How it’s calculated
ENERGY STAR publishes a simple sizing chart for room (window, wall and portable) air conditioners based on the floor area, assuming an 8 ft ceiling, followed by four adjustments.
| Area (ft²) | Capacity (BTU/h) |
|---|---|
| 100–150 | 5,000 |
| 150–250 | 6,000 |
| 250–300 | 7,000 |
| 300–350 | 8,000 |
| 350–400 | 9,000 |
| 400–450 | 10,000 |
| 450–550 | 12,000 |
| 500–700 | 14,000 |
| 700–1,000 | 18,000 |
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two regular occupants: add 600 BTU/h per additional person.
- Kitchen: add 4,000 BTU/h.
The chart’s 450–550 and 500–700 rows overlap; ENERGY STAR’s own example puts a 525 ft² room at 14,000 BTU/h, so this calculator switches to 14,000 above 500 ft².
Example: a very sunny 300 ft² kitchen used by 4 people: 7,000 × 1.10 + 2 × 600 + 4,000 = 12,900 BTU/h.
This is a rule of thumb for single rooms. Central systems should be sized with a proper load calculation (ACCA Manual J) that accounts for insulation, windows, climate and air leakage.
Frequently asked questions
How many BTU do I need per square foot?
Roughly 20 to 35 BTU/h per ft² from the ENERGY STAR chart, with smaller rooms needing more per square foot. Use the chart value, then adjust.
Is a bigger air conditioner better?
No. ENERGY STAR notes that an oversized unit cools quickly but leaves humidity behind, making the room cold and clammy, and wastes energy.
What is a ton of cooling?
12,000 BTU/h, about 3.52 kW of heat removal.
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Sources
- ENERGY STAR Qualified Room Air Conditioners (sizing chart) — U.S. EPA / DOE ENERGY STAR
- Room Air Conditioners — ENERGY STAR
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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