How to Measure Openings for Folding Doors (Bi-Fold Systems): A Complete Step-by-Step Guide by KozijnenPro

Folding 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 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) of your folding door .
• 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 doors 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 folding door 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 doors require near perfect openings, any sag or warping will cause the folding doors to drift or not latch properly.

Step 6 — Measure Stack Space and Clearance

When folded, folding door 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 folding door 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 folding door 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 door? 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

1. Measuring only once instead of three times across width and height.
2. Forgetting to check diagonal squareness.
3. Ignoring stack space or nearby obstructions.
4. Measuring from plaster or trim rather than structural reveals.
5. Failing to account for final floor height or drainage slope.
6. Confusing frame size with rough opening size.
7. 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.