How to Measure for Bifold Doors: A Step-by-Step Guide

Accurate measurements are essential when ordering made-to-measure bifold doors. Unlike an off-the-shelf door that can sometimes be adjusted to fit, an aluminium bifold system is manufactured specifically for the dimensions supplied.

Even a relatively small measuring error can result in a frame that is too large for the opening, excessive gaps around the installation or problems with the threshold and finished floor levels.

This guide explains how to measure an opening for an initial quotation and identify the details that must be resolved before the doors enter production.

Important: measurements taken by a homeowner are useful for quotations and early planning, but the final manufacturing dimensions should be checked by a competent installer, surveyor or suitably experienced construction professional.

What Are You Actually Measuring?

Before reaching for the tape measure, it is important to understand the difference between the structural opening and the finished opening.

Structural Opening

The structural opening is the gap formed within the brickwork, blockwork, timber frame or structural system before all finishes have been applied.

It may still need:

• Plaster or render

• Internal lining boards

• External cladding

• Floor screed

• Tiles, timber or another finished floor covering

• A threshold tray or drainage channel

Finished Opening

The finished opening is the final available space into which the complete bifold door frame will be installed after the relevant wall and floor build-ups have been allowed for.

This is generally the more useful measurement when confirming the final door size.

If your extension is still under construction, the finished opening may not yet exist. Your builder, architect or installer will need to calculate it from the drawings and confirmed floor, wall and threshold details.

What Equipment Will You Need?

For an initial measurement, you will need:

• A good-quality steel tape measure

• A spirit level or laser level

• A pencil and paper or digital notes

• A second person for wider openings

• A camera or phone for photographs

A fabric measuring tape is not suitable because it can stretch, bend or sag across a wide opening.

Record every measurement in millimetres. Do not mix millimetres, centimetres and metres within the same set of notes.

Step 1: Measure the Width in Three Places

Measure horizontally between the finished sides of the opening at three points:

1. Across the top

2. Across the middle

3. Across the bottom

Record all three measurements rather than writing down only one figure.

For example:

• Top width: 3,012mm

• Middle width: 3,008mm

• Bottom width: 3,004mm

If the measurements differ, the opening is not perfectly parallel. For quotation purposes, the smallest measurement is usually the safest figure to provide because the door frame must fit within the narrowest part of the aperture.

However, do not automatically subtract a fitting allowance yourself. The required deduction depends on the product, installation method and condition of the opening. The final frame size should be confirmed by the supplier or installer.

Step 2: Measure the Height in Three Places

Measure vertically from the intended finished floor or threshold level to the underside of the structural head.

Take measurements at:

1. The left side

2. The centre

3. The right side

Again, record all three measurements and highlight the smallest.

For example:

• Left height: 2,112mm

• Centre height: 2,109mm

• Right height: 2,105mm

A small variation may be manageable through the installation allowance. A significant difference could indicate that the floor, lintel or structural beam is not level and may need to be corrected before the doors are ordered.

Step 3: Check Whether the Opening Is Square

Width and height measurements alone do not confirm that an opening is square.

Measure diagonally:

• From the top-left corner to the bottom-right corner

• From the top-right corner to the bottom-left corner

If the two diagonal measurements are the same, the opening is likely to be square. If they differ, the frame opening is out of square.

A minor difference may be accommodated during installation, depending on the available tolerance. A substantial difference should be corrected before the door is manufactured.

Bifold doors rely on several panels remaining accurately aligned. Installing the frame into a badly distorted opening can affect:

• How smoothly the panels move

• Whether the locks engage correctly

• The contact between the panels and weather seals

• The appearance of the gaps around the frame

Step 4: Check the Sides Are Plumb

Use a spirit level or laser level to check whether both sides of the opening are vertically straight.

An opening can have similar width measurements at the top and bottom while still leaning to one side. This creates a parallelogram-shaped opening rather than a square or rectangle.

Check both the internal and external faces where possible, particularly on older properties where brickwork, render or plaster may be uneven.

Do not assume the existing door frame is straight. If you are replacing an older door, measure the actual prepared opening rather than copying the dimensions of the old frame.

Step 5: Confirm the Finished Floor Level

One of the most common measuring mistakes is taking the height from an unfinished floor.

If the room still needs screed, insulation, underfloor heating, adhesive, tiles or timber flooring, the final floor level could be considerably higher than it appears during the early construction stage.

Before confirming the door height, establish:

• The thickness of any remaining floor insulation

• The depth of the screed

• The thickness of the finished floor covering

• Whether underfloor heating is being installed

• The required internal threshold height

If you measure from the concrete slab and later add 70mm of floor construction, a door ordered to the original height will no longer fit the finished opening correctly.

Step 6: Consider the External Ground Level

The external patio, decking or paving level must also be considered, particularly if you want a low or visually flush threshold.

The external surface should normally be designed so that water drains away from the property rather than towards the doors.

A completely level transition between inside and outside may look seamless, but it must be supported by a suitable drainage and waterproofing design.

Check:

• The proposed external paving height

• The direction and gradient of the paving

• Whether a drainage channel is required

• The damp-proof course and waterproofing details

• The exposure of the doors to wind-driven rain

Do not raise external paving against the threshold after installation without considering drainage and the damp-proofing of the building.

