folding Doors (Bi-Fold Systems): A Complete Step-by-Step Guide by KozijnenPro
Folding (or bi-fold) doors open living spaces completely, connecting interiors to gardens or terraces with uninterrupted views. Their success, however, depends on one thing — accurate measurements. Because folding systems consist of multiple hinged panels moving along top and bottom tracks, even a small inaccuracy can lead to misalignment or poor sealing.
This guide by KozijnenPro explains how to measure openings for both new construction and replacement folding-door systems using metric units (mm) with imperial equivalents where helpful. Follow these professional steps to ensure your new aluminium (aluminum) folding doors fit precisely, fold smoothly, and seal perfectly.
Understanding Key Terms
- Rough opening: The unfinished structural opening in a wall, measured stud-to-stud (width) and subfloor to header (height).
- Finished opening: The visible frame opening after plaster, drywall, or cladding is complete.
- Head: The top frame component that carries some load of the folding system and guide track.
- Threshold (sill): The bottom track or frame element that the panels slide and fold along.
- Stack area: The space where panels rest when folded open.
- Jambs: The vertical sides of the opening; they must be plumb to prevent hinge strain.
- Tolerance gap: The 10–15 mm (3/8–5/8 in) clearance around the frame allowing for insulation, alignment, and expansion.
Needed tools on a table.
Tools You’ll Need
- Tape measure (5 m / 16 ft) or laser distance measurer
- Spirit or laser level
- Pencil or masking tape
- Notebook or smartphone for recording
- Optional: a helper for wide openings
Step 1 — Identify Door System and Scenario
Confirm the following before you begin:
- Number of panels: total leaves and opening direction (inward or outward).
- Installation type: new build (rough opening) or replacement (finished frame).
Width measurements of a folding door.
Step 2 — Measure the Width
Measure between the left and right jambs (or reveals) at top, middle, and bottom. Write down all three and use the smallest measurement. For very wide openings (4 m+), it can help to take an additional mid-span measurement across the lintel. Folding systems require uniform width along the entire length to ensure even panel spacing.
Height measurements of a folding door.
Step 3 — Measure the Height
Measure vertically from the subfloor or structural base to the underside of the header at left, center, and right — then use the smallest result.
If your final flooring (tile, timber, etc.) is not yet installed, record its thickness and subtract it from your measurement. This prevents panels from rubbing against the finished floor later. Example: rough height 2100 mm; tile buildup 15 mm → order height 2085 mm.
Diagonal measurements of a folding door.
Step 4 — Check Squareness
Measure diagonals corner-to-corner (top-left to bottom-right and top-right to bottom-left). If the two values differ by more than 6 mm (1/4 in), the opening is out of square. Slight skew can be shimmed, but significant misalignment can cause panel binding or gaps in the seals. For multi-panel folding doors, squareness is crucial — each hinge and roller alignment depends on it.
Step 5 — Verify Level and Plumb
Place a level on the threshold to check for slope or warping, then hold it against both side jambs to confirm they’re plumb. Record any deviation. If the sill slopes more than about 5 mm end-to-end, correct it before installation. Folding systems require near perfect openings, any sag or warping will cause the doors to drift or not latch properly.
Step 6 — Measure Stack Space and Clearance
When folded, bi-fold panels occupy a “stack” area on one or both sides of the opening. Measure how much clear wall space you have on the stacking side(s). Typical folded stack depth is 90–120 mm per panel, depending on profile thickness; a 4-panel system stacked to one side needs roughly 400–480 mm of wall clearance.
Also check for obstacles — radiators, switches, sockets, or furniture — that might block panels when open. Mark the swing path on the floor with tape to visualize how far the panels project.
Step 7 — Record Measurements and Apply Tolerance
Subtract approximately 10–15 mm (3/8–5/8 in) from both width and height to allow for shimming, insulation, and expansion. Example: opening 4000 × 2100 mm → order size about 3985 × 2085 mm.
Label every dimension clearly: note whether it’s a rough opening or finished opening, panel count and stacking side, threshold type, and inward/outward opening. Take photos of each opening and save them with your notes.
Ready to visualize your folding system? Enter your measurements in the KozijnenPro Configurator — explore panel counts, thresholds, colors, and pricing instantly.
Step 8 — Check Structural Requirements
Folding doors exert downward and lateral loads on both head and sill. Before ordering, verify that your header beam is strong enough to support the system’s weight without deflection. For top-hung designs, a steel or reinforced lintel is often recommended.
Also confirm the substrate below the sill is solid, level, and moisture-protected. A weak or uneven base can twist the track and affect rolling performance.
Step 9 — Document Everything
- Double-check all recorded values.
- Confirm swing direction (inward or outward).
- Note left- or right-hand stacking.
- Include your floor-finish thickness and threshold choice.
- Photograph each opening and attach images to your notes.
Common Mistakes to Avoid
- Measuring only once instead of three times across width and height.
- Forgetting to check diagonal squareness.
- Ignoring stack space or nearby obstructions.
- Measuring from plaster or trim rather than structural reveals.
- Failing to account for final floor height or drainage slope.
- Confusing frame size with rough opening size.
