Guide
BTU per square foot, by climate
The cooling and heating figures for every US climate zone.
Short answer
About 23 BTU per square foot for cooling and 34 for heating in a mixed climate such as Washington DC or St Louis, for an average house with 8 ft ceilings. Cooling runs from about 35 in the hottest zone to 15 in the coldest; heating runs the other way, from about 20 to 60.
BTU per square foot by climate zone
| Zone | Examples | Cooling | Heating |
|---|---|---|---|
| Zone 1, hot humid | Miami, Key West, Honolulu | 30 to 35 | 15 to 25 |
| Zone 2, hot | Houston, Orlando, Phoenix, New Orleans | 27 to 30 | 20 to 30 |
| Zone 3, warm | Atlanta, Dallas, Los Angeles, Charlotte | 24 to 27 | 25 to 35 |
| Zone 4, mixed | Washington DC, Seattle, St Louis, Nashville | 22 to 25 | 30 to 40 |
| Zone 5, cool | Chicago, Denver, Boston, Detroit | 20 to 23 | 35 to 45 |
| Zone 6, cold | Minneapolis, Burlington, Helena | 18 to 22 | 40 to 50 |
| Zone 7, very cold | Duluth, Grand Forks, International Falls | 16 to 20 | 45 to 55 |
| Zone 8, subarctic | Fairbanks, Barrow | 15 to 18 | 50 to 60 |
Not sure of your zone? Climate zones by state lists them.
What that means for a 1,500 sq ft house
| Zone | Cooling | Heating |
|---|---|---|
| Zone 1 | 47,813 BTU (3.98 ton) | 28,125 BTU |
| Zone 2 | 42,188 BTU (3.52 ton) | 35,625 BTU |
| Zone 3 | 37,688 BTU (3.14 ton) | 43,125 BTU |
| Zone 4 | 34,688 BTU (2.89 ton) | 50,625 BTU |
| Zone 5 | 31,688 BTU (2.64 ton) | 58,125 BTU |
| Zone 6 | 29,250 BTU (2.44 ton) | 65,625 BTU |
| Zone 7 | 26,250 BTU (2.19 ton) | 73,125 BTU |
| Zone 8 | 24,188 BTU (2.02 ton) | 80,625 BTU |
Why the "20 BTU per square foot" rule misleads
The old rule of thumb, 20 BTU per square foot, is a cooling figure for a mild climate. It undersizes air conditioning in the South and wildly undersizes heating in the North, where the heating load is two to three times the cooling load. It also ignores ceiling height, which matters because the load follows the volume of air, not the floor: a 10 ft ceiling adds a quarter. For a real house, the only authoritative number is a Manual J load calculation; these figures are for planning and for checking a quote.