How to measure for bifold doors

Should you measure yourself?

Honestly — usually not. Bifold installations are bespoke to the opening, and the cost of a wrong measurement is significant. If you're committing to ordering doors, book a free site survey: a surveyor will measure properly, identify any preparation work needed, and the measurements they take are the ones the manufacturer uses.

Where measuring yourself makes sense: getting an indicative quote before booking a survey, or sense-checking quotes from multiple installers. Self-measurements should never be the basis of the actual order.

Tools you need

• Tape measure — at least 5m, ideally 8m. The plastic ones are fine; the metal ones are better. Get a quality one — Stanley, Fisco, Stabila. Cheap tapes can be 2mm out over a few metres.
• Spirit level — at least 1m long, preferably 2m. A short level won't reveal subtle slope.
• Plumb bob or laser level — for checking vertical (plumb) on the reveals.
• Notebook — write everything down immediately, don't trust memory.
• A second person — for holding the tape on long measurements. Hugely reduces error.

The measurements that matter

1. Brick-to-brick width (3 measurements)

Measure the opening width at three heights: at the top, the middle, and the bottom. Measure from the inside face of the existing brickwork (or the structural opening if there's no brick yet) — not from any existing frame.

You'll get three slightly different numbers. Use the smallest. Bifold frames need to fit through the narrowest point of the opening. The manufacturer will then subtract 10–15mm for fitting tolerance.

2. Brick-to-brick height (3 measurements)

Same approach for height: measure at the left, centre, and right. The measurement is from the cill/floor finish to the underside of the lintel (or structural opening).

Again, use the smallest of the three numbers.

3. Depth of opening

Measure the depth of the reveal (how thick the wall is). This determines the frame depth available and affects whether the threshold can be flush or rebated.

For most cavity walls this is 250–280mm. Solid wall: 220mm typically. Modern timber-frame: 200–230mm.

4. Plumb check (verticality)

Hold a long spirit level vertically against each side of the opening. If the bubble shows the wall leaning more than 5mm out of plumb over the height of the opening, the frame will need packers to compensate.

5. Level check (horizontality)

Place a long spirit level across the cill and across the lintel underside. Any slope greater than 3mm over the width of the opening is significant and needs flagging.

6. Squareness (diagonal check)

Measure both diagonals of the opening — top-left to bottom-right, and top-right to bottom-left. The two diagonals should be within 5mm of each other. If they differ by more than 10mm, the opening is significantly out of square and the frame fitting will be more involved.

What surveyors measure that you probably won't

• Floor finish: what's the finished floor level (FFL) inside? Is there a step out to the patio? What's the relative level outside? This determines threshold detailing.
• Outside ground level: are you below DPC level on the outside? Drainage matters for weatherproofing.
• Existing lintel: is it correctly sized for the new opening width? Sometimes the existing lintel needs replacing — this is a separate job before bifold installation.
• Structural opening below DPC: any damp issues on the existing brickwork below the threshold? These need resolving before installation.
• Glazing access: can the panels physically be brought to the installation point? For larger panels (1.2m+ wide) this becomes a real constraint in some properties.

Common mistakes

1. Measuring from the existing frame, not the structural opening. You're removing the existing frame, so measure to the brickwork or structural opening behind it.
2. Using one measurement instead of three. Openings are almost never perfectly square. Three measurements, smallest wins.
3. Not accounting for finished floor level. A measurement to bare slab is meaningless if you're laying engineered oak on top.
4. Adding the fitting tolerance yourself. Don't. Give the raw measurements; the manufacturer subtracts the tolerance.
5. Measuring during work. If you're widening an opening, measure after the structural work is complete, not during.
6. Trusting one measurement on a long opening. Above 4m, openings often have subtle bow in the wall — 5mm over 4m is easy to miss with a single measurement.

When to book a professional survey

Always, before ordering. A free survey from a specialist installer takes 30 minutes, costs nothing, and gives you measurements that the manufacturer will warranty. It also identifies preparation work (lintel issues, level problems, DPC concerns) that you'd otherwise discover on installation day.