How to Measure for Bi Fold Doors: Opening Size & Checks

Accurate measuring is one of the most important parts of buying external bi fold doors. A complete aluminium doorset needs to fit inside a structural opening with enough tolerance for packing, fixing and weather sealing, while the frame itself still has to be installed square and level. This guide explains how to measure for bi fold doors in a UK home, extension or renovation.

Use the actual frame dimensions of products in our bi fold doors range rather than relying on a nominal description such as 6ft, 8ft or 10ft. If a new opening is being built, it is usually more efficient to choose the doorset first and form the opening to the manufacturer's documented requirements.

What Are You Measuring for Bi Fold Doors?

The key measurement is the structural aperture that will receive the complete frame. This is different from the visible glass size, individual panel width or the inside dimension of an old patio-door frame. If an existing doorset is still fitted, trims and finishes can hide the true opening.

Keep three sets of figures separate in your notes: the structural opening, the new doorset's overall frame dimensions and the fitting tolerance required around that frame.

Tools for Measuring a Bi Fold Opening

• A long, reliable tape measure marked in millimetres.
• A long spirit level or laser level.
• A straightedge.
• A pencil and notepad or digital survey sheet.
• A second person for wide openings.
• A camera or phone for recording the opening and threshold details.

For large openings, a laser can make repeated height checks easier, but critical order dimensions should still be verified carefully.

Step 1: Identify the Structural Opening

Before measuring, establish exactly where the structural sides, head and base are. Decorative trims, plasterboard, old frames, sealant and render can all make an opening look smaller or larger than the space available for a replacement frame.

If the old frame is staying until installation day, ask the installer whether any trim can be removed temporarily to confirm the hidden edges.

Step 2: Measure the Width at the Top

Measure horizontally from one structural side to the other near the top. Keep the tape straight and record the figure in millimetres. Do not round it to the nearest nominal feet size.

Write the reading down immediately rather than relying on memory.

Step 3: Measure the Centre Width

Repeat the width through the centre. If the figure differs from the top measurement, the sides may not be perfectly plumb or the masonry may be irregular.

A small amount of variation is common, but it needs to be understood before the frame is ordered.

Step 4: Measure the Bottom Width

Take a third measurement near the base. Do not assume the widest reading is the one that matters. The complete frame must fit through the tightest part of the aperture while still leaving the installation space required by the manufacturer.

On visibly uneven openings, take additional readings between these three points.

Step 5: Measure the Height on the Left

Measure from the agreed threshold or base datum to the underside of the structural head. Be precise about the bottom reference point, particularly if screed, flooring, cill components or a drainage system are still to be installed.

Record the current and planned finished levels separately where necessary.

Step 6: Measure the Centre and Right Heights

Repeat the height through the centre and on the right. A wide opening can have a noticeable change in height if the base or structural head is not level.

If one measurement is significantly different, investigate the cause rather than ordering immediately.

Step 7: Check the Diagonals

Measure from top-left to bottom-right and then top-right to bottom-left. Similar diagonal measurements indicate that the aperture is reasonably square. A large difference suggests the opening needs further assessment or correction.

The aluminium frame should not be twisted to follow an out-of-square wall opening. The doorset needs to be installed square for the panels, rollers and locks to operate correctly.

Step 8: Check Level and Plumb

Use a long level on the base, head and both sides. Record any slope or lean. This helps the builder or installer distinguish a sizing problem from an opening that simply needs remedial work.

If the base is not level, correct it before final installation rather than trying to compensate by twisting the frame.

Step 9: Confirm the Manufacturer Fitting Tolerance

Once the structural measurements are known, compare them with the exact frame size and the required fitting gap. Do not automatically deduct a fixed amount such as 10mm or 20mm because advice written for another system may not apply.

Ask the supplier or installer to confirm the correct aperture for the exact doorset before structural work is finalised.

Step 10: Set a Clear Threshold Datum

The threshold is where several trades can collide: the builder, flooring contractor, patio installer and door fitter may all work to different levels unless one datum is agreed. Mark the intended finished internal floor, frame base and external level on the drawings or site.

This is especially important when the design aims for a visually low transition between inside and outside.

Step 11: Account for Internal Floor Build-Up

Screed, underfloor heating systems, tiles, engineered flooring, adhesive and other finishes can raise the final floor considerably compared with the shell stage. Include the final build-up before the opening height is confirmed.

Do not trim or alter a complete aluminium doorset to compensate for a floor level that was measured incorrectly.

