AC unit size calculator

What square feet per ton really means

A ton of cooling is 12,000 BTU/h. "Square feet per ton" is the inverse of the load intensity the BTU calculator produces: 500 ft²/ton is the same thing as 24 BTU/h per square foot. Contractors carry the number in their heads because it is quick, and it is wrong exactly as often as the house differs from the average house the number came from.

That is why the tool moves the figure by vintage. A 2018 house with a sealed envelope and low-E glass can hold 575 ft² per ton in zone 4; the same footprint from 1968 with single panes and a 4-inch attic will struggle past 400.

Starting points by zone

Reading the oversize check

Switch the control to "check what's installed", put in the nameplate tonnage, and the tool prints the gap. Under about 15% over, you are inside the noise of a screening estimate. Past 25%, the symptoms are usually already there: short cycles, a house that reaches temperature but feels damp, and a compressor that has never run a full hour.

What the standard requires where you are

Sizing decides the capacity; the federal standard decides the minimum efficiency you may buy. Split-system air conditioners must meet 13.4 SEER2 nationally. In the Southeast — Alabama, Arkansas, Delaware, Florida, Georgia, Hawaii, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, Puerto Rico, South Carolina, Tennessee, Texas, Virginia, the District of Columbia and the U.S. Territories — and in the Southwest (Arizona, California, Nevada and New Mexico), units under 45,000 BTU/h must meet 14.3 SEER2 and units at or above 45,000 BTU/h 13.8 SEER2. Southwest installs carry an EER2 floor as well.

A dealer who cannot say which of those applies to your address is not the dealer to size your system. The SEER2 guide walks through the difference between the ratings and what changed in 2023.

• Zone 1
• Zone 2
• Zone 3
• Zone 4
• Zone 5
• Zone 6
• Zone 7

Sources: 10 CFR 430.32(c)(6)(i)(A) (read 2026-09-04)