How to use it
- Choose metric or US units and enter the room area and ceiling height.
- Pick how much sun the room gets, and enter how many people regularly use it.
- Tick the kitchen box if the room is a kitchen.
- Read the recommended capacity in BTU/h, kW and tons.
Where the numbers come from
The base capacity is a lookup from the floor area, following the room size table published in ENERGY STAR guidance for room air conditioners:
| Area (sq ft) | BTU/h |
|---|---|
| up to 150 | 5,000 |
| 150 to 250 | 6,000 |
| 250 to 300 | 7,000 |
| 300 to 350 | 8,000 |
| 350 to 400 | 9,000 |
| 400 to 450 | 10,000 |
| 450 to 550 | 12,000 |
| 550 to 700 | 14,000 |
| 700 to 1,000 | 18,000 |
| 1,000 to 1,200 | 21,000 |
| 1,200 to 1,400 | 23,000 |
| 1,400 to 1,500 | 24,000 |
| 1,500 to 2,000 | 30,000 |
| 2,000 to 2,500 | 34,000 |
The calculator then applies the sun adjustment of minus or plus 10 percent, adds 600 BTU/h for each person beyond two and 4,000 BTU/h for a kitchen. Taller ceilings scale the base value in proportion to the height above 8 ft. Metric areas are converted with 1 sq ft = 0.09290304 m2.
This is a rule-of-thumb estimate for a room with typical insulation. It is not a cooling load calculation.
Worked example
A 400 sq ft room is in the 350 to 400 band, so the base is 9,000 BTU/h. Very sunny: 9,000 x 1.1 = 9,900. Four people: two extra, adding 1,200 for 11,100. A kitchen adds 4,000 for 15,100 BTU/h, or about 1.26 tons.
Tips
- For open-plan spaces, add the connected areas together.
- Poor insulation, large west-facing windows or a hot climate may call for the next size up; a professional load calculation is better for whole-home systems.
- Compare the BTU rating on the label with this figure, not the unit’s price or size.