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Furnace BTU Sizing Calculator

Estimate required furnace heat output and input BTU per hour from home square footage, a climate factor, and AFUE before getting a professional load calculation.

Screening estimate only.A professional Manual J load calculation is authoritative for final furnace sizing and equipment selection. Confirm every result with a licensed HVAC professional.

Screening result

Enter square footage to estimate furnace heat output BTU/h.

Fine Print and Sources

Screening estimate only. These tools use only the values you enter and simple planning math. A professional Manual J load calculation is authoritative for equipment sizing, room loads, and final purchase decisions. Confirm with a pro before you buy.

Method: required heat output BTU/h equals square footage times the climate factor you pick (30, 40, 50, or 60 BTU per sq ft). Input BTU/h equals heat output divided by AFUE, since a furnace burns more fuel than it delivers as heat. Per-square-foot factors are broad screening rules and vary with insulation, windows, air sealing, and local design temperature.