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Air Conditioner Size Calculator (BTU)
Work out what size of room air conditioner you need, in BTU per hour and kilowatts, from the room's size, sun, number of people, and whether it is a kitchen, using the ENERGY STAR sizing chart.
Air Conditioner Size Calculator (BTU): The calculator looks up the floor area in the ENERGY STAR room air conditioner sizing chart, which runs from 5,000 BTU/h for 100 to 150 sq ft up to 34,000 BTU/h for 2,000 to 2,500 sq ft, assuming an 8-foot ceiling. It then applies ENERGY STAR's adjustments: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU/h for each regular occupant beyond two, and 4,000 BTU/h for a kitchen, and rounds down to the nearest thousand as ENERGY STAR advises. Higher ceilings are scaled up in proportion. Runs 100% locally in your browser with zero server file uploads.
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6,000 BTU/h
1.8 kW of cooling- ENERGY STAR chart for the floor area: 6,000 BTU/h
- Rounded down to the nearest thousand, as ENERGY STAR advises
Room area 208 sq ft (19.3 m²). Buy the size nearest this figure: a unit that is too big cools quickly but leaves the room damp and clammy, and wastes energy. This is a rule of thumb; insulation, windows, and climate matter too. 1 kW is about 3,412 BTU/h.
What changes the size you need
Floor area is the starting point, but heat comes from several places: sun through windows, especially facing south or west in the northern hemisphere; people, each giving off roughly as much heat as a 100-watt bulb; and appliances, which is why kitchens need much more. Poor insulation, a top-floor room under a hot roof, or large windows can each push the need up.
Running costs depend on the unit's efficiency and how long it runs; the electricity cost calculator estimates the bill from its wattage. On hot, humid days, the heat index calculator shows how hot it feels.
Reading the label
Capacities are given in BTU per hour in North America and in kW elsewhere; divide BTU/h by about 3,412 for kW. In parts of Asia, units are also sold by horsepower, an old label that roughly corresponds to 2.5 kW of cooling per horsepower in China.
How to use it
- Enter the room's length, width, and ceiling height in metres or feet.
- Choose how sunny the room is, how many people usually use it, and whether it is a kitchen.
- Read the recommended capacity, with each adjustment shown.
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Frequently asked questions
What does BTU mean for an air conditioner?
A British thermal unit is a unit of heat; an air conditioner's capacity in BTU per hour is how much heat it removes from the room each hour. 12,000 BTU/h, sometimes called one ton of cooling, is about 3.5 kW.
Is a bigger air conditioner better?
No. An oversized unit cools the air quickly and switches off before it has removed the humidity, leaving the room cold and clammy, and it cycles on and off more, wasting energy. ENERGY STAR recommends matching the size to the room.
How many BTU for a 12 by 12 room?
144 square feet falls in the 100 to 150 sq ft band, which needs about 5,000 BTU/h; add 10% if the room is very sunny, and 600 BTU/h for each extra person beyond two.
Does this work for central air conditioning?
Not reliably. Whole-house systems are sized with a detailed load calculation, Manual J in North America, that accounts for insulation, windows, ducts, and the local climate. Ask an installer to do one.
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