Furniture stores are used to dealing with this question and will give some help, but they will not always be correct.
I have found that moving companies are usually more expert at knowing how to calculate what furniture will fit through cramped spaces.

If you want to be certain that a sofa will fit through a doorway, look for one that is sold unassembled. There are now several brands that make lower cost RTA (Ready to Assemble) sofas for under $1000.

There is also one company that specializes in high end self-assembly upholstered furniture. Simplicity Sofas makes custom sofas, sleepers and sectionals ranging in price from $1800 – $10,000. Everything Simplicity Sofas makes is guaranteed to fit through 15 inch width doors and stairways.

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