A room air conditioner is sized by the heat it can take out of a room in an hour, in BTU per hour in North America, in kilowatts in Europe and Australia, and often in tons in India and the Middle East. Enter the floor area and the calculator reads the ENERGY STAR sizing chart, then applies its four adjustments.
Example: a sunny 12 × 15 ft bedroom (180 ft², 16.7 m²) where three people often sit. The chart gives 6,000 BTU/h for 150–250 ft². Very sunny: +10% = 6,600. One person above two: +600. Total 7,200 BTU/h, so an 8,000 BTU/h window unit, or the smallest 9,000 BTU/h mini-split.
The ENERGY STAR sizing chart
For an 8 ft (2.4 m) ceiling. The metric columns are the same rows converted.
| Floor area (ft²) | Floor area (m²) | Capacity (BTU/h) | Cooling (kW) |
|---|---|---|---|
| 100–150 | 9.3–13.9 | 5,000 | 1.47 |
| 150–250 | 13.9–23.2 | 6,000 | 1.76 |
| 250–300 | 23.2–27.9 | 7,000 | 2.05 |
| 300–350 | 27.9–32.5 | 8,000 | 2.34 |
| 350–400 | 32.5–37.2 | 9,000 | 2.64 |
| 400–450 | 37.2–41.8 | 10,000 | 2.93 |
| 450–550 | 41.8–51.1 | 12,000 | 3.52 |
| 550–700 | 51.1–65.0 | 14,000 | 4.10 |
| 700–1,000 | 65.0–92.9 | 18,000 | 5.28 |
| 1,000–1,200 | 92.9–111.5 | 21,000 | 6.15 |
| 1,200–1,400 | 111.5–130.1 | 23,000 | 6.74 |
| 1,400–1,500 | 130.1–139.4 | 24,000 | 7.03 |
| 1,500–2,000 | 139.4–185.8 | 30,000 | 8.79 |
| 2,000–2,500 | 185.8–232.3 | 34,000 | 9.96 |
Then adjust:
- Heavily shaded room: 10% less.
- Very sunny room: 10% more.
- More than two people regularly in the room: +600 BTU/h for each extra person.
- Kitchen: +4,000 BTU/h for the heat of the stove and oven.
The chart assumes an 8 ft ceiling. For a taller room the calculator scales the result by the extra height (a 10 ft ceiling adds 25%): that is our assumption, because the volume of air grows with it. ENERGY STAR itself only says that taller ceilings need more.
The same unit, labeled four ways
Mini-splits are sold in a few standard sizes. Shops in different countries label the same box differently:
| BTU/h (US, Canada) | kW of cooling (Europe, Australia) | Tons (India, Gulf) |
|---|---|---|
| 9,000 | 2.6 (often sold as “2.5 kW”) | 0.75 |
| 12,000 | 3.5 | 1 |
| 18,000 | 5.3 (often “5.0 kW”) | 1.5 |
| 24,000 | 7.0 | 2 |
| 36,000 | 10.6 | 3 |
So a “1.5 ton” split in Mumbai, an 18,000 BTU unit in Texas and a 5 kW unit in Madrid are the same class of machine. The nameplate value is what it removes from the room; the electricity it draws is listed separately, as input power in watts.
Three quick metric examples
| Room | Chart value | With adjustments | Smallest split that fits |
|---|---|---|---|
| 20 m² bedroom, normal sun | 6,000 BTU/h | 6,000 BTU/h (1.8 kW) | 9,000 BTU/h (2.6 kW) |
| 30 m² living room, very sunny | 8,000 BTU/h | 8,800 BTU/h (2.6 kW) | 9,000 BTU/h (2.6 kW) |
| 12 m² kitchen | 5,000 BTU/h | 9,000 BTU/h (2.6 kW) | 9,000 BTU/h (2.6 kW) |
What the chart does not know
The chart is for one room in an ordinary house. A top-floor room under a dark roof, a wall of west-facing glass, poor insulation or a very hot climate can push the real need up. For a whole house, a central system or an open-plan space over 2,500 ft² (232 m²), an installer should do a proper load calculation (Manual J in the US). Once you know the size, the appliance energy cost calculator shows what it will cost to run.
Standing in a store comparing boxes with no signal? Once zet4u is saved on your phone, the calculator works offline.
Frequently asked questions
How many BTU do I need per square foot?
About 20–25 BTU/h per square foot for an average room with an 8 ft ceiling, which is what the ENERGY STAR chart works out to (10,000 BTU/h for 400–450 ft²). In metric that is about 215–270 BTU/h, or 63–79 W of cooling, per square meter. Small rooms come out higher per square foot.
What size air conditioner for a 12 × 12 room?
A 12 × 12 ft room is 144 ft², the smallest row of the chart: 5,000 BTU/h. Add 10% if it is very sunny and 600 BTU/h for each person above two. A 3.5 × 3.5 m room (12.25 m²) gets the same answer.
Is a bigger air conditioner better?
No. An oversized unit cools the air quickly and switches off before it has pulled the moisture out, so the room feels cold and clammy and the unit cycles on and off. Choose the smallest size that covers what the calculator gives.
How do I convert BTU to kW?
Multiply BTU/h by 0.000293 (1 BTU/h = 0.293 W). A 12,000 BTU/h unit is 3.5 kW of cooling, which is also exactly 1 ton of refrigeration. This is cooling output, not the electricity the unit uses: that is usually a third or less of it.
Sources
- ENERGY STAR (US EPA): Room air conditioner fact sheet, with the sizing chart
- NIST Special Publication 811: Guide for the Use of the SI, Appendix B (British thermal unit)
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