- Not labeling swing direction, stacking side, or threshold type.
Regional Notes
In Europe, folding doors are specified in millimetres with 10–15 mm tolerance all around. In North America, doors are often described by nominal inch sizes (e.g., “144 × 82”); rough openings are typically larger than the frame by about 1/2 inch per side. Always clarify whether your provided dimensions represent the structural opening or the external frame size when ordering.
Conclusion
Measuring folding door openings accurately guarantees smooth operation, perfect weather sealing, and clean stacking every time. By following KozijnenPro’s method — three measurements per axis, squareness checks, verified thresholds, and recorded tolerances — you’ll have all the data needed for a flawless installation.
At KozijnenPro, each aluminium (aluminum) folding system is crafted precisely to your dimensions and engineered for lasting performance, security, and thermal efficiency.
Start today — visit www.kozijnenpro.nl to enter your measurements and configure your folding doors exactly as you envision them.
FAQ: Measuring Folding (Bi-Fold) Door Openings
Always measure from the inside unless requested otherwise. Interior measurements are more accurate because interior walls are straighter and less exposed to render, plaster, or brick variations. Measuring internally also gives access to the subfloor and true structural reveals.
However, if your project involves masonry reveals or external cladding, it’s smart to measure both inside and outside and label them clearly. KozijnenPro’s technical team can then confirm which reference to use during production. Providing both ensures your new aluminium (aluminum) folding door fits precisely and prevents costly adjustment on-site.
Allow a tolerance gap of 10–15 mm (⅜–⅝ in) on all sides between your measured opening and the manufactured frame size.
This tolerance gives room for thermal expansion, shimming, and insulation foam during installation. Without this clearance, the frame could be forced into place, which might distort its geometry and affect folding operation.
Always note whether your dimensions are for the rough opening (structural size) or the desired frame (manufacturing size). KozijnenPro doors are always produced to fit the tighter measurement, so labeling correctly ensures the factory calculates the frame precisely.
Use a spirit or laser level to check the sill’s levelness. If one side is higher, measure from the highest point to the top of the opening. That value guarantees enough clearance for your new door.
Installers can raise frames slightly with packers but can’t lower them if they’re too tall. If there’s more than a 5 mm difference, use leveling compound or request KozijnenPro’s adjustable sill option.
For top-hung systems, a perfectly level sill prevents panel drift; for bottom-rolling systems, it ensures panels fold smoothly without friction or misalignment.
Measure from the structural subfloor, not the finished floor surface. Then subtract the thickness of your flooring materials — such as tiles, wood, or laminate — so that your door’s threshold will align perfectly once flooring is installed.
For example, if your tile buildup adds 18 mm, deduct that from your total height measurement.
Check by measuring the two diagonals from corner to corner. If the difference is less than 6 mm (¼ in), your opening is within acceptable tolerance.
If it’s greater, note the difference — you may need minor structural correction or additional shimming during installation.
A perfectly square opening ensures all folding panels align smoothly, reducing hinge tension and improving longevity.
Even a small misalignment can cause panels to catch or misfold, so this step is critical to long-term performance and effortless operation.
Each bi-fold panel typically requires 90–120 mm of stack depth when folded. Multiply that by the number of panels stacking to one side.
For example, a 4-panel system stacked to the left needs roughly 400–480 mm of unobstructed wall space.
Also check the floor and wall area near the stack side — ensure no radiators, switches, sockets, or furniture will block panel movement.
If your opening is tight, KozijnenPro can advise on split-stack configurations (panels dividing left and right) to optimize space without sacrificing full-width opening capability.
Yes. Folding systems rely on precise load distribution across multiple hinges and pivot points.
If the beam sags even slightly, panels may rub or fail to align. Our systems depend on a solid, level threshold — any unevenness can twist the rollers.
Before ordering, confirm your header and subfloor can handle the load. KozijnenPro provides technical specifications for each system to help verify structural suitability.
If you’re replacing an existing frame, measure inside the current frame, not the old sashes or trims.
Take width and height measurements at three points (top, middle, bottom for width; left, center, right for height) and record the smallest values.
If interior trims are removable, take them off for a more accurate reading.
Also photograph your existing threshold and hinge setup — KozijnenPro can use these visuals to ensure your replacement frame fits perfectly without altering wall finishes.
You can confidently measure your openings using this guide, a good tape measure, and a spirit level.
However, for large spans (over 4 meters) or complex configurations (multiple panels or corner-folding systems), it’s best to have your measurements verified by a professional installer.
KozijnenPro offers professional measurement verification for peace of mind, ensuring that factory production matches your real-world conditions exactly.
This small step can prevent delays and guarantee your installation goes smoothly from start to finish.
The most frequent errors include:
Measuring from decorative surfaces (trim, plaster, or tile) instead of the structural reveal.
Forgetting to take multiple measurements across width and height.
Ignoring diagonal squareness checks.
Not accounting for flooring buildup or drainage slope.
Providing the frame size instead of the rough opening size.
Overlooking stack space or obstructions.
Failing to record swing direction or stacking side.
Avoid these and you’ll ensure your KozijnenPro folding doors are built to fit perfectly and perform flawlessly for years to come.