How furnace sizing actually works
Start with the heat your house loses on a cold day. The quick rule of thumb is heated square footage × BTU per square foot for your climate, nudged up or down for insulation quality and ceiling height. That figure is the output your furnace has to deliver into the ductwork, hour after hour, on the coldest night of the year.
Furnaces, though, are usually badged by their input rating, and no furnace turns every input BTU into useful heat. That is what AFUE measures: an 80% AFUE furnace delivers 80 BTU of heat for every 100 BTU of fuel it burns, so it needs an input rating well above your heat load. The formula is input BTU = required output ÷ (AFUE ÷ 100). A 60,000 BTU heat load needs a 75,000 BTU input furnace at 80% AFUE, but only about 63,000 at 95%. Since furnaces are sold in steps (40,000, 60,000, 80,000, 100,000 and up), you round up to the next size rather than buying a custom capacity.
Resist the urge to oversize "just in case". An oversized furnace short-cycles: it blasts to temperature, shuts off, and starts again a few minutes later. That gives you uneven room temperatures, more wear on the igniter and blower, and worse efficiency than the label promises. Right-sizing, sealing air leaks and adding attic insulation all shrink the load, which is why an insulation upgrade often lets you drop a furnace size.
This is a planning estimate only. Before you buy, have an HVAC professional confirm the size with a Manual J load calculation, which accounts for your actual windows, orientation, duct losses and local design temperature.