How to measure for bifold doors

Overall size: the cill is included

The width and height you enter are the overall size of the whole set: the frame, with its threshold, plus the cill underneath. Every bifold comes with a cill, so the designer always takes 25mm off the height you enter for the cill and makes the frame from the rest. Never add anything for the cill or the threshold, and never take anything off for them — measure, deduct 10mm, and choose them in the designer.

Our bifolds are made from 1300mm to 5500mm wide and 1925mm to 2475mm high overall — that's a frame up to 2450mm high, plus the cill. Opening bigger than that? Email sales@newupvcwindows.co.uk or call 0161 793 1166 before you order.

Which cill: 93, 158 or 183mm

The cill sits under the frame on the outside and throws rainwater clear of the wall. It comes in three sizes — 93mm (stub), 158mm (standard) and 183mm (large) — and the bigger the size, the further it reaches out in front of the frame. All three add the same 25mm to the height, so the choice never changes your sizes.

Choose one that reaches past the face of the wall, step or paving below, so drips land clear of it instead of running down the brickwork. That depends on how far back in the opening the frame is set — your installer can tell you — so if the cill has to reach a particular distance, check with us before you order.

Threshold: standard 53mm or low 40mm

The threshold is the bottom of the frame — the leaves run on a track in it — so it's part of the overall height, not extra. Choose the standard threshold (53mm) or a low threshold (40mm) for a flatter step through between inside and out.

A low threshold is meant to sit close to the finished floor, so plan the floor levels and the drainage outside with your installer before you order. If it will be set down into a recess, measure the height from the bottom of the recess — the base it will actually sit on. Whichever you choose, the floors either side mustn't stop the leaves swinging or block the drainage: paving or decking outside should sit below the threshold and fall away from the doors. If you are unsure, measure both ways and ask us before you order.

How many leaves?

Each leaf is the overall width divided by the number of leaves, and it has to come out between 650mm and 1100mm. That gives these overall widths:

Where your width suits more than one count, fewer leaves means wider glass and fewer frames across the view; more leaves means narrower leaves that stick out less when they're folded back. With 3 or 5 leaves every layout has a master door. With 2 or 4, some layouts don't — see below.

Which way they fold

The leaves fold in pairs and stack against one or both sides of the opening. Left and right are always as seen from outside, looking at the doors — even when they open inwards. The drawings show each layout from above with the outside at the bottom, so left and right match what you see from the garden; the arrows show which way each stack slides.

A stack with an odd number of leaves includes a master door: a single leaf that opens on its own, like an ordinary door, so you can get in and out without folding the rest. It's drawn in blue, with its swing dashed. A stack with an even number folds in pairs, so there's no single door to open.

Every layout you can order

Opening in or out

Opens out: the leaves fold out onto the patio and the room stays clear. You need a clear, level area outside where they swing and stack — no step up, plant pots, raised decking or rising lawn in the way.

Opens in: the leaves fold into the room — handy where the space outside is tight or drops away. Keep the floor clear where they swing and stack, and check for radiators, furniture, blinds and curtains.

Either way, folded leaves stand out at right angles to the frame, so leave about one leaf's width of clear space where the stack lands. Left and right are still described from outside.

What to type into the designer

Width and height in millimetres, after the 10mm deduction: the overall size, cill included. The number of leaves, the layout, in or out, the threshold, the cill, the colour and the handle colour are all chosen next to the drawing, and none of them changes the size boxes. The glass is clear toughened double glazing, so there's nothing to choose there. If you type the brick-to-brick figure without deducting, the doors will be 10mm too big each way.

Common mistakes

• Measuring the old frame, the old doors or the inside reveal instead of the brick opening.
• Using the largest of the three readings, or averaging them.
• Forgetting the 10mm clearance, or deducting 10mm only once for both directions.
• Measuring the height from the top of an old threshold or step that is coming out.
• Adding the cill or the threshold onto the height — the designer already allows for the cill.
• Choosing 2 or 4 leaves all folding one way, or 2 + 2, without realising there's no single door to walk through.
• Forgetting the space the folded stack needs, inside or out.
• Measuring a new or widened opening from drawings before the builder has finished it.

Once the doors arrive, the bifold fitting guide covers putting them in. We are supply-only — we do not install — but the sizes you enter here are the sizes we make.