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Enter the floor area of the room, how sunny it is and how many people use it. The calculator follows the ENERGY STAR sizing chart for room air conditioners and its adjustments, and converts the result to kW and tons.

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Choose the unit you measured in.
Length × width of the room.
Where do I find this?

Measure the room's length and width and multiply them. For an L-shaped room add the two rectangles.

The chart assumes a ceiling of about 8 ft (2.4 m); add capacity for higher ceilings.

Large south- or west-facing windows make a room very sunny.
Each person above two adds 600 BTU per hour.
Where do I find this?

Count the people who regularly use the room at the same time, for example in a living room or office.

Cooking adds heat; ENERGY STAR adds 4,000 BTU per hour.

Result

Enter your values to see the result.

Cooling capacity
– BTU/h
Cooling capacity
– kW
Cooling capacity
– tons
Area used
– ft²

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How it is calculated

  • Base capacity from the ENERGY STAR chart by floor area (e.g. 250–300 ft² → 7,000 BTU/h; 450–550 ft² → 12,000 BTU/h)
  • Heavily shaded −10 %, very sunny +10 %; +600 BTU/h per person above two; kitchen +4,000 BTU/h
  • kW = BTU/h ÷ 3,412; tons = BTU/h ÷ 12,000; 1 m² = 10.76 ft²

Example

A sunny 300 ft² living room used by four people: chart 8,000 BTU/h (300–350 ft²), +10 % for sun = 8,800, plus 2 × 600 = 10,000 BTU/h, about 2.9 kW.

What this estimate leaves out

The chart is a rule of thumb for single rooms with normal ceilings and insulation. Whole houses, poor insulation, big glass areas or hot climates need a proper load calculation (Manual J in the US, a heat load calculation in Europe) by an HVAC professional.

Sources: ENERGY STAR: room air conditioner sizing chart

Written and reviewed by QLines, Cologne · Last reviewed

Guide: cooling with a heat pump

FAQ

Frequently Asked Questions

How many BTU do I need per square foot?

Roughly 20 BTU per hour per square foot for normal rooms: the ENERGY STAR chart gives 6,000 BTU/h for 150 to 250 ft² and 12,000 BTU/h for 450 to 550 ft². Adjust for sun, people and kitchens.

What size air conditioner do I need for a 12 x 12 room?

12 × 12 = 144 ft², which needs about 5,000 BTU/h according to the ENERGY STAR chart, or 6,000 BTU/h if the room is very sunny or used by more people.

How do I convert BTU to kW?

Divide BTU per hour by 3,412: 12,000 BTU/h is about 3.5 kW of cooling. One ton of cooling is 12,000 BTU/h.

Is a bigger air conditioner better?

No. An oversized unit cools the air quickly but switches off before it removes humidity, so the room feels clammy, and frequent cycling wastes energy.

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