WeekendCalc

Furnace Size Calculator

Enter your heated floor area and climate. You get the heating output the house needs in BTU per hour, and the standard furnace size to buy at your efficiency.

Your project

Results

Furnace size to shop for (input rating)
100,000 BTU/hr
Heat the house needs (output)
80,000 BTU/hr
Input rating needed at 96% AFUE
83,333 BTU/hr
In kilowatts of heat
23.4 kW
  • This is a rule-of-thumb estimate based on BTU per square foot by climate. A proper sizing uses a Manual J load calculation by a contractor, which accounts for windows, insulation, air leakage and ductwork. An oversized furnace short-cycles, wastes fuel and gives uneven heat.

How this calculator works

A furnace has two ratings: its input (the fuel it burns, in BTU per hour) and its output (the heat it delivers). The efficiency (AFUE) is the ratio between them: a 96% furnace delivers 96,000 BTU for every 100,000 BTU it burns. The house needs a certain amount of heat in the coldest weather, which depends on its size, the climate and how well it is insulated.

output needed = area × BTU per ft² for the climate × insulation factor × (ceiling ÷ 8) input = output ÷ AFUE

Worked example: a 2,000 ft² house in a mixed climate (40 BTU per ft²) needs 80,000 BTU/hr of heat. A 96% AFUE furnace needs 83,333 BTU/hr of input, so the next standard size is 100,000 BTU/hr. An 80% AFUE furnace would need 100,000, which is also a 100,000 unit.

Tips before you buy

  • Do not just match the old furnace. Insulation upgrades, new windows and air sealing since it was installed often mean that a smaller furnace is enough now.
  • Oversizing is the most common mistake. The furnace short-cycles, which wears it out, wastes fuel and makes rooms uneven. Two-stage or modulating furnaces handle mild days much better.
  • Ask the contractor for a Manual J load calculation, and a check of the ductwork, instead of quoting by square footage alone.
  • High-efficiency (90%+) furnaces need a way to drain condensate, and use plastic venting. Factor it into the installed cost.
  • A heat pump may heat more cheaply than a furnace in moderate climates: ask for a quote.

Quick answers by size

1,000 sq ft60,000 BTU/hr1,500 sq ft80,000 BTU/hr2,000 sq ft100,000 BTU/hr2,500 sq ft120,000 BTU/hr3,000 sq ft140,000 BTU/hr3,500 sq ftOver 140,000 BTU/hr: two furnaces or a custom system

Tools & materials for this project

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Furnace filters (16x25x1)Programmable thermostatDuct sealing mastic tapeCarbon monoxide alarmWeatherstripping kit

Frequently asked questions

What size furnace do I need for 2,000 square feet?

About 80,000 BTU/hr of heat output in a mixed climate (40 BTU per ft²), which is a 100,000 BTU input unit at 80 to 96% efficiency. In a cold climate it could be 100,000 BTU output or more.

How many BTUs per square foot do I need?

About 25 to 35 BTU per square foot in hot climates, 35 to 45 in mixed climates, and 45 to 60 in cold ones. The calculator applies these as a first estimate.

What does AFUE mean?

Annual Fuel Utilization Efficiency: the share of fuel energy that becomes heat in the house. An 80% furnace wastes 20% up the flue, and a 96% furnace wastes only 4%.

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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.