What Size AC Do I Need? How a Load Calculation Answers It

The right air conditioner size comes from a heat gain calculation for your specific house, ACCA Manual J, followed by equipment selection under ACCA Manual S, not from square footage. The International Residential Code requires that approach, or another approved method, for heating and cooling equipment. ENERGY STAR’s design rules for new homes recommend that a single-speed air conditioner’s capacity land between 90% and 115% of the calculated cooling load.

Why square footage alone gets it wrong

Two houses with the same floor area can need very different equipment, which is why a question like “what size AC for 2,000 square feet” has no single correct answer. The Building America Solution Center, run by Pacific Northwest National Laboratory for the U.S. Department of Energy, says contractors can no longer rely on rules of thumb based on a rough estimate of square footage.

What changes the load is the house itself. ENERGY STAR’s guidance on hiring a contractor says the system should be sized based on the size of the house, the level of insulation and the windows, and a load calculation goes further than that, as the next sections show.

What the building code requires

The model code makes the calculation mandatory. Section M1401.3 of the 2024 International Residential Code (IRC) says heating and cooling equipment “shall be sized in accordance with ACCA Manual S or other approved sizing methodologies based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies.”

The energy code adds a ceiling. In Washington’s adoption of the International Energy Conservation Code (Section R403.7), equipment output may not exceed that of the smallest available size that covers the calculated loads, including the allowable oversizing limits.

The IRC allows two exceptions. Equipment that uses multistage or variable refrigerant flow technology is not limited to the Manual S capacities when the calculated loads fall within the manufacturer’s published capacity range. And when no standard size can satisfy both the total and the sensible heat gains, the next larger standard size is allowed.

States and cities adopt these codes on their own schedules and often amend them, so the local building department decides which edition applies.

What goes into the calculation

A Manual J calculation starts from design conditions: an indoor temperature to hold and an outdoor temperature to hold it against. ENERGY STAR’s design report for certified new homes uses 70°F indoors for heating and 75°F for cooling.

Outdoor design temperatures come from climate data for the location, and ENERGY STAR sets county-by-county limits on the values used. Its example is Frederick County, Virginia, with a cooling design limit of 93°F.

The same report lists the other inputs a calculation has to document:

• The number of occupants, set at the number of bedrooms plus one.
• The conditioned floor area.
• The window area, and the windows’ solar heat gain coefficient.
• The infiltration rate in summer and in winter, and any mechanical ventilation rate.
• The heat gain for each direction the house could face, plus the latent (moisture) load.

Floor area is one line on that list, not the whole calculation.

How much bigger than the load is acceptable

Some margin is allowed, and the limits depend on the equipment. ENERGY STAR’s National HVAC Design Report (Rev. 14, revised January 2025) has equipment selected under ACCA Manual S, but for now it permits cooling capacity above the Manual S limits, up to the “allowed” column below. It sets these ranges for cooling capacity as a share of the calculated cooling load:

Compressor type	Recommended	Allowed
Single-speed	90% to 115%	90% to 130%
Two-speed	90% to 120%	90% to 140%
Variable-speed	90% to 130%	90% to 160%

For furnaces, the same report recommends an output of 100% to 140% of the calculated heat loss.

Heat pumps are sized for two seasons at once. In what Manual S calls Condition B climates, where heating degree days are at least twice the cooling degree days and the cooling load is at least 95% sensible, the report allows a heat pump’s cooling capacity of 90% to 100% of the load plus 15,000 Btu/h. The Building America Solution Center describes four sizing approaches for cold climates, from sizing to the cooling load and relying on backup heat, to sizing for the full heating load at the local design temperature.

What oversizing does

A system that is too large does not simply cool faster. In the words of the Building America Solution Center, an oversized system “will turn on and bring the air temperature down below the thermostat set point quickly then shut off before the system has had time to remove moisture from the air,” which can cause high humidity problems, especially in humid climates.

On the heating side, DOE’s Federal Energy Management Program lists oversizing among the installation problems that cause efficiency losses, occupant discomfort and shortened equipment life.

When the house has changed

A house that has been air sealed, insulated or fitted with new windows since the old system went in has a different load. DOE’s Federal Energy Management Program notes that sealing and weather-stripping may allow a lower-capacity furnace to be selected.

That is one reason the size printed on the old equipment is not an answer. The calculation describes the house as it is now, including work done since the last replacement, and the inputs listed above are where those changes show up.

Ducts are part of the size question

The equipment and the ducts have to match. ENERGY STAR’s design report requires the duct system for certified new homes to be designed for the selected equipment under ACCA Manual D.

In an existing house, ENERGY STAR advises that a good contractor inspects the ducts for air leaks and insulation and measures airflow to make sure it meets the manufacturer’s specifications.

When a professional is needed, and what to ask

Sizing is part of any replacement, and it is the contractor’s job to show the work. Ask for the calculation itself, not only the tonnage:

• Can I have a copy of the Manual J report for this house, with the design temperatures, window areas and insulation levels it assumes?
• What capacity will the proposed equipment deliver at those design conditions, according to the manufacturer’s expanded performance data?
• What percentage of the calculated load is that, and does it fall within the ENERGY STAR ranges above?
• Were the ducts evaluated, and was airflow measured?
• If two contractors propose different sizes, what did each calculation assume? Comparing the inputs side by side shows where the two reports part ways.
• Does the local building department require the calculation with the permit application?