How to get a sofa through a narrow door
A sofa that will not go through the door is the most expensive mistake in furniture buying. It is also the easiest one to avoid, because the whole question comes down to four measurements and one piece of geometry most people have never been told about.
Measure the opening, not the door
Start here, because almost everyone gets it wrong. A door sold as 30 inches wide does not give you a 30 inch gap. Open it to 90 degrees and the slab is still sitting inside the frame, and the stop on the other side is sticking out too. A US interior door slab is 1 and 3/8 inches thick. The stop takes maybe another half inch.
So put the tape across the real gap: from the face of the open door to the stop on the far side. On a 30 inch door you will usually read about 28. That is the number to work with.
Measure the height the same way, from the floor to the underside of the frame. Most US interior doors are 80 inches tall, so the opening is a little under that.
Three measurements of the sofa
Length, depth and height. Take the depth at the deepest point, which is usually the front edge of the seat cushion, not the back panel. Take the height to the top of the back, and include the feet.
Then do the sum the shops never mention. A sofa has six ways of meeting a doorway. It can go through end first or side first, and it can be upright, on its back, or stood on one end. Each of those puts a different pair of measurements in the opening. You only need one of the six to work.
The usual winner is on its back, end first. Tip the sofa backwards so the back panel is on the floor, and the number that has to clear the door width is the sofa’s height, which is nearly always its smallest measurement. That single move gets more sofas through doors than anything else.
The doorway calculator tries all six and tells you which one to use.
The corner is what actually stops you
Here is the part that catches people. Getting through the door is easy. Swinging a long sofa round a right angle in a hallway is not, and it needs far more room than you would guess.
Between two hallways 36 inches wide, a rigid item 30 inches deep can only be about 42 inches long before it jams in the elbow. A three-seat sofa is twice that. On paper, most hallway corners are impossible.
Yet people do it every day. The reason is that real corners are almost never closed on four sides. There is a bedroom door at the elbow, or the hall opens out into a living room, and the far end of the sofa goes into that gap while you swing the near end round. Before you give up, go and stand at the corner and look at what opens off it. That space is your working room.
If nothing opens off it, the corner is a genuine no, and no amount of lifting and shoving will change the geometry.
Take the door off
Two minutes with a screwdriver. Tap the hinge pins up and out, lift the door away, and you have gained roughly the thickness of the slab: about an inch and a half. On a tight fit that is often the whole margin.
If you need a little more, the stop is a thin strip pinned to the frame. Pry it off carefully with a chisel or a stiff scraper and you get another half inch. It is painted in, so take your time, and keep the pins.
Strip the sofa
Everything that comes off, take off.
Cushions lift out and can cut two or three inches from the depth. Many sofa legs unbolt with a socket wrench, which drops the height. Some sofas have backs that unclip from the frame, and that turns the whole thing into two flat pieces. If the sofa is being delivered, ask the shop before it arrives; they know which models come apart.
When the tilt trick does not work
People try turning a sofa diagonally in the doorway. It feels like it should help. It does not. Lay a tape corner to corner across the face you are pushing through: that diagonal is what the opening has to swallow once the sofa is at an angle, and it is bigger than either side.
Tilting helps with a different problem. It trades height for length, so it gets a tall item under a low frame or up a stairwell. If your problem is width, square the sofa up and go straight in.
The last resort
If the sofa will not go, a window will sometimes take it, and furniture removers do this more often than you would think. Beyond that, some sofas are sold as sectionals for exactly this reason, and in a flat with a tight stair it is worth paying the difference.
Before you buy anything, run your numbers through the doorway calculator. It takes a minute, and it is cheaper than a return.