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Furnace BTU Sizing Calculator

Enter your home area in square feet and select your climate zone to size the furnace in BTU/h.

How it is calculated

The tool multiplies the heated floor area in square feet by a fixed heating factor for the climate zone you pick: BTU/h = area × factor. The factors are 30 for hot, 35 for warm, 40 for moderate, 45 for cold and 50 for very cold climates, in BTU per square foot. An unrecognised or missing zone falls back to moderate (40). The second figure divides the result by 12,000 BTU/h to give tons of capacity, with two decimals. Area must be above zero. This is only a rule of thumb: insulation, windows, ceiling height, air leakage and the furnace efficiency rating are not considered, so use it for a first estimate, not a final selection.

Worked example

A house has 2,000 sq ft of heated space in a cold climate zone, factor 45. Heating capacity: 2,000 × 45 = 90,000 BTU/h. In tons: 90,000 / 12,000 = 7.50 tons. If the same house were in a moderate zone, the factor of 40 would give 80,000 BTU/h.

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Common mistakes

  • Entering the area in square metres. The factors are per square foot, and 1 m² is about 10.76 ft².
  • Including unheated areas such as an unconditioned garage or an open attic in the area.
  • Choosing the zone by how cold winters feel instead of the real design conditions for your location; ask a local installer if unsure.
  • Reading the result as the furnace's rated input. Heat output depends on efficiency, so a unit's label capacity and its delivered heat differ.
  • Buying the exact figure. An undersized furnace runs constantly and an oversized one short-cycles, so treat this as a starting point for a proper load calculation.

About this calculator

This calculator estimates furnace heating capacity using the standard HVAC rule-of-thumb of BTU per square foot by climate zone — from 30 BTU/sq ft in hot climates, through 35 (warm), 40 (moderate) and 45 (cold), up to 50 BTU/sq ft in very cold zones — multiplied by your home's square footage to get the required output in BTU/h.

HVAC contractors use this kind of zone-based estimate as a quick first pass before running a full Manual J load calculation, which also accounts for insulation, window area and ceiling height. Getting the size right matters: an undersized furnace runs constantly and can't keep up on the coldest days, while an oversized one short-cycles, wasting energy and wearing out components faster.

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Comments (1)

  • Editorial team

    Each step between climate zones here changes the factor by 5 BTU per square foot, which for a 2,000 sq ft home is a 10,000 BTU/h swing. That is why the zone choice matters more than rounding the area. If you are replacing a furnace, compare the result with the output figure on the old unit's nameplate before trusting either. What zone are you in, and does the figure match your current unit?

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