AC tonnage calculator
Enter the floor area that the air conditioner cools. The calculator gives two ranges: the size the common rule of thumb produces, and the size a very efficient house needs. A real house falls between them, and a load calculation finds where.
- Common rule of thumb4.0 to 5.0 tons
- Very efficient house2.0 to 2.5 tons
Rule of thumb: one ton for every 400 to 500 square feet gives 48,000 to 60,000 BTU/h. In stocked sizes that is 4.0 to 5.0 tons.
Very efficient house: one ton for every 800 to 1,000 square feet gives 24,000 to 30,000 BTU/h. In stocked sizes that is 2.0 to 2.5 tons.
Both figures come from the US Department of Energy, which calls the rule of thumb inadequate because it ignores the climate and the house. Use the result to judge a contractor’s proposal, and ask for a Manual J load calculation before buying.
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AC size by square footage
Residential split systems come in half-ton steps from 1.5 to 5 tons. One ton is 12,000 BTU/h. This table gives the size range from two US Department of Energy (DOE) figures.
| Floor area (sq ft) | Common rule of thumb (tons) | Very efficient house (tons) |
|---|---|---|
| 1,000 | 2.0 to 2.5 | 1.0 to 1.3 |
| 1,500 | 3.0 to 3.8 | 1.5 to 1.9 |
| 2,000 | 4.0 to 5.0 | 2.0 to 2.5 |
| 2,500 | 5.0 to 6.3 | 2.5 to 3.1 |
| 3,000 | 6.0 to 7.5 | 3.0 to 3.8 |
| 3,500 | 7.0 to 8.8 | 3.5 to 4.4 |
Common rule of thumb: one ton per 400 to 500 square feet. Very efficient house: one ton per 800 to 1,000 square feet. The largest residential system is 5 tons. A larger figure means more than one system.
How the calculator works
tons = floor area ÷ square feet per ton
Both ranges appear in the US Department of Energy’s Consumer Guide to Home Heating and Cooling. DOE says the common rule of thumb ignores climate and the efficiency of the house. It says one ton can cool 800 to 1,000 square feet of a very efficient house. DOE says contractors should use ACCA Manual J.
The first range is a rule of thumb. The second is a DOE figure. Neither is a standard. The published standards are ACCA Manual J, which calculates the load of a specific house, and Manual S, which selects equipment to match it. The load is the cooling the house needs.
Checking a proposed size
Manual S limits how far equipment capacity may stray from the cooling load. These are the 3rd edition limits, as shown in ACCA’s proposed Addendum c of March 2025.
| Equipment | Capacity as a share of the cooling load |
|---|---|
| Single-speed, cooling load 24,000 BTU/h or less | 0.90 to 1.20 |
| Single-speed, cooling load above 24,000 BTU/h | 0.90 to 1.15 |
| Two-speed | 0.90 to 1.25 |
For a 36,000 BTU/h load, a single-speed unit may have 32,400 to 41,400 BTU/h of cooling capacity.
Limits of this calculator
- Floor area is not a load. Neither range uses your climate. Only a Manual J load calculation works from your house.
- Some houses need less than the lowest figure here. DOE’s Building America guidance treats more than 1,000 square feet per ton as a low-load home.
- Published size limits differ. The Building America guide gives 95% to 115% of the Manual J load. The 3rd edition itself is paywalled, so the limits above come from an ACCA proposed addendum.
Sources
- US Department of Energy, Consumer Guide to Home Heating and Cooling: https://www.energy.gov/sites/default/files/2021-08/ES-HomeHeatingandCooling_081221.pdf
- DOE Building America Solution Center, Air Conditioning guide: https://basc.pnnl.gov/resource-guides/air-conditioning
- ACCA, Manual J, residential load calculation: https://www.acca.org/standards/technical-manuals/manual-j
- ACCA, Manual S, equipment selection: https://www.acca.org/standards/technical-manuals/manual-s
- ACCA, proposed Addendum c to Manual S, March 2025: https://higherlogicdownload.s3.amazonaws.com/ACCA/c6b38bda-2e04-4f93-bd51-7a80525ad936/UploadedImages/Manual_S_Addendum_C_03062025.pdf
Related
For one room, or for heating, use the BTU calculator. To find the airflow a system moves, use the CFM calculator. Read how long a central air conditioner lasts on the air conditioner lifespan page. To date a unit, use the AC unit age lookup. To read the size of the unit you have now, use the model number decoder. For the sizes on sale and what an oversized unit does, read what size AC unit do I need.
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Calculator last reviewed on October 5, 2026. Method and sources.