Cooling capacity is measured in tons. One ton is 12,000 BTU per hour, a unit inherited from the amount of heat absorbed by a ton of ice melting over a day.
The starting rule
The common rule of thumb is 400 to 600 square feet per ton, and the spread in that range is the whole problem — it is the difference between a 2.5-ton and a 4-ton system on the same house.
A more usable starting point works from climate:
| Climate | Sq ft per ton |
|---|---|
| Hot and humid (Gulf, Deep South) | 400–450 |
| Hot and dry (Southwest) | 450–500 |
| Mixed (mid-Atlantic, Midwest) | 500–600 |
| Cool (Northern tier, Pacific Northwest) | 600–700 |
A 2,000 sq ft house is therefore roughly 4.5 tons in Houston and 3 tons in Seattle.
What moves it from there
Insulation and air sealing. A well-sealed house built in the last fifteen years can need a full ton less than an equivalent 1960s house.
Windows. Area, orientation and glazing. West-facing glass is the single largest afternoon load in most houses.
Ceiling height. The rule is per square foot but the load is per cubic foot. Nine- and ten-foot ceilings add roughly 10–20%.
Shade. Mature trees on the west side are worth real capacity.
Occupancy and appliances. Each person adds around 400 BTU/h of sensible and latent load; a kitchen in use adds considerably more.
Ductwork in an unconditioned attic, which can add 20–30% to the load through leakage and conduction alone.
Why oversizing is the worse error
An undersized system runs continuously on the hottest days and falls behind. That is uncomfortable and obvious.
An oversized system is worse, and the reason is humidity. Air conditioning dehumidifies by running long enough for moisture to condense on a cold coil. A system with too much capacity satisfies the thermostat quickly and shuts off, so it never runs long enough to remove moisture.
The result is a house at 72°F that feels clammy, plus short cycling that wears the compressor, uneven temperatures between rooms, and higher electricity use than the equipment rating suggests.
In humid climates this is the dominant sizing consideration, and it is the reason contractors who oversize "to be safe" produce houses people are unhappy in.
Available sizes
Residential equipment comes in half-ton steps: 1.5, 2, 2.5, 3, 3.5, 4, 5 tons. Calculated loads rarely land on one.
ACCA Manual S guidance is to select equipment between 100% and 115% of the calculated cooling load — that is, round to the nearest size and prefer the smaller one when the calculation falls between.
Variable-capacity and inverter systems change this. They modulate down to a fraction of rated capacity, so they tolerate a size larger without the humidity penalty, and they run long low-output cycles that dehumidify well.
The measurement that settles it
A Manual J load calculation is a room-by-room accounting of heat gain: envelope, glazing by orientation, infiltration, internal gains, duct losses.
It typically comes out 20–40% below what the square-foot rule gives, because the rule was built around older, leakier housing stock.
Any contractor sizing a system by floor area alone, or by matching whatever was there before, is guessing — and the previous system was very likely oversized too, which is how the error propagates through a house's history.
When the old system's size is useful information
It is worth knowing, and it is not the answer.
If the old system kept up comfortably and cycled reasonably, its size is a reasonable upper bound. If it ran short cycles and the house felt damp, it was oversized and the replacement should be smaller.
Any insulation, window replacement or air sealing done since it was installed lowers the requirement further.