BTU SizerHow Many BTU Per Square Foot?
Every sizing conversation starts with a rate per square foot, and almost every argument about sizing is really an argument about which rate.
The cooling rate is 20, almost everywhere
Twenty BTU per hour for every square foot of floor is the figure behind the ENERGY STAR room air conditioner chart, and it holds across most of the United States. It assumes an eight-foot ceiling, ordinary daylight and two people in the room.
What surprises people is how little it varies. From the Gulf coast to the Canadian border the cooling rate moves between about 18 and 22 — barely 20% — because summer indoor and outdoor temperatures are similar almost everywhere. Phoenix is hotter than Boston for longer, but the temperature difference an air conditioner works against on a given afternoon is much the same.
The heating rate is not
Heating doubles across the same map:
| Climate zone | Example | Heating BTU/h per sq ft |
|---|---|---|
| 1 | Miami | 27 |
| 2 | Houston | 32 |
| 3 | Atlanta | 37 |
| 4 | Baltimore | 42 |
| 5 | Chicago | 47 |
| 6 | Minneapolis | 52 |
| 7 | Duluth | 57 |
The load follows the gap between the temperature you want inside and the coldest the outside gets. In summer that gap is 15 to 25 °F everywhere. In winter it ranges from 25 °F to 70 °F, and the heating rate follows it directly. This is why a national rule of thumb works for cooling and is useless for furnaces.
The four corrections
A rate per square foot describes an average room. Yours is not one:
- Ceiling height. Multiply by height ÷ 8. You condition a volume, and every chart was written at eight feet.
- Sun. Add 10% for a room with unshaded south or west glass, take 10% off a shaded north one.
- Insulation. Add 15% for draughty or single-glazed, take 10% off a well-sealed modern build.
- People and appliances. Add 600 BTU/h per person past two. A kitchen gets a flat 4,000 because of the cooking.
Applied together these can move the answer by 40%, which is more than a full step on most size ladders. That is the difference between a rule of thumb and a guess.
Where the rate stops being enough
For one room and one machine, this is the calculation the whole industry uses and it lands within a size. For a whole-house system it does not see what actually drives the load: the area of west-facing glass, ducts running through an unconditioned attic, how leaky the shell is. That is what an ACCA Manual J calculation measures, and why a contractor’s number can legitimately differ from the one here.
Start with the air conditioner size calculator for cooling or the furnace calculator for heating — both apply these corrections for you and land on a size that is sold.