How Do I Know What Size Bifold Door to Buy?

How do I know what size bifold door to buy comes down to one thing: measuring your opening accurately. Bifold doors are made to measure, not sold in fixed sizes. This guide walks through the exact measuring method, what to do if your opening isn’t square, and when to book a professional survey instead.

You’ve probably already decided a bifold door suits your opening. What you need now is a simple, accurate way to work out the size, not a vague “measure your wall” instruction.

How do I know what size bifold door to buy is one of the most common questions we hear, and it deserves a clear, step-by-step answer.

What Does Size Actually Mean for a Made-to-Measure Bifold Door?

Bifold doors don’t come in fixed standard sizes. Each one is built for your exact opening, which is why accurate measuring matters.

This is different from buying a normal door off a shelf. A bifold door is made to your exact opening, so the manufacturer needs your real measurements, not a rough guess.

Here’s what most first-time buyers get wrong. They assume there’s a “size” to pick from a list, like choosing a T-shirt size. There isn’t. Every bifold door is built around your own opening. Our aluminium bifold doors range covers a wide span of configurations, all built to measure.

How Do You Measure for Bifold Doors?

How to measure for bifold doors starts with checking height and width at several points. Then you confirm the opening is square using a diagonal check. Getting this method right avoids the most common ordering mistakes.

Work slowly rather than rushing a single reading. Taking your time here saves far more time later, since a mistake at this stage is expensive to fix once the door has been made. This is really the heart of how do I know what size bifold door to buy, since everything else in this guide builds on getting these first readings right.

What Tools Do You Need?

A decent tape measure is the minimum tool for measuring bifold doors accurately. A laser measure makes the job faster and cuts the chance of a reading slipping.

A second person to help hold the tape is genuinely useful on a wider opening, since a sagging tape gives a wrong reading. A notepad or your phone’s notes app helps too, since you’ll take several readings and it’s easy to mix numbers up if you try to remember them all.

How Do You Measure Height and Width Correctly?

Measure height and width at several points across the opening, roughly every 500 to 750mm. Don’t take one reading and assume it applies everywhere. Older properties in particular often have openings that aren’t perfectly even from one side to the other.

If your readings vary, use the smallest figure, not the largest. Ordering to the largest measurement risks a door that’s too big to fit. The smallest figure keeps you safely within the real opening.

How Do You Check Your Opening Is Square?

Check squareness by measuring both diagonals, corner to corner, and comparing the two figures. Equal diagonals mean a genuinely square opening. Unequal diagonals mean the opening is out of true, which is worth flagging before you order.

Take your measurements from the exterior blockwork or brickwork, not from an existing frame. That’s a key part of how to measure for bifold doors correctly, since an old frame may already be slightly warped, which would throw off your figures.

Is There a Difference Between Opening Size and Frame Size?

Yes, the structural opening and the finished frame size are two different figures. Mixing them up is a common cause of ordering errors.

The structural opening is the raw gap in your wall. The frame size is the finished product once it’s fitted. Your supplier builds in the right tolerance during manufacturing, so the frame fits neatly inside your opening with room for sealing.

If you’re ever unsure which figure a supplier is asking for, just ask. It’s a simple question, and getting it wrong is one of the more common mix-ups first-time buyers run into. If you’d rather buy separately and arrange your own fitting, our supply-only cost guide explains what that route includes.

What Common Measuring Mistakes Should You Avoid?

The most common mistake is measuring the existing frame instead of the structural opening behind it. A warped or slightly misaligned old frame gives you the wrong figure before you’ve even started.

Another common error is taking just one measurement and assuming it applies across the whole opening. Walls settle unevenly over the years, especially in older properties, so a reading taken at the top can differ from one taken at the bottom by more than you’d expect.

Rounding numbers is a smaller but real mistake too. Rounding 1,797mm up to 1,800mm feels harmless, but on a made-to-measure product, every millimetre matters. Write down the exact figure, not a tidy approximation.

What Measurements Do I Need for Bifold Doors?

