Ask three contractors how big an air conditioner a house needs and at least two will divide the floor area by a number between 400 and 600 square feet per ton.
The rule exists because it is usually close enough on the housing stock it came from. On an insulated house it is not close enough, and the direction of the error matters.
The rule, with the adjustments people skip
Cooling, square feet per ton (12,000 BTU/h):
| Climate zone | Leaky / older envelope | Average | Tight, well-insulated |
|---|---|---|---|
| 1–2 (hot) | 400 | 500 | 600–700 |
| 3–4 (mixed) | 500 | 600 | 750–850 |
| 5–6 (cold) | 600 | 700 | 900–1,000 |
| 7–8 (very cold) | 700 | 800 | 1,000–1,200 |
Heating, BTU/h per square foot:
| Zone | Leaky | Average | Tight |
|---|---|---|---|
| 1–2 | 20–25 | 15–20 | 10–14 |
| 3–4 | 30–35 | 25–30 | 15–20 |
| 5–6 | 40–45 | 30–40 | 20–28 |
| 7–8 | 50–60 | 40–50 | 28–35 |
Two things move the answer more than floor area does: which zone and how tight the envelope is. A 2,000 ft² house in zone 5 is 3.3 tons if it is leaky and 2.2 tons if it is tight — a full ton and a half apart on the same floor plan.
That is also the point of this site. Every R-value you add moves the house right along that table, and the equipment should move with it.
Why oversizing is the expensive mistake
Undersized equipment runs constantly and fails to hold setpoint on the worst day of the year. That is obvious and it gets fixed.
Oversized cooling equipment does something less obvious: it satisfies the thermostat before it has removed the moisture.
An air conditioner dehumidifies only while the coil is cold and air is moving across it, and that takes several minutes of continuous running. A unit sized 40% too large hits setpoint in six minutes, shuts off, and the house sits at 74 °F and 65% relative humidity — cold and clammy. The occupant lowers the thermostat, which makes it worse.
Short cycling also:
- Wears the compressor, which draws its highest current at every start
- Delivers uneven temperatures, because the air never circulates long enough to mix
- Costs more to run, since the starting current is paid for many more times a day
Oversizing is the more common error, precisely because the rule of thumb was calibrated on leakier houses than the one being sized.
What the rule cannot see
A load calculation — ACCA Manual J — accounts for things the floor area does not:
- Window area, orientation and glazing type. A wall of west-facing glass can be a third of the cooling load on its own.
- Actual R-values of ceiling, walls and floor
- Measured or estimated air leakage
- Ceiling height — the rule assumes 8 ft, and a vaulted room breaks it
- Occupancy and internal gains
The gap between a rule-of-thumb figure and a Manual J result on a well-insulated house is routinely 25–40%, almost always with the rule reading high.
How to use the rule properly
It is still useful — as a sanity check, not a specification:
- Run the table above for your zone and envelope to get an expected range
- Get the contractor's Manual J output
- If the proposal is more than about 15% above your range, ask which inputs drove it
If a contractor will not produce a Manual J, that is the answer to the question. And if the house is about to be insulated or air-sealed, do that first and size afterwards — sizing equipment to the old envelope locks in the oversizing for the next fifteen years.
The order that saves money
- Air-seal the ceiling plane
- Insulate to the zone target — see r value chart for attic insulation
- Then run the load calculation
- Buy equipment to that number
Done in this order the equipment gets smaller, cheaper and better at controlling humidity. Done backwards, you pay for capacity that then makes the house less comfortable.