HVAC Tonnage per Square Foot
The per-zone square-feet-per-ton bands, and where they break down.
8 min
BTU calculator for heating and cooling
Put in the floor area, the climate zone and how tight the house is; get a screening load in BTU/h and the tonnage that sits nearest it. Cooling and heating are separate readings because they are separate loads.
Pre-filled with a 1,800 ft² house in climate zone 4. Change anything.
ft²
ft
Zone 1 is south Florida; zone 7 is northern Minnesota.
people
Above two, each person adds 600 Btu/h.
43,800BTU/h
Cooling load
What this calculates
A BTU is a British thermal unit — the heat it takes to raise a pound of water one degree Fahrenheit. HVAC equipment is rated in BTU per hour, and a "ton" of capacity is exactly 12,000 BTU/h. This tool turns a house into a BTU/h number by multiplying its conditioned floor area by a per-square-foot factor for the climate zone, then adjusting for ceiling height, envelope tightness, sun exposure and occupants.
The factors are the honest middle of the range a load calculation lands in. They are not a substitute for one: a real calculation counts every window by orientation, every wall by assembly, the duct location, the infiltration rate and the internal gains room by room. Use this to sanity-check a quote, not to order equipment.
The bands the tool uses
| Climate zone | Cooling | Heating | Roughly |
|---|---|---|---|
| 1 | 32 | 22 | South Florida, Hawaii |
| 2 | 30 | 26 | Houston, Phoenix |
| 3 | 27 | 32 | Atlanta, Los Angeles |
| 4 | 24 | 38 | St. Louis, Baltimore |
| 5 | 22 | 45 | Chicago, Denver |
| 6 | 20 | 52 | Minneapolis, Burlington |
| 7 | 18 | 60 | Duluth, northern Maine |
Units are BTU/h per square foot of conditioned floor area, at an 8 ft ceiling.
Worked example
A 1,800 ft² house in zone 4, 8 ft ceilings, average envelope, normal exposure, three people. Cooling: 24 × 1,800 = 43,200 BTU/h, times 1.00 for height and envelope, plus 600 BTU/h for the third occupant, = 43,800 BTU/h — 3.65 tons, so a 3.5-ton unit with the balance taken by a longer run time, not a 4-ton unit that short-cycles.
Heating for the same house: 38 × 1,800 = 68,400 BTU/h of output. At 80% AFUE that is about 85,500 BTU/h of input — the size a contractor should be quoting, not the 100,000 BTU/h box that happens to be on the truck. The furnace size calculator does that conversion for you.
Why oversizing costs you
Bigger is not safer
An oversized air conditioner satisfies the thermostat before it has run long enough to pull moisture out of the air. The house gets cold and clammy, the compressor starts and stops all day, and the equipment wears out early. On the heating side an oversized furnace overshoots, cycles on the high limit and leaves the far rooms cold.
If your result lands between two nominal sizes, size down and fix the envelope, not up. A load that reads far outside the bands in the table usually means the inputs are wrong — an unconditioned basement counted as floor area, or a leaky house entered as tight.
Climate-zone notes
Regional minimum efficiency standards follow roughly the same geography. Split-system air conditioners must meet 13.4 SEER2 nationally and 14.3 SEER2 under 45,000 BTU/h in the Southeast and Southwest. That does not change the load — it changes what you are allowed to install against it.
- Zone 1
- Zone 2
- Zone 3
- Zone 4
- Zone 5
- Zone 6
- Zone 7
Sources: 10 CFR 430.32(e)(1)(ii)(A) (read 2026-09-04)
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