Cooling
Air conditioner BTU calculator
Size a window, wall or portable air conditioner for one room. The calculator uses the ENERGY STAR sizing chart and its four adjustments, and shows the answer in BTU per hour and kilowatts.
Cooling capacity needed
350 ft² room, average sun, 2 people.
9,000 BTU/h
2.64 kW of cooling · 0.75 ton
- Look for a unit of9,000 BTU/ha chart size
- In kilowatts2.6 kWcooling capacity
| Chart size for 350 ft² (350 to under 400 ft²) | 9,000 |
|---|---|
| Sun: no change | 0 |
| People: 2 (no change up to 2) | 0 |
| Kitchen: no | 0 |
| Cooling capacity needed | 9,000 |
The chart assumes 8 ft (2.4 m) ceilings. A taller room has more air to cool, so check the size with a seller or installer.
| Area to be cooled | BTU/h | kW |
|---|---|---|
| 100 to under 150 ft² | 5,000 | 1.5 |
| 150 to under 250 ft² | 6,000 | 1.8 |
| 250 to under 300 ft² | 7,000 | 2.1 |
| 300 to under 350 ft² | 8,000 | 2.3 |
| 350 to under 400 ft² (your room) | 9,000 | 2.6 |
| 400 to under 450 ft² | 10,000 | 2.9 |
| 450 to under 550 ft² | 12,000 | 3.5 |
| 550 to under 700 ft² | 14,000 | 4.1 |
| 700 to under 1,000 ft² | 18,000 | 5.3 |
| 1,000 to under 1,200 ft² | 21,000 | 6.2 |
| 1,200 to under 1,400 ft² | 23,000 | 6.7 |
| 1,400 to under 1,500 ft² | 24,000 | 7.0 |
| 1,500 to under 2,000 ft² | 30,000 | 8.8 |
| 2,000 to under 2,500 ft² | 34,000 | 10.0 |
How to use the air conditioner calculator
- Enter the floor area the unit has to cool, or its length and width, in feet or metres.
- Say how sunny the room is, how many people normally use it and whether it is a kitchen.
- Read the capacity in BTU/h and kW, then look for a unit at or just above it.
Where the numbers come from
ENERGY STAR, the US Environmental Protection Agency's efficiency programme, publishes a sizing chart for room air conditioners: 5,000 BTU/h for 100 to 150 ft², rising to 34,000 BTU/h for 2,000 to 2,500 ft². It then adjusts for the room: take off 10% if the room is heavily shaded, add 10% if it is very sunny, add 600 BTU/h for every regular occupant beyond two, and add 4,000 BTU/h if the unit is used in a kitchen. This calculator applies the percentage to the chart size first and then adds the fixed amounts.
Units sold outside North America are usually rated in kilowatts of cooling. One kilowatt is 3,412 BTU/h, so a 9,000 BTU/h unit is about 2.6 kW and a 12,000 BTU/h unit (one "ton") about 3.5 kW.
Worked example
A 300 ft² kitchen-diner in full afternoon sun, used by four people: the chart gives 8,000 BTU/h, sun adds 800, two extra people add 1,200 and the kitchen adds 4,000, for 14,000 BTU/h, about 4.1 kW. That happens to be a chart size, so a 14,000 BTU/h unit fits.
Before you buy
ENERGY STAR warns that an oversized unit costs more, wastes energy and does not cool better: it chills the air before it has removed the moisture, so the room feels damp and clammy. Bigger is not safer, so pick the size that matches the chart rather than a step or two above it.
The chart is worked out for 8-foot ceilings and ordinary rooms. A loft room under a hot roof, a conservatory, a room with a lot of west-facing glass or a hot, humid climate may need more; a well-insulated, shaded room may need less. For central air or a heat pump serving several rooms, installers size from a room-by-room cooling-load calculation such as ACCA Manual J instead of a chart.
Shopping for a portable or window unit online? If a shop you have never used has a price that looks too good, look up its domain with WebTraceCheck's free DNS and domain lookup before you pay, to see when it was registered and where it is hosted.
Sources
- ENERGY STAR (US EPA), Room Air Conditioners, "Properly Sized Room Air Conditioners": the sizing chart, the shade, sun, occupant and kitchen adjustments, the 8-foot ceiling assumption and the note on oversized units.
- Conversion: 1 W = 3.412 BTU/h (International Table BTU); one ton of cooling = 12,000 BTU/h.
Questions
What size air conditioner do I need for a 12 × 12 room?
A 12 × 12 foot room is 144 ft², which falls in the ENERGY STAR chart's first row (100 to 150 ft²): 5,000 BTU/h, about 1.5 kW. Make it 5,500 BTU/h if the room is very sunny, and add 600 BTU/h for each regular occupant beyond two.
How many BTU per square foot does an air conditioner need?
For rooms of about 250 to 1,000 ft², the chart works out at roughly 18 to 28 BTU/h per square foot. Small rooms need more per square foot (50 at 100 ft²) and large open areas less (about 14 near 2,500 ft²), so use the chart row rather than a single rate.
What happens if the air conditioner is too big?
According to ENERGY STAR, an oversized unit costs more to buy, wastes energy and cools the room before it has removed the humidity, leaving it cool but damp and clammy. Choose the chart size for your room and adjustments.
How do I convert BTU to kW?
Divide BTU/h by 3,412 to get kW. A 9,000 BTU/h unit is about 2.6 kW, 12,000 BTU/h about 3.5 kW and 18,000 BTU/h about 5.3 kW. Multiply kW by 3,412 to go the other way.
Does the chart work for rooms with high ceilings?
The ENERGY STAR capacities are calculated for 8-foot (2.4 m) ceilings. A room with a much higher ceiling holds more air and often more sun-heated wall, so treat the chart size as a minimum and ask the seller or an installer.