Checks the whole route, not one door: a doorway, the tilt-and-pivot through it, then a 90° hallway corner — the place sofas actually get stuck. Uses the real geometry of a box turning a corner and tells you which orientation to carry it in.
Measure the narrowest point (door stops, handrails, radiators). Allow 2–3 cm (1 in) for hands and blankets — results within that margin are marked tight. Removing legs, feet or the door itself often adds several centimetres. Geometry assumes a rigid box; soft cushions can compress, frames cannot.