How to measure for replacement windows, step by step

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.

Bays are different. A bay's overall height is measured outside, with the new cill included, and the internal width of each face and the angle at each corner are measured inside the room. The bay designer prices the whole bay as one item, with the bay poles and cill welds, and works out the backspan so you can check your sizes. Our bay measuring guide takes you through it step by step.

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.

Measuring for aluminium bifold doors? They have their own bifold measuring guide, which also helps you choose how many leaves.

Cills: the size you enter includes it

The height you enter is the overall height: frame plus cill, if you choose one. A projecting cill (85-225mm) sits under the frame and throws water off the wall; when you pick one, the designer makes the frame shorter by the cill's height so the whole window still fits the size you entered. So never add anything for the cill, and never take anything off for it: just measure, deduct 10mm, and pick the cill 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 overall size we manufacture, including the cill if you choose one, and any frame extenders. Colour, glass, trickle vents, handles and cill are chosen next to the drawing, and they never change the size boxes. There is no “opening size” field; if you enter the brick-to-brick figure without deducting, the window 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.

Want a rough figure first? See what your sizes cost in our window price guide, with live prices by size.

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.
• Copying one size across every opening.
• Ordering a bay as separate windows. Use the bay designer and follow the bay measuring guide.

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.