BTU, tons and what the numbers mean
Air conditioners are rated by how much heat they remove in an hour, measured in BTU (British Thermal Units). Central systems use a larger unit, the ton: one ton of cooling is 12,000 BTU per hour, the heat needed to melt a ton of ice in a day. A 3 ton system is therefore 36,000 BTU per hour. Window units and mini-splits are sold in BTU; central air and heat pumps are sold in tons, in half-ton steps from 1.5 to 5.
The rule of thumb and why it needs adjusting
The quick estimate most contractors start from is 20 BTU per square foot. It assumes 8 foot ceilings, average insulation, a moderate climate and two people in the room. Almost no house matches all four, so the calculator adjusts for the things that move the number most:
| Factor | Adjustment | Why |
|---|---|---|
| Climate | -15% to +25% | Phoenix in July asks far more of a system than Seattle does |
| Sun exposure | -10% to +10% | Large west-facing windows are a heat source all afternoon |
| Insulation and windows | -12% to +15% | Single-pane glass and an unsealed attic leak the cooling you paid for |
| Ceiling height | Proportional above 8 ft | You cool a volume, not a floor; 10 ft ceilings hold 25% more air |
| People | +600 BTU each beyond two | A resting adult gives off about 400 to 600 BTU per hour |
| Kitchen | +4,000 BTU | Cooking appliances and the refrigerator add heat |
AC size by square footage
Typical sizes for a home with average insulation in a moderate climate. Hot climates move each row up one size; cool climates move it down.
| Area | BTU/hr | Central AC | Window or mini-split |
|---|---|---|---|
| 150 to 250 sq ft | 5,000 to 6,000 | - | 6,000 BTU |
| 300 to 400 sq ft | 7,000 to 8,000 | - | 8,000 BTU |
| 450 to 550 sq ft | 10,000 to 12,000 | 1 ton | 12,000 BTU |
| 700 to 1,000 sq ft | 14,000 to 18,000 | 1.5 ton | 18,000 BTU |
| 1,000 to 1,400 sq ft | 21,000 to 24,000 | 2 ton | 24,000 BTU |
| 1,400 to 1,800 sq ft | 28,000 to 30,000 | 2.5 ton | Two units or mini-split |
| 1,800 to 2,200 sq ft | 34,000 to 36,000 | 3 ton | - |
| 2,200 to 2,600 sq ft | 40,000 to 42,000 | 3.5 ton | - |
| 2,600 to 3,000 sq ft | 46,000 to 48,000 | 4 ton | - |
| 3,000 to 3,500 sq ft | 54,000 to 60,000 | 5 ton | - |
Bigger is not safer
Homeowners often ask for the next size up "to be sure". It is the most common and most expensive sizing mistake. An oversized system hits the thermostat temperature in a few minutes and shuts off before it has run long enough to pull moisture out of the air. The house reads 72 degrees and feels sticky. The compressor then cycles on and off dozens of times a day, which wears it faster and uses more electricity than a correctly sized unit running steady, longer cycles.
If the calculator lands between two sizes, pick the smaller one in a humid climate and the larger one in a dry climate. Humidity control matters more than peak capacity in Houston; in Phoenix the opposite is true.
What a contractor does differently: Manual J
This calculator is an estimate. Before installing central air, a good HVAC contractor runs a Manual J load calculation. It measures every wall, window and door, the direction they face, the insulation values, duct losses, the number of occupants and the local design temperature, and produces a cooling load in BTU per hour. The result usually lands within half a ton of a careful estimate like this one, but it is the number a system should be bought on. If a contractor sizes your system from square footage alone, ask for the Manual J; reputable ones have software for it and some will show you the report.
Using this number when you get quotes
- Ask each contractor what size they are quoting and why. Two quotes a full ton apart mean one of them did not measure.
- Check the SEER2 rating. Higher is more efficient; 15 to 16 is the current standard range, 18 and up is premium.
- If ducts are old, ask about duct sealing and static pressure. A new unit on leaky ducts loses a quarter of its capacity in the attic.
- Compare installers, not only equipment. HVAC contractors on ProBusiness101 list their services, photos and contact details so you can shortlist three and ask the same questions to each.