A standard British front door is 6'6" tall, which is 1981 mm, and comes in widths of 2'3", 2'6", 2'9" or 3'0". That is 686, 762, 838 and 914 mm. Your opening is unlikely to match any of them exactly, which is why so many replacement doors need a new frame as well.
British door sizes come from imperial measurements and have never fully converted. A joiner will quote you 2'9" and a website will list 838 mm, and they mean the same door.
You will also see metric-imperial sizes quoted as 726, 826 and 926 mm wide by 2040 mm tall. Those come from the European convention and turn up on stock doors sold in Britain. They are close to the imperial sizes without matching them, which is a common source of a door that will not fit.
External doors in Britain are 44 mm thick as standard. Internal doors are 35 mm. The difference matters when you replace a door, because a 44 mm door needs a rebate deep enough to take it and hardware sized to suit.
Houses move. A brick opening built in 1890 has had 130 years of settlement, and the wall above it has been pressing down for all of them. Measure a Victorian door opening at the head and at the threshold and you will often find 10 mm or more of difference.
Extensions and conversions add their own problem. An opening formed in the 1980s was built to whatever door the builder had, and that door may no longer be made.
The result is the same in both cases. A stock door either does not fit, or it fits with a gap you have to disguise. Trimming a door to size takes material off the stiles and rails that were sized to carry the door, and on an external door it also cuts through the paint seal on the edges.
We build to frame size, which is the door and its frame measured from the outside edge of the frame on one side to the outside edge on the other. The opening size is the hole in the wall.
To get from one to the other, subtract 2 to 3 cm from the opening in both width and height. That gap is what you pack and seal once the frame is in, and it is what lets you get the frame square in an opening that is not.
Order by frame size. Ordering by opening size gives you a frame that will not go in.
Measure the hole, not the old door. The old door may have been trimmed, and the frame around it may be rotten or out of square.
Width, in three places. At the top of the opening, at the middle and at the threshold. Write all three down and use the smallest.
Height, in three places. At the left edge, the centre and the right edge, measuring from the finished floor or the threshold up to the underside of the head. Use the smallest again.
Depth of the wall. Measure the thickness of the reveal from outside face to inside face. This decides what frame depth you need and whether the frame will sit flush with your plaster line.
Check for square. Measure both diagonals of the opening. If the two figures differ by more than about 5 mm, the opening is out of square, and you need the gap around the frame to absorb it.
Measure twice on separate days if the house is old. Timber and plaster move with humidity, and a reading taken after a wet week reads differently from one taken in August.
Then a made-to-measure door is the cheaper option, which surprises people who assume bespoke costs more.
The alternative is a stock door plus the work to make the hole fit it: packing out the reveal, re-plastering, new architrave inside, new cill outside, and a door that is not quite the proportion the opening wants. Budget for that work and a made-to-measure door often comes in lower.
Every door we make is built to the dimensions you enter rather than trimmed down from a standard size. You give us the frame size, we build to it, and the door arrives square in a frame that fits.