What measurements do I need for bifold doors comes down to four figures: overall width, overall height, both diagonal measurements, and the depth of your opening. Having all four ready before you contact a supplier speeds up the whole process.

• Overall width, measured at least three times across the opening
• Overall height, measured at least three times up the opening
• Both diagonal measurements, to confirm squareness
• Depth of the opening, relevant for track and threshold choice

It also helps to note anything nearby that might get in the way, such as a step down, a nearby wall, or pipework close to the opening. A supplier can usually work around these, but only if they know about them in advance.

Does Wall Thickness Affect the Size You Order?

Yes, wall thickness affects the reveal depth, which decides how the frame sits within your opening. A thicker wall gives more room for the frame to seat properly and seal well.

Most bifold door frames need a minimum reveal depth to fit and seal correctly. A very thin wall, sometimes found on an older extension or outbuilding, can limit which systems are genuinely suitable. This is worth checking before you fall in love with a specific configuration.

Your supplier will usually ask for wall thickness alongside your other measurements for exactly this reason. It’s a small detail, but it can rule certain options in or out before you get too far into the decision.

How Wide Should Bifold Doors Be for Your Opening?

How wide should bifold doors be depends entirely on your opening. That figure decides how many panels you need and how they’re arranged. A wider opening generally needs more panels, not simply bigger ones.

Opening width	Typical panel count
Up to 1.8m	2 panels
1.8m to 2.7m	2 to 3 panels
2.7m to 3.6m	3 to 4 panels
3.6m and beyond	4 or more panels

A wider opening also needs more wall space either side, so the folded panels have somewhere to stack. Check this against your kitchen or extension layout before you settle on a configuration. For a fuller comparison of configurations and brands, our best bifold doors buying guide covers that in more depth than a sizing guide can.

How Does Panel Configuration Affect Which Way Doors Stack?

Panels can stack fully to one side, or split so half fold to each side of the opening. This choice affects both your measurements and how the finished door actually feels to use.

A single stack to one side is simpler and often suits a smaller opening well. Splitting the stack across both sides can suit a wider opening, since it avoids piling every folded panel against a single stretch of wall.

Think about which side of the opening you actually want clear once the door is open, whether that’s for a clear view, furniture placement, or simply how the space feels. This decision is worth making before your final measurements are confirmed, not after.

What Is the Minimum and Maximum Size for a Bifold Door?

Most bifold systems have a minimum width of around 1.4 metres. A narrower gap doesn’t leave enough room for a workable panel arrangement. Maximum widths vary by manufacturer, though many systems reach 6 metres or more across several panels. How wide bifold doors should be at the very top of that range depends heavily on the specific system, so always confirm with your supplier directly.

Each panel is limited too, usually to around 1.2 metres wide, no matter how wide the full opening is. That’s why a very wide opening needs more panels rather than simply wider ones.

If your opening sits near either end of that range, mention it early. Not every system handles both extremes equally well. For a slimmer sightline that still handles a wide opening well, our slimline aluminium bifold doors guide explains how that profile works across wider spans.

Can You Order a Bifold Door Before the Opening Is Built?

Yes, but it needs careful planning, since the door is still made to measure against a final, agreed size rather than a rough estimate. Ordering too early, before the opening is fully confirmed, is a genuine risk worth avoiding.

On a new extension, the opening size is usually confirmed once the structural work reaches a certain stage, with the exact dimensions agreed between your builder and the door supplier. Ordering the door too far ahead of this point risks a mismatch if the final build varies even slightly from the original plan.

Most suppliers are used to working alongside builders on this exact timing question, coordinating so the door arrives once the opening is genuinely ready. It’s worth raising this coordination early in your project, not assuming it will simply work itself out.

What If Your Opening Isn’t Square or Level?

An opening that isn’t square or level can often still take a bifold door, but it usually needs correcting first. A big difference between your two diagonal measurements is the clearest sign something needs attention. This is exactly the kind of detail that makes how do I know what size bifold door to buy a bigger question than it first looks.

A small variation of a few millimetres is common, even in a well-built opening, and rarely causes a problem. A bigger gap between the two diagonals suggests the opening has moved or settled over time, which is common in older properties. Raise this with a builder or your installer before you order anything.

