What Size AC Do I Need?

Step by step

1. Gather the basics: square footage, the age of the house, any insulation or window upgrades, and where the ducts run.
2. Note problem rooms, such as west-facing bedrooms or a hot upstairs, and how the current system performs on 100°F days.
3. Ask each contractor for a Manual J load calculation and a copy of the results.
4. Compare the recommended size with the calculation, not just with the old unit's label.
5. Ask whether the installer will verify airflow, which affects how large a system California's Energy Code allows.

What does AC tonnage mean?

AC size is measured in tons of cooling, and one ton equals 12,000 BTUs per hour, as Fixr explains. Home central systems commonly range from about 1.5 to 5 tons; This Old House's price tables run across that span. A ton describes how much heat the system can remove, not how much it weighs.

Why doesn't a square-foot rule work?

The Department of Energy says the common estimate of one ton for every 400 to 500 square feet fails to account for local climate and the efficiency of the house, and that one ton can condition 800 to 1,000 square feet of a very efficient home. Two houses of the same size can need very different systems.

Insulation, window area and direction, shade, air leaks, ceiling height, and duct location all change the load. That's why the DOE says contractors should use ACCA Manual J.

What goes into a Manual J load calculation?

Manual J, published by the Air Conditioning Contractors of America (ACCA), is the standard method for calculating how much heating and cooling a home needs. The contractor enters the home's dimensions, insulation levels, window sizes and orientation, air leakage, duct location, and the local design weather conditions, room by room.

The result tells the contractor how many BTUs each room needs, which guides both the equipment size and the duct design. Ask for a copy. A contractor who sizes by the old unit's label or by square footage alone is skipping this step.

What happens if your AC is too big or too small?

Bigger isn't better. The Department of Energy says oversized cooling systems can cost too much, provide inadequate comfort, and lead to mold and poor indoor air quality. This Old House adds that an oversized system cools quickly but cycles on and off often, which hurts efficiency, comfort, and equipment life.

An undersized system has the opposite problem: This Old House says it must work harder, raising bills and wear. On the hottest Sacramento afternoons, it may never catch up.

What does California's Energy Code say about AC size?

California's 2025 Energy Code requires heating and cooling load calculations for entirely new or completely replaced systems. For replacements, the code caps system size unless the installer verifies airflow of at least 350 cfm per ton. Without that verification, a cooling-only system's capacity is limited to the calculated cooling load plus 6,000 BTUs per hour, according to the Energy Commission's 2025 compliance manual.

For variable-speed or multi-speed systems, the lowest-speed capacity must be no more than 80% of the design load in most cases. That helps the system run longer, gentler cycles.

How are mini splits sized?

Each indoor head is sized for the room or zone it serves, and the outdoor unit is matched to the combined load. A head that's too big for a small bedroom will short-cycle, just like an oversized central system. Ask for room-by-room numbers from the Manual J calculation.

What makes Sacramento homes different?

Sacramento's heat is the main factor. NOAA's 1991–2020 normals show an average July high of 94.4°F near downtown and about 23 days a year at 100°F or hotter. Manual J uses local design weather, so a calculation done for Sacramento will differ from one for a coastal city.

Ducts are the other factor. If your ducts run through the attic, they sit in very hot air all summer, and ENERGY STAR says typical duct systems lose 20 to 30 percent of their air. Sealing leaky ducts, which the Energy Code requires during replacements, can matter as much as the equipment size.

Frequently asked questions

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