What Size Heat Pump, AC or Furnace
Do I Need?

The Short Answer

The right size is the one that matches your home's heat loss on a cold design day and its heat gain on a hot one. Those two numbers come from a load calculation, usually ACCA Manual J, and the equipment is then chosen to fit them, usually with ACCA Manual S. Floor area is one input among many, which is why this guide has no square-foot-to-tonnage table: no primary source we could find publishes one for central systems. This guide is about homes; for offices, shops and other businesses, see our guide to HVAC for commercial buildings.

Tons and Btu per Hour

Heating and cooling capacity is a rate: how much heat the equipment moves in an hour, measured in British thermal units per hour (Btu/h). Air conditioners and heat pumps are also sold in tons. One ton of refrigeration is 12,000 Btu/h, about 3.5 kW (NIST Guide to the SI, Appendix B.9, read October 22, 2026). So a 3-ton air conditioner moves 36,000 Btu/h, and a 24,000 Btu/h unit is a 2-ton unit. Furnace capacity is given in Btu/h.

Why Square-Foot Rules of Thumb Mislead

A rule that gives one tonnage per so many square feet treats a leaky house with single-pane windows the same as a tight one with new insulation, and a house in Phoenix the same as one in Seattle. The heat a home gains or loses depends on its climate, its walls, attic and windows, which way the glass faces, how much air leaks in, and where the ducts run. ENERGY STAR's quality installation guidance asks homeowners to make sure the contractor uses the actual characteristics of the home to choose a right-sized system and does not just rely on a rule of thumb (ENERGY STAR, HVAC quality installation, read October 22, 2026).

The size of the old unit is a rule of thumb too. If it was oversized when it went in, or the house has since had new windows, insulation or air sealing, a like-for-like replacement carries the old mistake forward.

Manual J, Manual S and Manual D

The Air Conditioning Contractors of America (ACCA) publishes the three manuals most residential sizing follows:

• Manual J calculates the load: how many Btu/h the home loses and gains at its local design temperatures. ACCA calls the 8th edition (ANSI/ACCA 2 Manual J, 2016) the national ANSI-recognized standard for producing HVAC equipment sizing loads for homes (ACCA, Manual J).
• Manual S selects the equipment to meet that load. ACCA describes it as a guide for selecting and sizing residential heating, cooling, dehumidification and humidification equipment, with size limits that account for regional outdoor conditions. The current 3rd edition (ANSI/ACCA 3 Manual S, 2023) added wider size tolerances for variable-capacity heat pumps (ACCA, Manual S).
• Manual D sizes the ducts, so the airflow the equipment needs can reach each room (ACCA, Manual D, ANSI/ACCA 1 Manual D, 2016). More on ducts in our ductwork installation guide.

All three manuals were read on ACCA's site on October 22, 2026.

What the Codes Say

The International Residential Code, a model code many states and cities adopt, says in section M1401.3 that heating and cooling equipment shall be sized in accordance with ACCA Manual S or other approved sizing methods, based on building loads calculated in accordance with ACCA Manual J or other approved methods (IRC M1401.3, 2024 edition as adopted in Texas, via UpCodes, read October 22, 2026). Its exception lets multistage and variable-capacity equipment exceed the Manual S limits when the calculated loads fall within the manufacturer's published capacity range.

Energy codes carry a similar rule. Washington's residential energy code, section R403.7, requires sizing with Manual S from a Manual J or other approved load calculation (WAC 51-11R-40360, read October 22, 2026). Each city or county adopts its own code edition and amendments, so the permit office is the place to confirm what it asks for on a replacement. In Chicago, for example, the City's Express Permit energy form asks for new heating and cooling equipment in residential buildings up to four stories to be sized with Manual S from a Manual J load calculation; our Chicago HVAC installation page links the form.

What Goes Wrong When the Size Is Off

Too big

ENERGY STAR warns that oversized equipment may cycle too frequently, resulting in less comfort and a shortened lifespan (ENERGY STAR, HVAC quality installation). It adds that a conventional system that is too large may not keep the home comfortable because of frequent on/off cycling, causing humidity control problems and inefficient operation, and that equipment with variable-speed compressors does a better job of compensating for oversizing (ENERGY STAR, clean heating and cooling, both read October 22, 2026). As that page puts it, bigger is not always better when buying new heating and cooling equipment.

Some inspectors check for it. Miami-Dade County's inspection guidelines check that cooling equipment is no larger than 1.15 times the calculated load, as our Miami page explains with its source. In hot-humid climates like Miami's and Dallas's, removing moisture matters as much as lowering the temperature, so the humidity problems an oversized unit causes are felt most.

Too small

An undersized system runs flat out on the hottest or coldest days and still falls behind. For heat pumps, Pacific Northwest National Laboratory (PNNL) puts it plainly: with undersized heating capacity the system will not be able to keep the house warm enough during the coldest times, and the backup heat will make up the difference at a much lower efficiency (PNNL Building America Solution Center, Cold Climate Heat Pump Sizing and Selection, updated May 14, 2024, read October 22, 2026).

