AC tonnage is not set by square footage. It is set by how much heat the system has to remove per hour. And that depends on where the building is.
The tonnage formula
Cooling Load = Envelope + Occupants + Equipment + Lighting
Tonnage = Cooling Load (BTU/hr) / 12,000
The envelope load dominates in most buildings:
Envelope = Area × Climate Factor × (Ceiling Height / 8)
Climate factors by region
Climate BTU/ft² Example cities
──────────────────────────────────────────────
Cool 20–25 Seattle, Portland, Minneapolis
Moderate 25–30 Denver, Chicago, DC
Hot-humid 30–35 Houston, Dallas, Atlanta, Miami
Hot-dry 35–40 Phoenix, Las Vegas, Tucson
These factors account for the design outdoor temperature, solar gain patterns, and humidity levels typical for each region.
Same house, four cities
A 2,000 sq ft house with 9 ft ceilings, 4 occupants, 500W equipment, 200W lighting:
City Climate BTU/ft² Envelope Internal Total Tons
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Seattle Cool 22 49,500 2,400 51,900 4.3
Denver Moderate 27 60,750 2,400 63,150 5.3
Dallas Hot 32 72,000 2,400 74,400 6.2
Phoenix Hot-dry 38 85,500 2,400 87,900 7.3
Same house. Tonnage nearly doubles from Seattle to Phoenix.
Why "400 sq ft per ton" fails
The most common rule of thumb:
Tonnage = Area / 400 to 600
2,000 sq ft / 400 = 5.0 tons
This gives 5 tons everywhere.
Seattle: needs 4.3T, gets 5.0T → oversized by 16%
→ short cycling, poor humidity control, wasted energy
Phoenix: needs 7.3T, gets 5.0T → undersized by 32%
→ runs continuously, never reaches setpoint on peak days
The rule was calibrated for a moderate climate. It fails everywhere else.
Ceiling height multiplier
The formula uses volume, not area:
8 ft ceilings: 2,000 × 8 = 16,000 ft³
10 ft ceilings: 2,000 × 10 = 20,000 ft³
12 ft ceilings: 2,000 × 12 = 24,000 ft³
The ceiling height adjustment:
Factor = actual height / 8
8 ft: 1.0× (baseline)
9 ft: 1.125×
10 ft: 1.25×
12 ft: 1.5×
A house with 12 ft ceilings in Phoenix needs 50% more tonnage than the same footprint with 8 ft ceilings.
When internal gains change the picture
In Phoenix, envelope is 85–90% of total load. Internal gains barely matter.
In Seattle, envelope is 60–70%. Internal gains are 30–40%.
Seattle office with heavy equipment:
Envelope: 49,500 BTU/hr (70%)
Internals: 21,000 BTU/hr (30%)
Total: 70,500 BTU/hr = 5.9 tons
Seattle bedroom (same area):
Envelope: 49,500 BTU/hr (93%)
Internals: 3,600 BTU/hr (7%)
Total: 53,100 BTU/hr = 4.4 tons
Same area. Different internal loads. 1.5 tons difference. In hot climates, this gap shrinks because the envelope load overwhelms everything else.
