If you're planning a room upgrade, knowing how to measure for a bifold door is the first thing you need to nail down before you even look at styles or finishes. It's one of those tasks that feels a bit intimidating because if you get it wrong, you're stuck with a very expensive piece of glass and aluminum that doesn't fit the hole in your wall. But don't worry—it's actually pretty straightforward as long as you're patient and follow a specific sequence.
The biggest mistake people make is assuming their door opening is a perfect rectangle. Spoiler alert: it almost never is. Houses settle, walls lean, and floors can be surprisingly wonky. That's why we measure multiple times and in multiple places.
Grab your tools and get ready
Before you start stretching that tape measure, you need the right gear. Don't try to do this with a fabric tape measure you found in a sewing kit; those things stretch and will give you a "close enough" reading that isn't actually close enough.
You'll need a high-quality steel tape measure, a spirit level (the longer the better), a pencil, and a piece of paper to jot everything down. I also find it helpful to have a second pair of hands if the opening is particularly wide. Trying to hold a tape measure steady across a four-meter gap by yourself is a recipe for a poked eye or an inaccurate reading.
Measure the opening, not the old door
If you're replacing an old set of doors, ignore them. Seriously. Don't measure the existing door leaves or even the old frame. You need to measure the brick-to-brick opening (often called the aperture).
The goal is to find the smallest point of the opening. If you buy a door that fits the widest part of a wonky opening, it won't fit into the narrowest part. It's much easier to fill a small gap with shims and sealant than it is to grind away your brickwork because the door is two millimeters too wide.
How to measure the width
Start with the width. You want to take three separate measurements: one at the top, one in the middle, and one at the bottom.
Run your tape measure from one side of the structural opening to the other. Make sure the tape is flat and straight. If your middle measurement is 2400mm but the bottom is 2395mm, that 5mm difference matters. Write all three numbers down clearly.
Once you have those three figures, look for the smallest one. That's your "base" width. However, you don't actually order the door at that exact size. You usually subtract about 10mm to 15mm from that total to allow for a fitting tolerance. This "wiggle room" ensures the frame can be leveled up perfectly even if the walls aren't 100% vertical.
Getting the height right
Measuring the height follows the same logic as the width, just vertically. Take three measurements: one on the left side, one in the center, and one on the right side.
When measuring height, you need to be mindful of your flooring. Are you measuring from the lintel down to the raw subfloor, or is the finished flooring (like tiles or wood) already in place? If the floor isn't finished yet, you'll need to know exactly how thick the final floor will be. Most bifold systems need to account for the "finished floor level" so the track sits at the right height and the doors don't drag on your expensive new carpet.
Just like with the width, take the smallest of your three height measurements and subtract about 10mm to 15mm for your fitting tolerance.
Checking for "squareness" and level
A door opening can have the same width at the top and bottom but still be shaped like a parallelogram. This is why you need to check if the opening is "square."
The easiest way to do this is to measure the diagonals. Measure from the top-left corner to the bottom-right corner, then from the top-right to the bottom-left. If the two numbers are the same (or within a few millimeters), you're in good shape. If they're significantly different, your opening is "racked," and you'll need to be extra careful with your tolerances.
While you're at it, grab your spirit level. Check the side walls to see if they are "plumb" (perfectly vertical) and check the floor and lintel to see if they are "level" (perfectly horizontal). If the floor has a massive slope, you might need to do some leveling work before the doors can be installed. Bifold doors rely on a very precise track system; if that track isn't level, the doors will either slide open on their own or be incredibly difficult to push.
Why tolerances matter
You might be tempted to order a door that fits the opening exactly to avoid gaps. Don't do it.
Bifold doors are heavy and mechanical. They need a frame that is perfectly square and level to function. The 10mm-15mm gap you leave around the edge (the fitting tolerance) allows the installer to use packers to shim the frame into a perfect position. Once the door is secure, that gap is filled with expanding foam and covered with trim or sealant, so it ends up being completely airtight and invisible anyway.
Considering the threshold
When you're learning how to measure for a bifold door, you'll eventually run into the "threshold" question. Do you want a weathered threshold or a low/flush threshold?
- Weathered thresholds have a small lip that the door seals against. These are great for keeping out the rain and wind. They usually sit a bit higher.
- Low thresholds are designed to be flush with the floor for a seamless transition between inside and out.
If you want that "flush" look where the inside floor and outside patio are at the same level, you have to plan for it during the measuring stage. This often involves "sinking" the track into the floor, which affects your vertical height measurements. Talk to your supplier about how much depth their specific track system requires.
A quick sanity check
Before you walk away and call the door company, do a quick sanity check. 1. Did you measure in millimeters? Most manufacturers work in mm. Using inches or centimeters can lead to rounding errors. 2. Did you double-check the numbers? Measure everything twice. It takes five minutes and can save you thousands of dollars. 3. Is the opening clear? Make sure there are no obstructions like protruding bricks, old mortar, or wires that might interfere with a clean measurement.
Final thoughts
Learning how to measure for a bifold door isn't rocket science, but it does require a bit of discipline. It's all about finding the smallest point of the opening and then giving yourself enough room to install the frame properly.
If you're at all nervous about it—especially if your house is very old or the opening is massive—many door companies offer a professional survey service. They'll send someone out to take the measurements for you, which usually puts the liability on them if the door doesn't fit. But if you're a confident DIYer or working with a builder, following the "three-point check" and subtracting your tolerances will get you exactly where you need to be.
Just remember: measure three times, subtract your tolerance, and always keep that spirit level handy. Your future self, enjoying a breeze through perfectly sliding doors, will thank you.