UK door sizes explained: imperial and metric
The UK door trade never fully went metric. It quotes millimetres, because that is what the law and the tape measure use, but the sizes themselves are still inches. That is why the numbers look arbitrary until you convert them, and then suddenly do not.
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What size are UK internal doors?
Most UK internal doors come from two families. Imperial sizes are quoted in millimetres but cut to inches, so 1981 x 762 x 35mm is the door everyone calls a 78 x 30. Metric sizes are 2040mm tall and 40mm thick, in widths from 526mm to 926mm.
Why is a 1981 x 762mm door called a 78 x 30?
Because it is 78 inches by 30 inches, converted. An inch is 25.4mm, so 30 inches is 762mm exactly and 78 inches is 1981.2mm, which the trade rounds to 1981. The imperial name survived because joiners, merchants and every existing house still work in it, while the millimetre figure is what appears on the label and the order.
The same logic runs through the whole imperial family. Every width below is a whole number of inches, converted and rounded to the nearest millimetre.
| Imperial name | Metric size | Width in mm | Common? |
|---|---|---|---|
| 78" x 18" | 1981 x 457 x 35mm | 457mm | Cupboards and airing cupboards |
| 78" x 21" | 1981 x 533 x 35mm | 533mm | Narrow cupboards |
| 78" x 24" | 1981 x 610 x 35mm | 610mm | Yes |
| 78" x 27" | 1981 x 686 x 35mm | 686mm | Yes |
| 78" x 28" | 1981 x 711 x 35mm | 711mm | Occasional |
| 78" x 30" | 1981 x 762 x 35mm | 762mm | Yes, the most common of all |
| 78" x 33" | 1981 x 838 x 35mm | 838mm | Yes |
| 80" x 32" | 2032 x 813 x 35mm | 813mm | Occasional |
What are metric door sizes?
The metric family is a genuinely separate set of sizes, not a conversion of the imperial one. Metric doors are 2040mm tall and 40mm thick, and the widths run in round 100mm steps. They turn up in newer housing and in some European ranges.
| Metric size | Width in mm | Common? |
|---|---|---|
| 2040 x 526 x 40mm | 526mm | Cupboards |
| 2040 x 626 x 40mm | 626mm | Occasional |
| 2040 x 726 x 40mm | 726mm | Yes |
| 2040 x 826 x 40mm | 826mm | Yes, the most common metric size |
| 2040 x 926 x 40mm | 926mm | Wide openings |
How do I tell which family my doors are?
- Measure the height of an existing door. Around 1981mm means imperial; around 2040mm means metric.
- Measure the thickness. 35mm points to imperial, 40mm to metric — though this is a guide, not a rule, because both thicknesses exist in both families.
- Measure the width and look it up in the tables above. If it lands on a whole number of inches, it is imperial.
Why are fire doors a different thickness?
Fire doors need a dense core to hold back fire for the rated period, and that core makes them thicker and considerably heavier than a standard door of the same face size. FD30 leaves are usually 44mm and FD60 leaves usually 54mm, in the same face sizes as the standard families.
The practical consequence is that you cannot simply swap a fire door into a lining built for a 35mm door. The rebate depth, the hinge positions and the latch all change, and the lining itself has to be suitable.
What if my opening does not match a standard size?
Take the next standard size up and trim it, within the allowance the maker states for that specific door. Where the gap is bigger than the allowance, the options are a made-to-measure door or re-lining the opening to a standard size the market actually sells, which is usually cheaper and always easier to replace in future.
Common questions
What is the most common internal door size in the UK?
1981 x 762 x 35mm, known in the trade as a 78 x 30. It is the standard size for a room door in most UK housing built to imperial sizes, which is the large majority of the existing stock.
Is a 78 x 30 door the same as an 1981 x 762mm door?
Yes, they are the same door. 78 x 30 is the size in inches and 1981 x 762mm is the same size converted to millimetres and rounded. You will see both used for the same product, sometimes on the same label.
Are metric and imperial doors interchangeable?
No. Metric doors are 2040mm tall and 40mm thick against an imperial door's 1981mm and 35mm. The height difference alone is far beyond any trimming allowance, so the two families need different linings.
What thickness are UK internal doors?
Standard internal doors are commonly 35mm in the imperial family and 40mm in the metric family. Fire doors are thicker because of their core: FD30 leaves are usually 44mm and FD60 leaves usually 54mm.
What size is a standard cupboard door?
Cupboard and airing-cupboard doors are usually the narrow sizes in the same height family as the room doors: 457mm or 533mm wide in imperial, 526mm in metric. The height stays the same as the rest of the house.
Ready to order?
Every door lists its sizes in both imperial and metric, so you can check yours against what you measured before you commit.