Furnace Size Calculator

A furnace is sized by the heat a house loses on a cold day, and the quick version of that is the square-foot rule: a set number of BTU per square foot for the climate zone, adjusted for insulation and ceiling height. Enter the heated floor area and pick a zone, and it returns the heating load, the input rating a furnace needs at the AFUE you give and the nearest common furnace size. A second mode takes a heating load you already have and does only the efficiency and sizing step. The rule of thumb is a check rather than a design, so the equipment that actually goes in should be sized from a Manual J load calculation done by the installer.