Heat Pump Sizing Calculator
Size a heat pump in tons from your home area and climate, and estimate how much supplemental heat you may need on the coldest days.
How it works
Cooling load = area × cooling factor · Tons = load ÷ 12,000 · Cold-weather output = nominal × derate% · Supplemental = heating load − cold-weather output
A heat pump is normally sized to the cooling load, because an oversized unit dehumidifies poorly in summer. In winter its heating output shrinks as the outdoor temperature drops — often to 60–80% of nameplate at the design low — so the heating load may exceed what the unit can deliver on the coldest days. The gap is covered by supplemental heat, usually electric resistance strips or a dual-fuel furnace, which this estimate flags.
Worked example
An 1,800 ft² home at 25 BTU/ft² cooling needs 45,000 BTU/hr (3.75 tons), rounded to a 48,000 BTU/hr unit. Its heating load at 35 BTU/ft² is 63,000 BTU/hr; at 70% output the unit gives 33,600 BTU/hr, so about 29,400 BTU/hr of backup heat is needed at design conditions.
Frequently asked questions
Should I size a heat pump for heating or cooling?
Size to the cooling load in most climates so the unit dehumidifies well in summer, then add supplemental heat to cover the shortfall on the coldest winter days. In very cold climates with modern cold-climate heat pumps, some contractors size closer to the heating load.
Why does a heat pump lose capacity in the cold?
It moves heat from outdoor air, and there is simply less heat to extract as that air gets colder. Output and efficiency both fall as the temperature drops, which is why backup heat is common in cold regions.
What is supplemental or auxiliary heat?
It is a secondary heat source — electric resistance strips or a gas furnace in a dual-fuel setup — that switches on when the heat pump alone cannot meet the load. This estimate shows roughly how much of it your design day may require.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.