Enter your home size, climate zone, and insulation level. You get a recommended furnace output range in BTU, with the math shown so you can sanity-check any contractor quote.
A furnace is sized in BTU per hour: how much heat it can put out. Too small and it never keeps up on the coldest days. Too big and it short-cycles, wears faster, and heats unevenly. The goal is a furnace matched to your home's actual heat loss.
The fast method multiplies your heated square footage by a climate-zone factor. Warmer regions need roughly 32 to 40 BTU per square foot; colder regions need 50 to 60. Insulation, windows, and air sealing move you within that band, which is why the calculator returns a range rather than one number.
For a sizing-critical install, especially a high-efficiency or modulating system, ask your contractor for a room-by-room Manual-J load calculation. It accounts for window area, orientation, ceiling height, and measured air changes. Use this calculator to get in the right range and to check that a quote is not wildly oversized.