Sizing basics
BTU per square foot by climate zone
Twenty BTU per square foot is right in St. Louis and wrong in Miami and Minneapolis. Here are the figures by climate zone, what changes them, and when to stop using them.
Why the figure changes with climate
A house gains heat in summer in proportion to how hot it is outside, how strong the sun is, and how humid the air is, and it loses heat in winter in proportion to how cold it is. A rule that ignores the climate can only be right in one place. The US Department of Energy and the International Energy Conservation Code divide the country into numbered climate zones by county, from zone 1 in southern Florida and Hawaii to zone 7 in northern Minnesota and the mountains (zone 8 is Alaska), and the zone is the most useful single piece of information for a first estimate.
The per-square-foot figures this site uses, and that the central AC size calculator and the furnace size calculator apply, are rule-of-thumb loads for an average-insulated house with 8 ft ceilings. They are compiled from published climate-zone sizing charts and are set out on the BTU per square foot table.
The figures by zone
| Zone | Example cities | Cooling, BTU/hr per sq ft | Heating, BTU/hr per sq ft | Design temperature |
|---|---|---|---|---|
| 1 | Miami, Honolulu | 24 | 20 | 50 °F |
| 2 | Houston, Phoenix, Orlando | 22 | 25 | 35 °F |
| 3 | Atlanta, Dallas, Los Angeles | 20 | 30 | 25 °F |
| 4 | Washington DC, St. Louis, Seattle | 18 | 38 | 15 °F |
| 5 | Chicago, Denver, Boston | 15 | 45 | 0 °F |
| 6 | Minneapolis, Burlington | 13 | 52 | −10 °F |
| 7 | Duluth, Fargo | 12 | 60 | −20 °F |
The cooling figures are before people and the kitchen; the heating figures are the heat the house loses on the design night, an output figure. The two columns run in opposite directions, and they cross in zone 3: north of it, heating sets the size of a heat pump; south of it, cooling does.
What the figures give for a real house
For a 1,500 sq ft house with average insulation, three occupants and a kitchen, the calculators give:
| Zone | Cooling load | AC size | Heating load (output) | Furnace input at 95% |
|---|---|---|---|---|
| 1 | 40,600 BTU/hr | 3.5 tons | 30,000 BTU/hr | 40,000 |
| 2 | 37,600 BTU/hr | 3.5 tons | 37,500 BTU/hr | 40,000 |
| 3 | 34,600 BTU/hr | 3 tons | 45,000 BTU/hr | 60,000 |
| 4 | 31,600 BTU/hr | 3 tons | 57,000 BTU/hr | 60,000 |
| 5 | 27,100 BTU/hr | 2.5 tons | 67,500 BTU/hr | 80,000 |
| 6 | 24,100 BTU/hr | 2.5 tons | 78,000 BTU/hr | 100,000 |
| 7 | 22,600 BTU/hr | 2 tons | 90,000 BTU/hr | 100,000 |
Two things stand out. The air conditioner shrinks and the furnace grows as you go north, so a single “house of this size needs this much” rule is wrong for one of them almost everywhere. And the rounding to sizes that are sold matters: in zone 6 the cooling load of 24,100 BTU per hour is just over 2 tons, so the next half ton, 2.5, is 24% above it. In a case like that a Manual J often shows 2 tons is enough.
The adjustments
The table describes an average house. The calculators adjust it with five factors, all shown in their results:
Insulation has the largest effect and is the one you can change. A poorly insulated house in zone 5 has a heating load of about 54 BTU per hour per square foot, close to the average house in zone 6; air sealing and attic insulation can move it back by more than a climate zone. Ceiling height is the second: a house with 10 ft ceilings needs a fifth more than the table.
The same house in two climates
A 2,000 sq ft house with average insulation, 8 ft ceilings, three people and a kitchen, in Atlanta (zone 3) and in Chicago (zone 5).
- Atlanta cooling: 2,000 × 20 + 600 + 4,000 = 44,600 BTU per hour, so 4 tons
- Chicago cooling: 2,000 × 15 + 600 + 4,000 = 34,600 BTU per hour, so 3 tons
- Atlanta heating: 2,000 × 30 = 60,000 BTU per hour of output
- Chicago heating: 2,000 × 45 = 90,000 BTU per hour of output
The Chicago house needs a smaller air conditioner and a furnace half again as big. For a heat pump, the Atlanta house can be sized for cooling and cover most of its heating; the Chicago house will need backup heat or a cold-climate model, which the heat pump size calculator works out with the guide to heat pumps in cold climates.
Rooms, zones and emitters
The same per-square-foot figures size individual rooms. The mini-split size calculator applies the cooling figure to each zone, with a factor for the room’s use; the baseboard heater calculator and the radiator calculator apply the heating figure to a room to find the watts or feet of emitter it needs. A 168 sq ft bedroom in zone 4 has a heating load of 6,384 BTU per hour, which is 8 ft of standard electric baseboard.
Season energy is a different question
Per-square-foot figures describe the design hour, the hottest or coldest typical moment. They say nothing about how much energy the season uses, which depends on how many hours are hot or cold. That is the job of degree days: a zone 5 city has about 6,300 heating degree days a year, a zone 3 city about 2,800, and the guide to heating and cooling degree days explains how to turn a design load into a fuel bill.
When to stop using the rule
The per-square-foot method is a starting estimate. Stop using it, and ask for a Manual J load calculation, when you are buying equipment, when the house is unusual (a lot of glass, a cathedral ceiling, a well-insulated new build, a basement that is half the floor area), or when the result sits close to a size boundary. The guide on how to size an air conditioner explains what Manual J adds and how to read one.