Will a couch fit through the door?
When you carry something straight through a doorway, only its cross-section matters, meaning the two dimensions you're not carrying along. That cross-section can be turned and tilted within the door frame.
Three cross-sections
A box has three cross-sections: length × height, width × height and length × width. Carry it along whichever dimension leaves a cross-section that fits. If you carry a long sofa lengthways, only its depth × height has to pass. That's why sofas often go through on their end or on their back.
Tilting within the frame
A cross-section that's too tall to go upright can sometimes go through tilted at an angle. The exact test for whether a p × q rectangle fits inside an a × b rectangle at some angle comes from W. Carver's answer to a 1956 American Mathematical Monthly question. J. E. Wetzel gives a modern proof (Mathematics Magazine, 2000). Carry Clearance uses that condition and checks it against a brute-force rotation sweep in its tests.
Example
A cabinet 82 in tall, 30 in wide and 20 in deep can't pass upright through a 32 × 80 in opening, because it's taller than the opening. Laid down and carried lengthways, only its 30 × 20 in cross-section has to pass, and it clears easily. If it must stay upright (some appliances), it won't fit, and Carry Clearance says so.
What isn't credited
- Pitching the item in the direction of travel and "threading" it through a thin wall. That can work for real sofas with sloped backs (retailers measure a "diagonal depth" for this), but it depends on the shape. A box model can't verify it.
- Removing the door, its stops or the item's legs, unless you enter the measurements as they'll be after removal.
- Room to line up. You need space in front of and behind the door to carry the item straight on. If a hallway turn is right at the door, use the "turn from a hallway through a door" step.
Sources
- W. B. Carver, solution to L. Ford's rectangle problem, Amer. Math. Monthly (1956–57); J. E. Wetzel, "Rectangles in rectangles", Mathematics Magazine 73(3), 2000.
- U.S. Access Board, ADA Standards §404.2.3 (clear width).