Heat Pumps in Cold Climates

A heat pump has to cover two loads with one machine, and in a cold climate they rarely match. PNNL notes that in most areas of the United States, right-sizing a heat pump for cooling will result in an undersized heating capacity. Three terms come up in any cold-climate quote:

• Balance point: the lowest outdoor temperature at which the heat pump can provide all of the home's heating. Below it, a backup source has to help, and PNNL calls electric resistance backup an inefficient and typically expensive form of heat (PNNL).
• Capacity at low temperatures: a heat pump delivers less heat as it gets colder, so the number that matters is its output at your design temperature, not its nominal tonnage. ENERGY STAR's cold-climate criteria test at 5°F, where a unit must keep at least 70% of its heating capacity at 47°F and reach a coefficient of performance of at least 1.75 (ENERGY STAR heat pump criteria, read October 22, 2026).
• The NEEP list: Northeast Energy Efficiency Partnerships keeps a cold-climate air-source heat pump product list, now under specification Version 4.0 (effective January 1, 2023), meant to help designers confirm that equipment has the required heating capacity at design temperature without unnecessary oversizing (NEEP ccASHP list, read October 22, 2026).

PNNL says a properly sized cold-climate heat pump can meet 100% of a home's heating load in nearly any location in the United States, and that variable-capacity equipment lets designers size closer to the heating load because an oversized unit can still run at lower capacity instead of cycling on and off. The choice between covering the whole heating load and sharing it with a furnace changes the size, so decide which you want before comparing quotes. Our heat pump installation guide covers the rest of the job, and the heating cost calculator compares a year of heating with a heat pump, a gas furnace and electric resistance in your state. For rooms or homes without ducts, our ductless mini-split guide covers the options.

What You Can Gather Before a Quote

• Past utility bills, ideally a year or more of gas, oil or electricity use. They are a rough check on the load, not a replacement for it: in a Massachusetts Clean Energy Center pilot, gas use was a useful check on installers’ Manual J results but was skewed by an old boiler and by occupants who kept the heat high
• What you know about the house: insulation levels in the attic and walls, window type and age, and any air sealing or energy audit, including a blower door result if you have one. PNNL calls a blower door test the most accurate way to measure a home’s air leakage
• Rooms that run too hot or too cold now, which can point to duct problems a bigger unit will not fix
• The size and model numbers of your current equipment, so you can see whether the new calculation agrees with it
• Your outdoor design temperatures, the heating and cooling conditions the calculation sizes for

Sources for the first two items: the MassCEC whole-home heat pump pilot update (June 16, 2020) and PNNL. For design temperatures, some of our city pages list the figures from the state's own design data:

The California figures come from the California Energy Commission's 2022 Reference Appendices and the Washington figures from the state energy code's design temperature table; each city page links its source.

Questions to Ask About the Load Calculation

• Will you do a room-by-room load calculation, and which method: Manual J or another?
• Can I have a copy of the calculation, with the design temperatures and the assumptions about insulation, windows and air leakage it uses?
• How does the equipment you propose compare with the calculated heating and cooling loads, and why that size?
• For a heat pump: what is its heating capacity at our design temperature, what is the balance point, and what provides backup heat below it?
• Will the existing ducts carry the airflow the new equipment needs, and was that checked against Manual D?
• If the size differs from my current unit, what changed?

ENERGY STAR also asks homeowners to make sure the contractor verifies the system size with Manual J and uses current design and installation standards: ACCA Manuals J and S, and ACCA Standard 5 (ENERGY STAR). When quotes propose different sizes, the calculation behind each is the fairest way to compare them. For the efficiency numbers on the same quotes, see SEER2, EER2 and HSPF2 explained, and for what each job involves, our guides to AC installation and furnace installation.

Sources

• NIST SP 811, Appendix B.9: ton of refrigeration (12,000 Btu/h)
• ACCA: Manual J, Residential Load Calculation (ANSI/ACCA 2 Manual J, 2016)
• ACCA: Manual S, Residential Equipment Selection (ANSI/ACCA 3 Manual S, 2023)
• ACCA: Manual D, Residential Duct Systems (ANSI/ACCA 1 Manual D, 2016)
• International Residential Code M1401.3, equipment and appliance sizing (via UpCodes)
• Washington WAC 51-11R-40360, section R403.7, equipment sizing (via Cornell LII)
• ENERGY STAR: HVAC quality installation
• ENERGY STAR: clean heating and cooling
• ENERGY STAR: air-source heat pump key product criteria, including cold-climate criteria
• PNNL Building America Solution Center: Cold Climate Heat Pump Sizing and Selection (updated May 14, 2024)
• NEEP: cold-climate air-source heat pump specification and product list
• Massachusetts Clean Energy Center: whole-home heat pump pilot update (June 16, 2020)

All sources read October 22, 2026.