Say this out loud before using the chart. Square feet per ton is not a Manual J method and will not satisfy a code official, a utility rebate, or a permit reviewer. Treat what follows as a sanity check, never as a design.
The chart
Square feet of conditioned floor area per ton of cooling, by rough climate band. Wider bands mean the envelope matters more, not that the number is vaguer.
| Climate | Typical sq ft per ton | What moves it inside the band |
|---|---|---|
| Hot & dry (Phoenix, Las Vegas, inland CA) | 350 – 500 | West glass and roof exposure dominate; shading and window upgrades swing it hard. |
| Hot & humid (Gulf Coast, Florida, Houston) | 400 – 550 | Latent load is a large share; infiltration and ventilation air matter more than area. |
| Mixed (Atlanta, Dallas, Kansas City, mid-Atlantic) | 500 – 700 | The classic 500–600 figure comes from here. Vintage and attic insulation set the spread. |
| Marine & mild (coastal CA, Pacific Northwest) | 700 – 1,000+ | Low design temperature difference; a tight modern envelope pushes past 1,000 routinely. |
| Cold (upper Midwest, Northeast, mountain) | 600 – 900 | Cooling is the smaller season; heating sizing usually drives the equipment choice instead. |
To use it: divide conditioned floor area by the figure for your climate. A 2,100 sq ft house in a mixed climate at 600 sq ft/ton gives 3.5 tons. Keep that number in pencil.
Where it breaks
The rule assumes an average house. That is a real thing on average and a fiction in any particular driveway. The four inputs that most often blow it up:
- Window area and orientation. A wall of west-facing glass can add a ton on its own in a hot climate. The chart cannot see it, because floor area does not change when you swap a wall for a window.
- Design temperature, not average temperature. Load tracks the difference between indoor and outdoor design conditions. Two towns with the same summer “feel” can be 15°F apart on the 1% design dry-bulb.
- Envelope vintage. A 1965 house and a 2020 house of the same floor area, same town, are routinely a full ton and a half apart.
- Ceiling height and volume. Everything in the chart is per square foot. A great room with 18-foot ceilings does not read as different floor area, but the infiltration and the volume are.
Put those together and the honest error band on a rule of thumb is about ±30% — which, at these sizes, is one to two size increments. That is the difference between a system that dehumidifies properly and one that short cycles all summer. The consequences are laid out in what oversizing actually costs you.
The one job it does well
A rule of thumb is a smell test, and it is a good one. If a contractor quotes 5 tons for a 1,600 sq ft house in Ohio, the chart says roughly 2.5 to 3, and that gap is worth a conversation. You are not trying to out-size the contractor — you are asking them to show the load calculation that supports their number.
That is a reasonable ask, and a contractor who does the work properly will have one ready. “It is what was there before” is not a load calculation; the old unit may well have been oversized too, which is how a house inherits the same mistake three replacements in a row.
Two minutes gets you a real number
The reason the rule of thumb survived is that the alternative used to mean desktop software and an afternoon. It does not anymore.
- The free Manual J calculator takes eight inputs in the browser — ZIP, floor area, vintage, glass, insulation — and uses the published 99%/1% design temperatures for that ZIP rather than a regional guess. It is a whole-house block load, and it tells you so.
- The HVAC Load Calculator app runs full Manual J 8th Edition methods offline, with real assemblies and a Manual S capacity band you can defend.
Either one replaces the chart entirely. Keep the chart for the smell test.
FAQ
How many square feet per ton of air conditioning?
The common figure is 500 to 600, but the defensible range runs from about 350 in the hot southwest to 900 or more in mild coastal and northern climates with a tight envelope. That spread is why the rule cannot settle a purchase on its own.
Is the square foot per ton rule accurate?
Accurate enough to catch a wildly wrong quote, not accurate enough to size equipment. It commonly lands within half a ton — but half a ton is a whole size increment, and the misses cluster in the houses where sizing matters most.
Why do contractors still use rules of thumb?
Speed, plus the historical cost of load software. That reason has largely evaporated now that a block load takes a couple of minutes on a phone, but the habit outlasted the constraint.
Can I use a rule of thumb to check my contractor’s quote?
Yes — that is the one job it does well. If the rule says 3 tons and the quote says 5, ask for the Manual J that supports the number rather than arguing about sizes.