How to measure for bi-fold doors

Getting the measurements right is the single most important part of ordering bi-fold doors. Made-to-measure doors are built to the sizes you give, and they can’t be returned because they’re the wrong size. This guide explains what to measure, how to do it and what to check with your installer before you order.

Measure the structural opening, not the old door

The structural opening is the hole in the wall the new frame will sit in: from brick (or block) to brick across, and from the finished floor level up to the underside of the lintel. If there’s an existing door or window in the way, its frame tells you very little. Old frames are often packed, sealed with thick mastic, or set into a rebate, so the real opening can be bigger or smaller than the frame suggests. Where possible, measure once the old frame is removed, or carefully remove plaster or trims at the edges to find the true brickwork.

For a new opening in an extension, measure what has actually been built rather than relying on the drawings. Openings are often a few millimetres out of square or slightly narrower at one end than the other.

Take three widths and three heights

1. Width: measure across the opening at the top, middle and bottom.
2. Height: measure from the finished floor level to the underside of the lintel at the left, centre and right.
3. Use the smallest width and the smallest height. A frame sized to the largest reading won’t go in where the opening is tightest.
4. Check the diagonals if you can. If the two diagonals differ a lot, the opening is out of square and your installer may need to allow more tolerance.

Know your finished floor level

Height is measured from the finished floor level, meaning the level of the floor once screed, insulation and the final floor covering are in. If your floor isn’t finished yet, agree the final level with your builder. This matters even more if you want a low or flush threshold, because the threshold height and drainage have to be planned with the floor build-up and the outside paving.

Allow a fitting tolerance

The door frame must be a little smaller than the opening so it can be packed level, fixed and sealed. Installers have their own preferred gap, so ask the person fitting the doors what to deduct before you enter sizes in the configurator. The width and height you enter in our bespoke bi-fold configurator are the overall frame size in millimetres, not the opening size.

Check the panel sizes work

Each bi-fold panel must be between 500 mm and 1,000 mm wide. Once you have your frame width, divide it by the number of panels to see roughly how wide each panel will be. The configurator does this check for you and warns you if you need to change the panel count. Our bi-fold sizes guide has worked examples. Bespoke bi-folds can be 1,200–7,000 mm wide and 1,800–2,400 mm high.

Decide opening direction and stacking

While you’re measuring, look at where the panels will go when the doors are open. Bi-folds can open out or in, and the panels can stack to the left, right or split to both sides. Check that stacked panels won’t hit a wall, drainpipe, step, garden furniture or a kitchen island. Also think about which side you’ll use most, as many people like a single access door at one end for everyday use.

Could a stock size work?

If your opening is close to 2.0 m, 2.4 m, 3.0 m or 3.6 m wide and suits a 2,090 mm high frame once you’ve allowed your installer’s tolerance, a stock-size bi-fold can be quicker. Stock bi-folds are often ready in 5–10 working days once paid, subject to availability. If you’re forming a new opening, building it to suit a stock size is worth considering.

Common measuring mistakes

• Measuring the old frame instead of the brickwork.
• Using the largest reading rather than the smallest.
• Measuring from the current floor, not the finished floor level.
• Forgetting to deduct the installer’s fitting tolerance.
• Mixing up centimetres and millimetres. Our configurator works in millimetres, so 3 metres is 3000.

Get your installer to check

Because we supply only, the sizes you order are your responsibility. The safest approach is to have the person fitting the doors take, or at least check, the final measurements. We also check every order request before invoicing and will ask if anything looks unusual, but we can’t see your opening. Read supply-only bi-fold doors: what you need to know for more.

Next steps

Once you have your frame sizes, configure your bi-fold door, add it to your basket and send an order request. We’ll confirm everything and email an invoice for payment by bank transfer. Nothing is made until you’ve approved the specification and paid.

Frequently asked questions