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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 to use

  1. Enter your values in the fields above.
  2. Press Calculate to see your result instantly.
  3. Use the Share button to copy a link to your result.

About this calculator

This calculator estimates furnace heating capacity using the standard HVAC rule-of-thumb of BTU per square foot by climate zone — roughly 30–35 BTU/sq ft in warm southern climates up to 50–60 BTU/sq ft in the coldest northern 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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