Step 7: Check the Structural Support Above

The bifold door frame must not support the weight of the wall, roof or upper floor above the opening.

A suitable lintel, steel beam or engineered structural member should carry those loads independently.

The structural head also needs to resist excessive deflection. A beam that moves downward under load can press against the top of the door frame and cause:

• Stiff or uneven operation

• Panel misalignment

• Difficulty engaging the locks

• Damage to the frame or seals

If the opening is new, unusually wide or supporting significant loads, its structural design should be confirmed by a qualified structural engineer.

A door survey confirms the dimensions of an opening. It does not replace the structural design of the building above it.

Step 8: Measure the Reveal Depth

The reveal is the section of wall surrounding the door frame. Its depth affects where the frame can be positioned and how the installation will appear internally and externally.

Measure the wall depth at several points and note any changes caused by:

• External insulation or cladding

• Uneven brickwork

• Internal plasterboard linings

• Cavity closers

• Existing frames or subframes

The chosen frame position can affect weather protection, internal plaster details, external trims and the amount of space available for the panels to fold.

Step 9: Check for Obstructions

Measure and photograph anything that could interfere with the door frame or folded panels.

Common obstructions include:

• Drainpipes

• External lights

• Steps and raised patios

• Kitchen units or islands

• Radiators

• Electrical sockets

• Curtain tracks and blinds

• Low ceilings or structural returns

This information will help determine whether the doors should open inward or outward and whether the panels should stack to the left, right or both sides.

Step 10: Record the Opening Direction

Measurements alone are not enough to produce a complete bifold door specification.

You will also need to decide:

• How many panels the door should have

• Whether the doors open inward or outward

• Whether the panels stack to the left, right or both sides

• Which panel acts as the everyday traffic door

• Which side the traffic door is hinged on

When describing the opening direction, state clearly whether you are viewing the doors from inside or outside. This avoids confusion over left and right.

A simple sketch showing the room, garden and desired panel movement is often more useful than a written description alone.

Should You Deduct a Fitting Allowance?

Do not automatically subtract 5mm, 10mm or any other amount from your opening measurements unless the manufacturer, supplier or installer has told you to do so.

The required allowance can depend on:

• The bifold door system

• The width and condition of the opening

• The type of fixing being used

• Whether the opening is perfectly square

• The installation materials required around the frame

Provide the smallest recorded opening width and height and allow the person responsible for the final survey to calculate the manufacturing dimensions.

Measuring a New Extension

For a new extension, the opening may be measured at several different stages of construction.

Early measurements are useful for budgeting, but they should not normally be treated as final manufacturing dimensions while:

• The structural opening is incomplete

• The beam or lintel has not been installed

• The floor build-up has not been confirmed

• The external paving level remains undecided

• Wall finishes could still change the opening size

Use the architectural or structural drawings to obtain an initial quotation, then verify the physical opening once it has been formed and the relevant levels are known.

Measuring for Replacement Bifold Doors

If you are replacing an existing door, do not assume its frame size is the same as the available structural opening.

The old frame may conceal:

• Packers and large perimeter gaps

• A timber subframe

• Damaged or uneven brickwork

• Old render or plaster finishes

• A threshold that will be removed or altered

The most reliable final measurements are often available only after the existing frame has been removed. Where that is impractical, an experienced surveyor can assess the likely opening using visible frame dimensions and construction details.

What Information Should You Provide for a Quote?

For an initial SmartaView quotation, provide:

• The smallest measured width in millimetres

• The smallest measured height in millimetres

• All three width and height measurements if they differ

• The two diagonal measurements

• Whether the dimensions are structural or finished

• The proposed finished internal floor level

• The proposed external ground level

• Your preferred number of panels

• The opening and stacking direction

• The preferred traffic door position

• The threshold type

• Clear photographs of the opening from inside and outside

Architectural drawings can also be supplied where the opening has not yet been built.

Final Measurements Before Manufacturing

An initial quotation can usually be produced from homeowner, builder or architectural measurements. However, final manufacturing dimensions carry a much greater level of responsibility.

Before approving the doors for manufacture, confirm in writing:

• The final frame width and height

• The fitting allowance used

• The threshold and finished floor relationship

• The opening direction viewed from a stated side

• The panel arrangement and traffic door

• The frame colour inside and outside

• The glazing and hardware specification

Once a bespoke aluminium bifold door has entered production, changing these details may not be possible.

Configure Your Bifold Doors with SmartaView

Browse SmartaView’s aluminium bifold doors and enter your approximate opening dimensions to compare available systems, panel layouts, colours and glazing options.

Use your measurements for initial planning and pricing, then have the final manufacturing dimensions verified before approving the order. Measuring carefully at the beginning is far easier and less expensive than resolving a made-to-measure door that does not fit.

Further reading

• How Wide Can Bifold Doors Be? Sizes, Panels & What's Possible
• How Much Do Bifold Doors Cost in the UK? (2026 Guide)
• Planning Permission for Bifold Doors: Do You Need It?

Ready to measure up? Follow SmartaView's how-to-measure guide, then explore our aluminium bifold doors and see your price online. Any questions? Speak to our team.