Measure the shortest wall and the door swing, then pick a mattress that still leaves aisle space. This is a room-fit table — not a firmness quiz. Shop links are Amazon search URLs with tag whatsizetobuy-20 only.
| Shortest wall (inside) | Mattress that usually fits | Leave this aisle |
|---|---|---|
| Under 8 ft / 2.4 m | Twin or Twin XL | One side path to the door |
| 8–9.5 ft / 2.4–2.9 m | Full | Nightstand on one side only |
| 9.5–11 ft / 2.9–3.4 m | Queen (default couple) | Nightstands both sides if the long wall is 11 ft+ |
| 11–12.5 ft / 3.4–3.8 m | King | Confirm the door can open past the foot |
| 12.5 ft / 3.8 m or more | King or split king | Split king if stairs cannot take a 76-inch box |
US mattress footprints we use here: Twin 38×75, Twin XL 38×80, Full 54×75, Queen 60×80, King 76×80 (inches). Measure the actual room, not the listing photo.