Heat Pump Size Calculator

How the heat pump size estimate works

Multiply the conditioned floor area by the BTU per square foot for the climate, choosing the low end of the range for a tight, well insulated house, the middle for an average one and the high end for a leaky or older one. Scale it by the ceiling height. Divide by 12,000 for tons, then take the smallest nominal size, in half-ton steps from 1.5 to 5 tons, that covers it.

Worked example: 1,800 sq ft in zone 4 (moderate), average insulation, 8 ft ceilings. The range is 20 to 25 BTU per square foot, so the middle is 22.5: 1,800 × 22.5 = 40,500 BTU per hour, 3.38 tons. The nominal size that covers it is 3.5 tons.

In a cold zone (5 to 7) the calculator adds a heating check. A cold-climate heat pump at the ENERGY STAR minimum keeps 70% of its 47°F capacity at 5°F, so to deliver 54,000 BTU per hour at 5°F it needs a 47°F heating rating of at least 54,000 ÷ 0.70 = 77,143 BTU per hour. If you have a design heating load from a Manual J or the heat loss calculator, type it in and the check uses it in any zone.

BTU per square foot by climate zone

Carrier's figures by climate, matched to the IECC climate zones by this calculator. Carrier gives about 20 BTU per square foot as standard in a moderate climate; the calculator takes 20 to 25 there, the lower half of Carrier's general 20 to 30 range. The example places come from the DOE Building America county guide.

Heat pump size chart by square footage

Carrier's chart, a general estimate it says is no substitute for a Manual J load calculation. A ton is about 12,000 BTU per hour of heating or cooling, and most homes use 1.5 to 5 tons.

One unit, two loads

A furnace only heats and an air conditioner only cools, but a heat pump does both, so its size has to fit two loads. In a hot climate the cooling load usually sets the size. In a cold one the heating load is often far bigger than the cooling load, and a unit big enough for January can be too big for July, short cycling and leaving the air damp. Variable-speed and staged units turn down to soften that.

That is why this page shows the size range for the zone and, where it matters, the heating rating to look for. If those two point to very different sizes, that is the question for the load calculation.

Cold climates: read the capacity at 5°F

A heat pump moves heat from the outdoor air, and there is less of it to move on a cold night, so its heating output falls as the temperature drops. ENERGY STAR's cold climate criteria set a floor: at 5°F the unit keeps at least 70% of its 47°F heating capacity, with a COP of at least 1.75. The model's spec sheet shows what it actually delivers at 5°F and at other temperatures.

The Building America guide from the Department of Energy and PNNL sets out approaches running from sizing for cooling, with the warmest balance point (the coldest outdoor temperature at which the unit carries the whole house) and the most backup heat, to sizing for the design heating load at the location's design temperature, with the coldest balance point. Backup electric resistance heat makes up any shortfall at much lower efficiency, so the guide says to keep it to a minimum. It also warns not to select equipment on nameplate capacity.

Where Manual J comes in

BTU per square foot cannot see the windows, the duct runs, the attic or the local design temperatures. ANSI/ACCA Manual J can: it works the heating and cooling loads room by room, and Manual S then selects equipment whose capacity at the design conditions fits them. This page is an estimate, not a substitute for that calculation. Use it to get a first number, to check a quote that looks far off, and to know what to ask the contractor.

For HVAC and home services reps: what to write down

The size comes from the load calculation. The sale comes from what the homeowner tells you: which rooms are cold or hot, what the bills look like in winter and summer, what heats the house now, whether they want to keep the furnace as backup, and who else decides. Write it down while you are in the house, and read it back before you send the quote.

Sources, read on 2026-10-29:

• Carrier: What size heat pump do I need? (square feet to BTU and tons chart, 1.5 to 5 tons, Manual J)
• Carrier: 2-ton heat pump guide (BTU per square foot in moderate, hot and cold climates)
• ENERGY STAR: air-source heat pump key product criteria (cold climate: 70% of 47°F capacity at 5°F, COP 1.75)
• DOE Building America Solution Center (PNNL): Cold climate heat pump sizing and selection
• DOE Building America: Guide to Determining Climate Regions by County, Vol. 7.3 (example counties)
• PNNL: Guide to Determining Climate Zone by County, Building America and IECC 2021 Updates (look up your county)

Keeping the furnace, or replacing it? The furnace size calculator sizes the gas side. For a design heating load from the walls, windows and air leaks, use the heat loss calculator; for a single room or a room air conditioner, the BTU calculator.

Related tools

• Furnace Size CalculatorEstimate the furnace BTU output and input a house needs from its size, climate zone, insulation and the furnace's AFUE, as a rule of thumb before a load calculation.Open the tool
• Heat Loss CalculatorEstimate a room's or home's heat loss in BTU per hour from walls, windows, ceiling, floor and air leakage, to size heating before a full load calculation.Open the tool
• BTU and AC Size CalculatorEstimate the BTUs and air conditioner tonnage a room or home needs from its size, sun, shade and occupants, as a rule of thumb before a load calculation.Open the tool
• Square Footage CalculatorWork out square footage for rectangles, L-shaped rooms, triangles and circles, add rooms together, and convert to square yards, square meters and acres.Open the tool