The quick answer: one ton of cooling (12,000 BTU) per 500–700 square feet — 700 in cool northern climates, 600 in moderate ones, 500 in the hot South. A 1,800 sq ft home in a moderate climate lands on a 3-ton unit. The chart below covers the common cases, then the factors that move the number.
AC size chart by square footage
| Home size | Cool climate | Moderate | Hot South |
|---|---|---|---|
| 1,000 sq ft | 1.5 ton | 2 ton | 2 ton |
| 1,500 sq ft | 2 ton | 2.5 ton | 3 ton |
| 2,000 sq ft | 3 ton | 3.5 ton | 4 ton |
| 2,500 sq ft | 3.5 ton | 4 ton | 5 ton |
| 3,000 sq ft | 4 ton | 5 ton | 5 ton |
(1 ton = 12,000 BTU. Residential systems come in half-ton steps from 1.5 to 5; needs beyond 5 tons mean two systems or zoning.)
What moves your number up or down
Up: vaulted ceilings (volume, not floor area, is what you’re cooling), big west-facing windows, poor insulation, 1970s-or-older construction, lots of occupants, a kitchen that runs hot. Down: new construction with modern insulation, heavy shade, a finished basement in the square footage (basements barely need cooling). Each factor shifts things half a ton in practice — which is why two same-size houses can need different systems.
Why bigger is NOT safer
Oversized ACs short-cycle: they blast the thermostat to temperature in minutes and shut off before dehumidifying the air — leaving the house cold-but-clammy, wearing out the compressor with constant starts, and spiking bills. Undersizing by a half ton is a milder error than oversizing by one. This is the single most common HVAC upsell, and the defense is one sentence: “Please show me the Manual J.”
The Manual J: the real answer
A Manual J load calculation measures your actual house — window areas, insulation values, orientation, duct condition — and outputs the correct size. Any contractor bidding a replacement should perform one (or at minimum a room-by-room worksheet), not eyeball your old unit’s label; the old unit was often sized wrong too. Our chart and the sizing built into the AC install calculator get you a solid first-pass number and a fair price range — use them to sanity-check bids, then let a Manual J set the final tonnage. Considering switching systems entirely? A heat pump sizes the same way and replaces the furnace too.