A Guide to Measuring for Bifold Doors (with Trade Checklist)
Measuring bifold doors is not complicated. But it must be precise.
If you get it wrong by even a few millimetres, the doors won’t sit square. They may bind on the track. The glass may need excessive packing. Worst case? You are reordering a full aluminium set.
At 6 Day Doors, we manufacture high-performance aluminium bifold doors for the trade with fast lead times. We speak to installers every day who need clarity before ordering. This guide explains how to measure bifold doors properly, what tolerances to allow, and exactly what information we need to produce an accurate trade quote.
Whether you are replacing patio doors or creating a new structural opening, this checklist will save you time and hassle.
Step 1: Confirm the Structural Opening
Before measuring bifold doors, confirm what you are actually measuring.
You are measuring the structural opening — not the plastered finish, not the trim, not the old frame.
The structural opening is the raw brick-to-brick or blockwork gap where the new frame will sit.
If the old doors are still in place:
- Remove internal architrave if possible
- Check behind plaster lines
- Confirm true masonry dimensions
If it’s a new build or extension:
- Measure directly from the finished structural aperture
- Confirm lintel is installed and bearing correctly
Always measure in millimetres.
Step 2: Measure Width (Three Points)
When measuring bifold doors, never assume the opening is straight.
Measure the width at:
- The top
- The middle
- The bottom
Write down all three sizes.
Use the smallest width when calculating your order size.
Walls bow. Brickwork moves. Openings are rarely perfect.
Step 3: Measure Height (Three Points)
Repeat the same process for height.
Measure from:
- The underside of the lintel
- Down to the structural floor level
Take measurements:
- Left
- Centre
- Right
Again, use the smallest height.
Step 4: Check the Diagonals
This is the step many installers skip.
Measure from:
- Top left to bottom right
- Top right to bottom left
If the diagonals are equal, the opening is square.
If they differ significantly, the opening is out of square.
A small difference can be corrected with packers.
A large difference may require building work before installation.
Step 5: Apply Correct Tolerances
This is critical when measuring bifold doors.
Never order the frame the exact same size as the opening.
Standard trade tolerance:
In general, order the bifold door frame 10mm narrower and 10mm shorter than the smallest structural opening measurement.
For example:
- Structural opening width: 3000mm
- Order width: 2990mm
- Structural opening height: 2100mm
- Order height: 2090mm
This 10mm tolerance allows:
- Frame adjustment
- Packing and plumbing
- Expansion room
- Easier installation
Without tolerance, you risk damaging the frame trying to force it into place.
If in doubt, visit one of our showrooms or speak to our team of experts on 020 3835 2007.
Structural Lintel Notes (Often Overlooked)
Bifold doors are heavier than standard French or patio doors.
Large multi-panel aluminium sets place significant load on the head.
Before ordering, confirm:
- The lintel is suitable for the span
- There is no visible sag
- There are no cracks in masonry above
- Bearing is adequate on both sides
If installing 4, 5 or 6-panel bifold doors, the structural support must be correct. Aluminium frames are not load-bearing.
If in doubt, confirm with a structural engineer.
Step 6: Consider Floor Levels & Threshold Choices
Threshold choice directly affects your height measurement.
Ask yourself:
- Is the floor finished?
- Will tiles be added later?
- Is carpet being installed?
- Will the patio slab be raised?
A common mistake when measuring bifold doors is forgetting final floor build-up.
Common Threshold Options
- Standard threshold – fully rebated, more weather resistant
- Low threshold – improved accessibility
- Flush threshold – internal and external levels aligned
If the external patio is not yet laid, allow for its final height.
Remember: outer frame sizes include the bottom track height. Always confirm this before ordering.
Step 7: Decide Opening Configuration (Viewed from Outside)
This causes more confusion than measurement errors.
All bifold doors are specified viewed from outside.
When ordering, confirm:
- Do the doors fold inwards or outwards?
- Do they stack left or right?
- Do they split from the centre?
- Is there a traffic door?
A traffic door is the everyday access leaf. It opens like a standard hinged door without folding the full set.
For trade projects, traffic doors are often essential.
You can explore configuration options on our Aluminium Bifold Doors page before confirming your layout.
Trade Checklist: Before Requesting a Quote
Here is the exact information we need at 6 Day Doors to price accurately:
- Structural opening width (smallest measurement)
- Structural opening height (smallest measurement)
- Desired tolerance confirmation (standard 10mm unless specified)
- Opening configuration (viewed from outside)
- Inward or outward opening
- Traffic door required?
- Threshold type
- Frame colour
- Glazing specification (double or triple)
- Site postcode (for delivery logistics)
Providing all of this avoids delays.
You can submit details directly via our Get a Quote – Bifold Doors page.
Quick Example: Measuring a 4 Panel Set
Let’s say you are installing a 4 panel aluminium bifold door.
Opening width measurements:
- Top: 3604mm
- Middle: 3600mm
- Bottom: 3598mm
Opening height measurements:
- Left: 2098mm
- Centre: 2100mm
- Right: 2097mm
Smallest width = 3598mm
Smallest height = 2097mm
Order size:
- Width: 3588mm
- Height: 2087mm
That 10mm tolerance gives you safe adjustment room.
Glazing & Installation Notes
Installing bifold doors is similar to other glazed products, but there are key differences.
Glazing
- Remove glazing beads carefully
- Insert bridge packers
- Toe and heel each unit from the bottom upwards
- Refit beads in original positions (width beads first)
- Insert wedge gaskets between glass and bead
Correct glazing prevents sash drop and operational issues.
If you need further details, our Glazing and Technical Specifications pages provide additional trade guidance.
Finishing & Sealing
Once installed:
- Use low modulus neutral cure silicone around the perimeter
- Ensure drainage channels are clear
- Test master door operation first
- Then check all sliding panels
- Confirm smooth travel along the stainless-steel track
Never block drainage slots.
Common Measuring Mistakes
Here are the most frequent errors we see:
- Ordering frame to exact opening size: No tolerance means no adjustment room.
- Forgetting finished floor height: Especially in extensions.
- Specifying opening direction incorrectly: Always view from outside.
- Ignoring structural movement or sagging lintels: Heavy aluminium sets require solid support.
- Guessing measurements: Always measure twice. Sometimes three times.
If you are comparing systems, you may also find our article Common Myths About Bifold Doors Busted helpful.
Care After Installation
Once fitted:
- Keep the stainless-steel track clean
- Remove dust and debris
- Wipe down with a sponge cloth
- Avoid harsh chemical cleaners
Proper care ensures smooth performance long term.
Final Trade Advice
Measuring bifold doors is straightforward when you follow a method:
- Measure structural opening
- Use smallest dimensions
- Check diagonals
- Allow 10mm tolerance
- Confirm configuration from outside
- Account for thresholds and floor build-up
Simple steps. But vital ones.
At 6 Day Doors, we manufacture trade aluminium bifold doors with fast lead times and consistent quality. If you provide clear structural opening measurements and configuration details, we can turn around accurate quotes quickly and keep your project on schedule.
When you are ready, submit your measurements on our Get a Quote for Bifold Doors page and our team will handle the rest.
Measure carefully. Order confidently. Install smoothly.
