The doorway is a red herring
A doorway is a flat opening and you can present a sofa to it at any angle you like. What defeats furniture is the turn — the point where a staircase changes direction, or where a hallway meets a doorway at right angles.
That is a three-dimensional problem, which is why "it came in, so it will go out" is not reliable. Walls get added, bannisters get replaced, and the route that admitted something years ago may not exist any more.
The four measurements
Take these with a tape, not by eye:
- The clear width at the narrowest point of the route — post to wall on a staircase.
- The depth of the half-landing, or of the hallway at the corner.
- The ceiling height directly above the turn. This is the one everybody forgets and the one that decides it.
- The diagonal of the item — its longest face measured corner to corner, not the dimensions on the listing.
If it will not go
Three honest options, and it is far better to pick one in advance than to find out with the thing wedged on your stairs:
- Take it apart. Many sofas have removable feet, backs or arms, and most wardrobes were flat-pack once.
- Go through a window. Genuinely possible, occasionally the right answer, and it needs planning, equipment and usually permission.
- Do not take it. Sell it and buy something that fits the place you are actually living in.
Measure before you buy, not after
If you are buying for a place you have not moved into yet, measure its turn first. Discovering the problem in a shop is free. Discovering it on a landing is not.