Step 12: Plan External Paving and Drainage

External paving should work with the threshold and drainage design rather than being installed independently after the doors. Raising paving too high can interfere with drainage or weather protection.

Record the planned external level and any drainage channel position during the survey.

Step 13: Confirm Inward or Outward Opening

Measuring the frame is only part of choosing the right configuration. Confirm whether the panels open inward or outward and check that the stack will have enough space. An outward-opening system needs clear patio or garden space; an inward-opening design uses room space.

Measure nearby kitchen units, furniture, walls, steps and other obstructions if they could affect the panel travel.

Step 14: Confirm Left or Right Handing

Some systems are offered in left- and right-hand configurations. The terminology can relate to the traffic door or stack position, so use the manufacturer's diagram rather than a generic definition.

A correctly sized frame with the wrong handing can still be unsuitable for the room.

Measuring a New Extension Opening

If the wall has not yet been built, select the intended doorset early. For example, the LPD Doors AluVu 8ft Aluminium Bi Fold Door has an exact frame dimension stated on its product page. The builder can use that current product data together with the manufacturer's required tolerance when forming the opening.

Coordinate the structural beam or lintel, bearings, frame position and threshold before masonry and flooring are finalised.

Measuring an Existing Patio-Door Opening

When replacing sliding or French doors, the old frame can hide part of the aperture. The true structural opening may be larger than the visible glass or frame opening. Have the installer establish the structural sides and head before ordering.

If the new bifold set is wider than the existing opening, widening the wall is a structural project and may require a new beam or lintel.

Measuring After Structural Work

Where an opening has just been widened, take final measurements after the structural work is complete and the aperture has been checked. Do not rely only on design drawings if the actual site dimensions can now be verified.

This final survey should confirm width, height, diagonals, level, threshold and required fitting tolerance.

Create a Bi Fold Door Survey Sheet

For a clear record, note the project address or room, top/centre/bottom widths, left/centre/right heights, both diagonals, frame size being considered, threshold datum, finished internal floor, external paving level, handing, panel configuration and installer name.

Photographs of the opening and a sketch showing the viewing position for handing can reduce misunderstandings.

Compare Real Bi Fold Door Frame Sizes

JJ Building Materials currently lists AluVu options including the LPD Doors AluVu 6ft Aluminium Bi Fold Door, the 8ft system above and the LPD Doors AluVu 10ft Aluminium Bi Fold Door. Compare the exact metric dimensions rather than treating the nominal feet description as the construction size.

Common Bi Fold Measuring Mistakes

• Measuring the visible glass instead of the structural opening.
• Taking only one width and one height.
• Using the largest measurement instead of understanding the full aperture.
• Ignoring a sloping base or structural head.
• Using unfinished screed as the final floor datum.
• Deducting a generic fitting gap from another brand's advice.
• Forgetting cill, threshold or drainage components.
• Ordering before structural alterations are complete.
• Choosing the wrong handing despite correct dimensions.
• Letting external paving cover the intended weathering detail.

Bi Fold Door Measuring Checklist

• Structural opening clearly identified.
• Top, centre and bottom widths recorded.
• Left, centre and right heights recorded.
• Both diagonals checked.
• Base and head level checked.
• Sides checked for plumb.
• Finished internal floor level known.
• External paving and drainage level planned.
• Exact frame dimensions confirmed.
• Manufacturer fitting tolerance confirmed.
• Opening direction selected.
• Handing and panel stack checked.

Frequently Asked Questions

Do I measure brick to brick for bi fold doors?

The structural opening is the relevant starting point for a full frame replacement, but visible brick, render or trim may not expose the true aperture. Confirm the structural edges.

How much smaller should bifold doors be than the opening?

Use the fitting tolerance specified for the exact doorset. There is no universal deduction that should be applied to every brand.

Should I measure before or after plastering?

The important measurement is the finished structural aperture used for installation. Coordinate reveals and finishes so the required frame space is known before ordering.

Do floor tiles affect bifold measurements?

Yes. Finished floor build-up affects the threshold and available height, so planned tiles, adhesive and other layers should be included.

Can a bifold frame fit an out-of-square opening?

Limited variation can be packed out within the permitted tolerance, but the frame itself must be installed square. Significant defects should be corrected first.

Should my installer take the final measurements?

For a high-value external doorset, having the person responsible for fitting verify the final dimensions is a sensible way to reduce risk.

Choose Your Bi Fold Doors

Use your verified measurements to compare our aluminium bifold doors, then read our bi fold doors buying guide for panel configuration, glazing, threshold and maintenance considerations.