Decision checkpoint: if your home is an older property with visible signs of settlement, get the opening properly checked before ordering, rather than measuring around the problem.

Should You Measure Yourself or Request a Professional Survey?

Measuring yourself works well if you’re confident, careful, and the opening is straightforward. A professional survey removes the guesswork entirely, especially on an older or unusual opening.

A survey usually takes under an hour and checks the structure alongside the measurements. What size bifold doors do I need becomes a far easier question once a professional has actually seen your opening. A wrong measurement discovered after manufacturing is a far more expensive problem than simply booking a survey upfront. Most first-time buyers underestimate how precise this needs to be. A few millimetres can decide whether a door seals properly.

Some suppliers offer a paid measuring service too, sitting between a full survey and doing it entirely yourself. That’s a fair middle ground if your opening looks straightforward but you’d rather not take full responsibility for the final numbers.

What Happens After You Submit Your Measurements?

Once you submit your measurements, your supplier usually confirms them before manufacturing starts. This step is what catches most measuring errors before they become expensive.

This often means a follow-up call or a written summary of your figures, giving you a chance to check everything before the door goes into production. Once confirmed, changes become harder and more costly, so it’s worth reviewing this stage properly rather than rushing to approve it.

Your door is then built to those confirmed measurements. This usually takes several weeks, depending on configuration, glazing, and colour. Installation itself typically takes one to two days for a straightforward opening. For labour costs specifically, our aluminium bifold door installation cost guide breaks down what professional fitting usually adds.

How Much Does Getting the Size Wrong Actually Cost?

Getting the size wrong after manufacturing usually means a full remake, since bifold doors can’t be resized once built. This is the single biggest financial risk in the entire process.

A remake means paying for a second door on top of the first, plus the lost time waiting for it to be built again. Manufacturers rarely offer a refund for a measuring error that’s genuinely the customer’s own mistake, which is exactly why the checklist and method in this guide matter as much as they do.

This is also the strongest argument for a professional survey on anything beyond a straightforward, modern opening. The survey fee is small compared with the cost of a full remake.

What Size Bifold Doors Do I Need for a Wide Kitchen Extension?

What size bifold doors do I need for a wide kitchen extension usually means 3 to 5 panels, spanning openings from around 2.7m up to 4.5m or more. This is where the sizing question connects most directly with a real extension project.

A homeowner extending into the garden with a rear opening of around 4 metres would typically need a 4- or 5-panel set, depending on how the panels stack. Getting the width right matters more here than anywhere else in this guide, since a genuinely wide opening leaves less room for error. Our separate aluminium bifold doors cost guide breaks pricing down by configuration, once you have a rough panel count in mind.

Do Different Manufacturers Use Different Sizing Standards?

Yes, panel widths, minimum openings, and reveal depth requirements all vary between manufacturers. This is exactly why comparing two suppliers on price alone can be misleading.

One manufacturer’s system might handle a 6.5 metre opening across six panels, while another maxes out at 6 metres across the same panel count. Neither is wrong, but the difference matters if your opening sits right at the edge of what’s possible. Hardware quality can vary between manufacturers too, not just the raw size limits, which is worth keeping in mind alongside the numbers themselves.

Always check the specific limits for the system you’re actually being quoted, rather than assuming general bifold door figures apply universally. A supplier working from their own product range should be able to confirm this quickly.

If your project is commercial premises rather than a home, our aluminium commercial doors range covers a different specification entirely.

Choosing the Right Bifold Door Size?

How do I know what size bifold door to buy ultimately comes down to accurate measuring, confirmed by your supplier before manufacturing begins. Getting this right the first time avoids a costly remake and keeps your project on schedule. If you take one thing from this guide, let it be this: use the smallest reading, not the largest, and get a professional survey if you’re at all unsure.

If you’re still deciding whether aluminium suits your project, our aluminium bifold doors good guide covers that broader question in more depth.

Get an Accurate Quote. We cover Buckinghamshire and Oxfordshire, with every project measured and confirmed before any figure is finalised.