Standard Window and Door Sizes in UK Homes

Is there such a thing as a standard size?

In a new build estate, windows and doors are often ordered in repeating sizes, because the builder designed the openings around them. In almost any house built before the 1980s, which covers much of the Wirral and Chester, that is not true. Openings were formed by bricklayers on site, lintels were cut to suit, and two houses in the same terrace can differ by a few centimetres in every direction.

That is why a size chart can only ever give a rough idea. A replacement frame has to fit the actual hole in the wall with a small, even gap all round for fixing and sealing. A window made to a nominal size and forced into the wrong opening either leaves wide gaps packed with foam, or needs the brickwork cut. Neither is a good result.

How windows are measured

An installer measures the brick opening, not the old frame. The width is taken at the top, middle and bottom, and the height at the left, centre and right, because openings are seldom perfectly square. The smallest figure in each direction governs the size, less a fitting tolerance. The sill projection, the depth of the reveal and whether there is a cavity closer also matter.

If you are measuring yourself to get a rough idea, measure the visible outside of your existing frame from brick to brick. That will be close to the finished size, but treat it as a guide only. Internal measurements can mislead, because plaster, window boards and old timber sub frames hide the true opening.

• Measure width in three places and height in three places
• Use the smallest reading in each direction
• Note any obvious lean, bulge or cracked lintel above
• Photograph each window from outside so you can compare later

Bay window sizes

Bay sizes are one of the most searched questions, and one of the hardest to answer with a single figure. A bay is several frames joined at angles, and the overall width, the depth it projects from the wall, and the angle of the side panels all vary. A square bay projects at right angles, a splayed bay has angled sides, and a bow window is a gentle curve of narrower units.

Because each bay also carries some load from above, the survey looks at the structure as well as the frames. Measurements are taken for every facet, and the corner posts are specified to suit. Our bay window replacement guide and our bay window page go into the fitting side in more detail.

French door and patio door sizes

French doors are a pair of doors opening from the middle, and people often ask for their dimensions because they are replacing an old window with a new doorway, or swapping an old door set for a new one. The width you can achieve depends on the opening, and whether side panels are added either side of the pair. Where an opening is being widened or a window lowered to make a door, a new lintel may be needed and building regulations can apply.

Sliding and bifold doors have their own considerations. The number of panels, which way they stack and how much clear opening you want all change the frame size. Our French doors buying guide and our comparison of French and patio doors cover the choices.

Front and back door sizes

Front and back doors are usually closer to common sizes than windows, but frames are still made to fit the opening. Side panels and top lights change the overall size, and the threshold height matters for level access. Where an old door was cut down or packed out over the years, the true opening may be different from what you see.

Why we always survey

Every window and door we fit is made to measure from figures taken at a free survey, not ordered from a size chart. That is how we can give a fixed written price that does not change on the day. Measuring yourself is useful for a rough idea, and for comparing quotes like for like, but the order should always be placed from the installer's own survey.

If you would like us to measure up, we cover the whole Wirral, Chester and everywhere within roughly 30 miles, and the survey costs nothing.

Common questions

Can I order replacement windows from my own measurements?

You can measure for a rough idea, but a reputable installer will survey before ordering, so the frame fits the real opening and the price is fixed.

Should I measure the old frame or the opening?

The opening. Measure brick to brick from outside, in three places each way, and use the smallest figures.

Are bay windows a standard size?

Rarely. Width, projection and side angles all vary, so every facet is measured at the survey.

Can a window be turned into French doors?

Often, by lowering the opening. A new lintel may be needed and building regulations can apply, which we check at the survey.

Do you charge for measuring?

No. Surveys are free, with a fixed written price afterwards.