Furnace BTU Sizing Calculator

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.

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.