How do I know a sofa will fit through my door?

Measure the delivery path, not just the wall. You need the sofa's width, depth, and diagonal depth (listed on most spec sheets), and your narrowest constraint: door widths, hallway turns, stair landings, and elevator doors. A sofa can usually pass a doorway if its diagonal depth is less than the door opening, tilted vertically. If the math is close, choose a modular design or one with removable legs and arms — the return fee for a sofa that will not fit is the most avoidable cost in furniture.