PickGlobeAir conditioner size calculator
Enter the room in metres to see the cooling capacity in kW and BTU/h.
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- Size to the room: an oversized unit costs more, wastes energy and can leave the room damp.
- ENERGY STAR's table assumes an 8 ft (2.4 m) ceiling and adds or takes off for sun, extra people and a kitchen.
- This sizes cooling only. Heating a room with radiators uses a different method.
How we calculate
We look up your floor area in ENERGY STAR's room air conditioner sizing table, which assumes an 8 ft (2.4 m) ceiling, then apply its adjustments: reduce by 10% for a heavily shaded room, increase by 10% for a very sunny one, add 600 BTU/h for each person beyond two who regularly use the room, and add 4,000 BTU/h for a kitchen. ENERGY STAR does not say in what order to apply them: we apply the sun adjustment to the table size first and then add the people and kitchen amounts. The result is shown in BTU/h, in kW (1 BTU/h = 0.2931 W) and in tons (1 ton = 12,000 BTU/h), and we name the next size up on ENERGY STAR's own list of capacities.
Three worked examples:
- A 15 x 12 ft room (180 sq ft, 16.7 m2), normal sun, two people: the table gives 6,000 BTU/h, which is about 1.76 kW or half a ton.
- A 4.6 x 3.7 m room (17.0 m2, 183 sq ft), very sunny, four people: the table gives 6,000 BTU/h; +10% is 6,600; two extra people add 1,200, so 7,800 BTU/h (2.29 kW). The next size on ENERGY STAR's list is 8,000 BTU/h.
- A 5 x 4 m kitchen (20 m2, 215 sq ft), normal sun, two people: 6,000 from the table plus 4,000 for the kitchen is 10,000 BTU/h (2.93 kW, 0.83 ton), which is on ENERGY STAR's list.
ENERGY STAR and Natural Resources Canada both warn that an oversized unit costs more, wastes energy and can leave the room damp, because it cools the air before it removes humidity. We do not size heating: the UK's radiator method uses room volume and insulation, and we found no government or standards-body figure to publish. We also give no number for ceilings above 8 ft because ENERGY STAR gives none.
How air conditioners are sized
| Item | Rule |
|---|---|
| Floor area to capacity | ENERGY STAR gives a capacity for each floor area, assuming an 8-foot (2.4 m) ceiling: 100 to 150 sq ft needs 5,000 BTU/h, up to 2,500 sq ft needing 34,000 BTU/h. Our table is its table. |
| Sun and shade | Heavily shaded room: reduce the capacity by 10%. Very sunny room: increase it by 10%. |
| People | If more than two people regularly use the room, add 600 BTU/h for each extra person. |
| Kitchen | For a unit in a kitchen, add 4,000 BTU/h. |
| Bigger is not better | ENERGY STAR and Natural Resources Canada both warn that an oversized unit costs more, wastes energy and cools the air before it removes humidity, so the room can feel clammy. |
| Units | 1 BTU/h = 0.2931 W, so 12,000 BTU/h (one ton of refrigeration) is about 3.5 kW. 1 square metre = 10.76 square feet (NIST). |
| Ceilings and heating | ENERGY STAR gives no numeric factor for ceilings above 8 feet, so we do not guess one: ask a professional for tall rooms. This tool sizes cooling only. For heating (radiators) the UK method depends on the room volume and insulation, and we found no government or standards figure to publish. |
Checked on 27 October 2026.
Sources and last checked
- Room air conditioner sizing, checked 27 October 2026: ENERGY STAR: Room air conditioners, sizing (read 2026-10-27); Natural Resources Canada: Buying an air conditioner (read 2026-10-27); Natural Resources Canada: EnerGuide label for room air conditioners (read 2026-10-27); NIST Special Publication 811, SI units and conversion factors (read 2026-10-27)
A guide, not an engineer's heat-load calculation. Your room, windows, insulation and climate change what you need, so check the unit's rating and ask an installer for tall, open-plan or unusual rooms. Read more: How many BTU does a room air conditioner need.