Internal door sizes: metric, imperial, and how to tell which your house takes

Most UK internal doors are 1,981mm tall and 35mm thick — that is the old imperial six foot six, carried over from when doors were sold in feet and inches. Width is where the choice sits, and these are the ones you will actually meet:

Houses built through the twentieth century — the Victorian and Edwardian terraces, the 1930s semis, the post-war estates — take the imperial sizes above. Newer builds increasingly use the metric standard instead: 2,040mm tall, typically 726mm or 826mm wide, and 40mm thick. Metric doors are noticeably taller, and the two systems do not mix — a metric door cannot be cut down to fit an imperial frame without ruining it.

The quickest check is to measure a door you already have. Roughly 1,981mm tall and it is imperial; roughly 2,040mm and it is metric. If it measures neither, you are probably in an older property where nothing is standard — which brings us to the interesting part.

A Victorian terrace has had a century and a half to move, and its door openings have moved with it. It is normal for us to measure an opening in an older house and find the width differs by 10mm between the top and the bottom, or that no two doors in the hallway are the same size. The original doors were often made to fit by a joiner on site, so "standard" never applied in the first place.

This is workable — a door can usually be trimmed to fit — but only within limits. Most veneered doors allow only a few millimetres of trimming per edge before you cut through the lipping and expose the core, and the manufacturer's stated trim allowance is a hard limit, not a suggestion. Solid timber doors are more forgiving. If an opening is badly out of square, the honest answer is sometimes to scribe the door to the opening, and that is a fitting skill rather than a measuring one.

Measure the structural opening, not the old door — an old door may have been trimmed, and it tells you what fitted then, not what fits now.

We measure every opening ourselves before we order anything — it is included in the job, and it is why a door we fit does not come back. If you are buying doors to fit yourself, the numbers above will get you to the right size in a standard house. If your house is pre-war, or the measurements you are getting do not match anything on this page, that is precisely the situation where a measuring visit costs nothing and a wrongly ordered door costs the price of the door.