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Heat Pump Size Calculator
Quick answer: For a rough first estimate, multiply conditioned floor area by a climate factor of about 25 to 55 BTU per hour per square foot, then adjust for insulation and ceiling height. Divide by 12,000 for tons. It is a planning number only; a licensed contractor should run a room-by-room load calculation before you buy.
How it works
The calculator starts from floor area and applies a climate factor that rises as winters get colder, because a heat pump must replace the heat a house loses on the coldest typical day. It then scales for insulation and air sealing, since a drafty older house can lose a fifth more heat than an average one, and for ceiling height, because taller rooms hold more air. The result is shown in BTU per hour, tons and kilowatts. Floor-area rules tend to run high, so treat the number as an upper planning range. A proper ACCA Manual J (or CSA F280 in Canada) calculation looks at windows, walls, orientation, ducts and infiltration, and very often lands lower. Once you have a figure, compare it with the unit's capacity at your design low temperature, not only the headline rating at 47°F. Read cold-climate heat pumps for why that matters, and convert units with the BTU to tons calculator.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: 1,800 sq ft, moderate climate (35), good insulation (0.85), 8 ft ceilings. Load ≈ 1,800 × 35 × 0.85 ≈ 53,550 BTU/h, about 4.5 tons. A room-by-room calculation on a tight house could come in well below that, which is why a contractor should confirm it.
FAQ
Why not just match my old furnace size?
Many older furnaces were oversized. A heat pump sized to an old furnace can short-cycle and cost more.
What happens on days colder than the design low?
Output falls and backup heat may run. See backup heat strips.
Who should do the final sizing?
A licensed HVAC contractor using Manual J or CSA F280.
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