Heat Pump Size Calculator (BTU & Tons)
Size a heat pump on its cooling load — the same area × climate-band method as an AC — and remember the catch that furnaces do not have: heat-pump capacity drops as it gets colder outside.
Calculator
Size a heat pump at about 3.0 tons (38,100 BTU/h) for cooling. Heating capacity drops as it gets colder — confirm the unit's capacity table and a Manual J before buying.
A heat pump is an air conditioner that can run in reverse, so it is nominally sized on the same cooling load — area × climate band, plus the usual sun, occupancy, kitchen and ceiling adjustments. The number this tool returns is the cooling tonnage, expressed in BTU/h, tons and airflow.
Heating is where heat pumps differ from furnaces. A furnace fires the same regardless of outdoor temperature, but a heat pump’s output derates as it gets colder — a nominal 3-ton unit may deliver well under 3 tons of heat at 17 °F. That is why cold climates use cold-climate heat pumps, backup heat, or a dual-fuel setup, and why you must check the unit’s capacity table at your design temperature.
Formula
Cooling load drives the nominal size:
cooling_BTU = area × climate_band\n × sun_factor\n × ceiling / 8 (only when ceiling > 8 ft)\n + 600 × max(0, occupants − 2)\n + kitchen_BTU\ntons = round(cooling_BTU / 12,000 to the nearest 0.5)
Heating capacity is not a fixed multiple of this: consult the manufacturer’s capacity-vs-temperature table. Basis: 1 ton = 12,000 BTU/h. See Sources.
Worked example
A 1,500 sq ft home in a moderate climate (25 BTU/sq ft), average sun, three occupants, 8 ft ceiling:
base = 1,500 × 25 = 37,500 BTU/h\npeople + 600 × (3 − 2) = 38,100 BTU/h\ntons = 38,100 / 12,000 ≈ 3.2 → 3.0 tons
The unit is nominally 3 tons for cooling. At a 17 °F design temperature that same nameplate might deliver only ~2 tons of heat, so a cold-climate model or supplemental heat may be required. Confirm with the capacity table and a Manual J.
Cold-weather capacity
Capacity derating. Heat-pump heating output falls with outdoor temperature while the home’s heat loss rises — the two curves cross at the balance point. Below it you need backup heat. Cold-climate (ccASHP) units hold capacity far better and are the right choice in cold zones.
Refrigerant is a pro job. Heat pumps are sealed refrigerant systems. Charging or servicing refrigerant requires EPA Section 608 certification; DIY charging is illegal. This tool sizes capacity only and does not cover refrigerant work.
Reference table
| Climate band | BTU/sq ft | Cooling load for 1,500 sq ft | ≈ Tons |
|---|---|---|---|
| Cool / northern (~20 BTU/sq ft) | 20 | 30,000 BTU/h | 2.5 |
| Moderate / mixed (~25 BTU/sq ft) | 25 | 37,500 BTU/h | 3.0 |
| Warm / southern (~30 BTU/sq ft) | 30 | 45,000 BTU/h | 4.0 |
| Hot / desert (~35 BTU/sq ft) | 35 | 52,500 BTU/h | 4.5 |
Nominal cooling tonnage. Heating output derates in the cold — always read the unit’s capacity table.
Frequently asked questions
Is a heat pump sized on heating or cooling?
Nominally on cooling, using the same area × climate-band method as an AC. Then you verify heating output at your design temperature, because capacity derates as it gets colder.
Why does heat-pump capacity drop in cold weather?
There is less heat in cold outdoor air for the refrigerant to absorb, so output falls as temperature drops — right when your home needs the most heat. Cold-climate models and backup heat address this.
What is the balance point?
The outdoor temperature where the heat pump’s output equals the home’s heat loss. Below it, supplemental heat is needed. Estimate it with the balance-point calculator.
Do I need a bigger heat pump than an AC for the same house?
The nominal cooling tonnage is the same. In cold climates you may choose a cold-climate unit or add backup heat rather than simply oversizing, since oversizing hurts summer humidity control.