Furnace Size Calculator
Choosing the right furnace size is critical for comfort and efficiency. An oversized furnace short-cycles and wastes fuel, while an undersized one runs constantly without reaching your desired temperature. This calculator determines the proper BTU capacity based on your home's size, climate zone, insulation quality, and ceiling height.
The calculation uses heating load per square foot values that range from 25 BTU/sq ft in mild climates (Zone 1) to 52 BTU/sq ft in very cold climates (Zone 5). These values are based on ACCA Manual J guidelines and represent the BTU needed to maintain 68-70°F indoors during the coldest design day in each zone.
Insulation quality has a major impact. A poorly insulated home can require 30% more heating capacity than the baseline, while an energy-efficient home with modern insulation and windows may need 30% less. If you're unsure of your insulation level, "average" is appropriate for most homes built between 1980-2010.
Standard residential furnaces come in sizes from 40,000 to 140,000 BTU (input rating). The AFUE rating (Annual Fuel Utilization Efficiency) determines how much of that input becomes actual heat. A 95% AFUE furnace converts 95 cents of every fuel dollar to heat. Modern high-efficiency furnaces achieve 95-98% AFUE, while older models may be as low as 60-80%.