How to measure windows for replacement uPVC

Brick-to-brick, not the inside reveal

The size we need is the structural opening — brick to brick on the outside. The inside reveal (plaster to plaster, or the daylight size of the old glass) is smaller. If you type the reveal size into the designer, the new frame will be short of the brickwork and the fitter will be packing a wide gap.

A bay is not one width. Measure each face as its own opening, brick-to-brick on that face, and order a separate window for each. Do not add the faces together or measure the room side of the bay and hope it works.

Windows and doors are measured the same way — almost

For both, ignore the old frame or door leaf. Measure the opening in three places, use the smallest figure, then deduct 10mm from width and 10mm from height. That deducted pair is what you enter.

Window height starts at the top of the cill you are keeping (or the brickwork if the cill is coming out) and finishes at the underside of the lintel. Door height starts at the finished floor — or the threshold you are keeping — and finishes at the same lintel line. Do not measure the old door slab and add a bit; the slab is smaller than the opening. uPVC doors are designed on the uPVC door page with the same millimetre boxes as windows.

Cills: what to deduct, and what not to

The height you enter is the frame height, not frame-plus-cill. A projecting cill (85–225mm) is an extra piece that sits under the frame and throws water off the wall. Do not add that projection onto the height, and do not take it off either — pick the cill as a separate option in the designer.

Keeping a stone, concrete or brick cill? Measure height from the top of that cill and choose “no cill” or a stub cill, so the new frame sits on what is already there. Replacing the cill as well? Measure from the brickwork the new cill will sit on, deduct the 10mm, and choose a projecting cill. If you are unsure, measure both and ask us before you order.

What to type into the designer

Width and height in millimetres, after the 10mm deduction. Those two numbers are the frame we manufacture. Colour, glass, trickle vents, handles and cill are chosen next to the drawing — they do not change the size boxes. There is no “opening size” field; if you enter the brick-to-brick figure without deducting, the frame will be 10mm too large each way.

Every window is made to your sizes. Standard window sizes are only a reference for older stock frames — do not pick one from a chart and skip the tape. The made-to-measure page explains why the opening, not a catalogue size, is what matters.

Common mistakes

• Measuring the old frame or the glass instead of the brick opening.
• Using the largest of the three readings, or averaging them.
• Measuring the inside reveal and calling it brick-to-brick.
• Forgetting the 10mm clearance, or deducting 10mm only once for both directions.
• Entering centimetres in a millimetre box (120 instead of 1200).
• Adding the cill projection onto the height, or burying a kept stone cill under a new projecting one.
• Treating a bay as one window, or copying one size across every opening.

Once the frames are here, the install-a-window guide covers packing, fixing and glazing. We are supply-only — we do not install — but the sizes you enter here are the sizes we make.