What size air conditioner do I need?
Cooling load in BTU and tons, matched to the sizes central and window units come in.
Air conditioners are sold in tons, and one ton is exactly 12,000 BTU per hour. The name is a leftover from the days when cooling was measured against the rate ice melts, and it has nothing to do with the weight of the equipment.
Before you buy anything. This is a square-foot estimate, not an ACCA Manual J load calculation. A Manual J uses your building’s real construction, orientation and air leakage, and on a well-sealed house it usually comes out smaller than this. Use this to sanity-check a quote, not to replace one.
BTU and tons, converted
| Tons | BTU per hour | Typical area, zone 3 |
|---|---|---|
| 1.5 | 18,000 | up to 720 sq ft |
| 2.0 | 24,000 | up to 960 sq ft |
| 2.5 | 30,000 | up to 1,190 sq ft |
| 3.0 | 36,000 | up to 1,430 sq ft |
| 3.5 | 42,000 | up to 1,670 sq ft |
| 4.0 | 48,000 | up to 1,910 sq ft |
| 4.5 | 54,000 | up to 2,150 sq ft |
| 5.0 | 60,000 | up to 2,390 sq ft |
The humidity problem
An air conditioner removes heat and moisture at the same time, but not at the same rate. Moisture removal needs run time: the coil has to stay cold long enough for water to condense on it and drain away. A unit that is too large satisfies the thermostat quickly, shuts off, and never gets to that part of its job.
The symptom is a house that reads 72 degrees on the thermostat and still feels clammy, often with condensation on windows and a musty smell in summer. The cause is almost always an oversized system, and no amount of thermostat adjustment fixes it. The full explanation is here.
Cooling load by house size and climate
Climate zone 1: Miami, Key West, Honolulu
| Sq ft | Cooling BTU | Heating BTU | AC size | Tons |
|---|---|---|---|---|
| 500 | 15,938 | 9,375 | 18,000 | 1.5 |
| 600 | 19,125 | 11,250 | 24,000 | 2.0 |
| 800 | 25,500 | 15,000 | 30,000 | 2.5 |
| 1,000 | 31,875 | 18,750 | 36,000 | 3.0 |
| 1,200 | 38,250 | 22,500 | 42,000 | 3.5 |
| 1,500 | 47,813 | 28,125 | 48,000 | 4.0 |
| 1,600 | 51,000 | 30,000 | 54,000 | 4.5 |
| 1,800 | 57,375 | 33,750 | 60,000 | 5.0 |
| 2,000 | 63,750 | 37,500 | 60,000 | 5.0 |
| 2,500 | 79,688 | 46,875 | 60,000 | 5.0 |
| 3,000 | 95,625 | 56,250 | 60,000 | 5.0 |
| 3,500 | 111,563 | 65,625 | 60,000 | 5.0 |
| 4,000 | 127,500 | 75,000 | 60,000 | 5.0 |
Climate zone 3: Atlanta, Dallas, Los Angeles, Charlotte
| Sq ft | Cooling BTU | Heating BTU | AC size | Tons |
|---|---|---|---|---|
| 500 | 12,563 | 14,375 | 18,000 | 1.5 |
| 600 | 15,075 | 17,250 | 18,000 | 1.5 |
| 800 | 20,100 | 23,000 | 24,000 | 2.0 |
| 1,000 | 25,125 | 28,750 | 30,000 | 2.5 |
| 1,200 | 30,150 | 34,500 | 36,000 | 3.0 |
| 1,500 | 37,688 | 43,125 | 42,000 | 3.5 |
| 1,600 | 40,200 | 46,000 | 42,000 | 3.5 |
| 1,800 | 45,225 | 51,750 | 48,000 | 4.0 |
| 2,000 | 50,250 | 57,500 | 54,000 | 4.5 |
| 2,500 | 62,813 | 71,875 | 60,000 | 5.0 |
| 3,000 | 75,375 | 86,250 | 60,000 | 5.0 |
| 3,500 | 87,938 | 100,625 | 60,000 | 5.0 |
| 4,000 | 100,500 | 115,000 | 60,000 | 5.0 |
Climate zone 5: Chicago, Denver, Boston, Detroit
| Sq ft | Cooling BTU | Heating BTU | AC size | Tons |
|---|---|---|---|---|
| 500 | 10,563 | 19,375 | 18,000 | 1.5 |
| 600 | 12,675 | 23,250 | 18,000 | 1.5 |
| 800 | 16,900 | 31,000 | 18,000 | 1.5 |
| 1,000 | 21,125 | 38,750 | 24,000 | 2.0 |
| 1,200 | 25,350 | 46,500 | 30,000 | 2.5 |
| 1,500 | 31,688 | 58,125 | 36,000 | 3.0 |
| 1,600 | 33,800 | 62,000 | 36,000 | 3.0 |
| 1,800 | 38,025 | 69,750 | 42,000 | 3.5 |
| 2,000 | 42,250 | 77,500 | 48,000 | 4.0 |
| 2,500 | 52,813 | 96,875 | 54,000 | 4.5 |
| 3,000 | 63,375 | 116,250 | 60,000 | 5.0 |
| 3,500 | 73,938 | 135,625 | 60,000 | 5.0 |
| 4,000 | 84,500 | 155,000 | 60,000 | 5.0 |
Assumes 8 ft ceilings, average insulation, two occupants and no kitchen gain. Every zone is on the full chart.