TonnageMapHow many tons your house actually needs, and what the swap costs.

Tons by Square Foot

The rule of thumb, where it breaks, and what a real load calculation changes.

What size AC for 2000 square feet in zone 4

2,000 ft² in zone 4 (mixed)
3.5tons
42,000 BTU/hr, which is 571 ft² per ton — the rule of thumb here, not a Manual J
2,000 ft² in zone 4 (mixed)3.5 tonsNominal size to b…3.5 tonsCalculated need4.5 tonsSame house in zon…2.8 tonsSame house in zon…
Nominal size to buy3.5tons
Calculated need2,000 ft² × 21 BTU/ft²3.5tons
Cooling load42,000BTU/hr
Square feet per tonthe number the rule of thumb quotes — and it moves with climate571ft²/ton
Oversize from rounding upwithin the normal rounding step0%
Same house in zone 1–24.5tons
Same house in zone 6–71.64× difference for the identical building2.8tons

Notes

  • "400 square feet per ton" is a Florida number being quoted everywhere. This house needs 4.5 tons in zone 1–2 and 2.8 tons in zone 6–7 — a factor of 1.64 for the identical building. Any single square-feet-per-ton figure is a climate assumption in disguise.
  • Buying bigger "to be safe" is the classic mistake. An oversized unit reaches the thermostat setpoint quickly and shuts off before it has run long enough to pull moisture out of the air. The result is a house that is cold and clammy, and a compressor that wears out early from short cycling. Undersized is uncomfortable on the five hottest days; oversized is uncomfortable all summer.
  • This is a rule of thumb, and the difference from a Manual J is money. The real calculation accounts for window area and orientation, insulation levels, air leakage, ceiling height, duct location, shading and how many people live there. A house with new triple glazing and a tight envelope can need a full ton less than this figure; a 1960s house with single glazing and an uninsulated attic can need a ton more.
  • Get the Manual J before you buy, not after. It costs a few hundred dollars against a $8,000–15,000 installation, and it is the only thing that turns the number above into a specification. A contractor who sizes by square footage alone is guessing with your money.
  • Heat pumps change the question. Sized for cooling, a heat pump may be short of capacity on the coldest days; sized for heating in a cold zone, it is badly oversized for cooling. Variable-capacity equipment or a modest backup heat source is the usual resolution, and it needs deciding at design time.

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AC tons by square foot and climate

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