Furnace Size Calculator

What size furnace do I need?

BTU/h
%

Your furnace needs at least

48,000BTU/h output minimum

Manual S allows furnace output from 100% to 140% of the design heating load. Gas work and venting belong to a licensed contractor, and local code decides.

Allowed furnace output and input at this AFUE67,200 max outputDesign load: 48,000 BTU/hAllowed oversize: 19,200 BTU/hFuel input above output: 84,000 BTU/h max input
Maximum output67,200 BTU/h
Input at this AFUE, minimum60,000 BTU/h
Input at this AFUE, maximum84,000 BTU/h

Turn your heating load into the allowed furnace output range and the input rating you will see on a spec sheet.

How it works

Start with a Manual J design heating load. Manual S allows furnace output between 100% and 140% of that load. Dividing output by AFUE gives input.

Before you order

  • Get a real load calculation. Rules of thumb per square foot commonly oversize furnaces.
  • A furnace too large short-cycles and runs less efficiently.
  • Airflow, duct size and venting matter as much as the BTU number.

Common questions

What is the difference between input and output?

Input is the fuel burned. Output is the heat delivered. An 80% AFUE furnace turns 100,000 BTU input into about 80,000 BTU output.

Can I just match my old furnace?

Old furnaces are often oversized, and insulation or window changes alter the load. Have the load calculated first.