A sectional is two walls, not one number.

The sizes

A sofa has one width. A sectional has a length along each wall, and quoting it as a single number is how people end up with a piece that measured fine and arrived wrong. These are the sizes our sofa & sectional guide publishes, compiled from Architectural Graphic Standards, Time-Saver Standards and current US retail.

For scale: a standard sofa is 84 × 38 and a grand sofa 96 × 40. The 108 × 72 L is nine feet along one wall and six along the other, and it is the size most people picture when they say "sectional".

The corner, and why the orientation is the decision

Put an L in a corner and it consumes a rectangle: its long leg's length along one wall, its return's length along the other. Everything outside that rectangle is clear floor, and it comes in two strips — one beside the long leg, one past the end of the return. The width of each strip is exactly the wall left bare.

Rotating the piece ninety degrees swaps which wall gets which leg, and the result is not symmetrical. Take the room our living-room guide works through: 13 × 16 feet, which is 156 × 192 inches, and a 108 × 72 L.

• Long leg on the 16-foot wall. 192 − 108 = 84″ of strip beside it; 156 − 72 = 84″ of strip past the return. Two real walkways.
• Long leg on the 13-foot wall. 156 − 108 = 48″ beside it; 192 − 72 = 120″ past the return. One walkway and one small room.

Same furniture, same room, and the first arrangement is plainly better. That is the whole argument for an L in a corner over a straight sofa in a room this shape: the route runs around the seating group rather than through it. The calculator reports both orientations for exactly this reason.

What the strips have to be

• Primary walkway: 36″, and never below 30. That is the route through the room, and the test is whether someone can walk it while another person sits in the chair beside it.
• Secondary path: 24–30″ — behind an occupied seat, or between a sofa back and the wall. Fine as a second route, wrong as the only one.
• Under 24″ is not a path. Shoulder breadth is the binding dimension, and it is bigger than people expect.
• Sofa to coffee table: 14–18″. A pairing distance, not a route — it should not be judged against the 36″ figure, and it comes out of the open floor rather than out of the sectional.
• Conversation distance: 4–8 feet between facing seats. This is a convention rather than a published standard, so verify it in your own room. A U-sectional's two returns sit inside it by construction, which is most of why the shape works.

The five ways a sectional goes wrong

• The chaise on the wrong side. A chaise sectional's long leg flips left or right, and the two versions are not interchangeable — one of them lands the chaise across the doorway. Decide the side on the plan before the order. A return shipment of a sectional is a genuinely bad day.
• Measured as a bounding box in the showroom, laid out as legs at home. The number that matters is the run along each wall, and it is the number the listing usually buries.
• The rug one size down. Both legs of the L should land on it, which means a 9 × 12 in most rooms and an 8 × 10 at the absolute minimum. A rug that only reaches one leg makes the group look like an accident.
• Forgetting the U does not go in a corner. A U-shape puts its back run on one wall and pushes both returns into the room. It wants a wall, not a corner, and it wants the floor in front of it.
• Buying the size the wall takes rather than the size the room takes. A 140 × 84 U will sit on a 13-foot wall happily and leave you a 12-inch corridor to get to it.

What the bounding box does not know

The rectangle this calculator uses is deliberately pessimistic. The inside notch of an L is real, usable floor — it is where the coffee table goes, and it is a large part of why a sectional suits rooms a straight sofa does not. Whatever clear floor the arithmetic reports, you have a little more than that in practice.

What it cannot see is everything a plan can: the door and its quarter-circle of dead floor, the window you would rather not sit with your back to, the radiator, and where the television has to go. Sizes here come from the sofa & sectional guide; the layouts they sit in come from the living room layout guide.