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  • BlueLeavesDesign

How to Calculate the Perfect AC Size


  • When buying an AC, consider :

  • Size of your room

  • Tonnage of the air conditioning unit

  • The direction of your window

  • The energy that it will consume

  • Most air conditioners are rated in BTU per hour (British thermal unit). They are also by tonnage. One ton is equal to 12,000 BTU per hour.

  • In India, we usually measure in tonnage.

How to calculate :

  • Area Method : To calculate, multiply the length and breadth of the room (in feet), take square-root of the answer and divide by 10. The resulting number will give the desired tonnage.

  • Volume Method : Alternatively, multiply the length, breadth and height of a room (in feet). Divide the answer by 1000. This is the tonnage required.

Examples :

  • Using Area Method : To make it clearer, suppose

  1. The room is 10 feet wide by 15 feet long.

  2. That means 10×15=150 square feet.

  3. Take the sqrt(150)/10

  4. We get 1.247

  5. Choose an AC which is 1.2 Tonnes.

So basically for a room which is 150 square feet, we need to get 1.2 ton AC.​

  • Using Volume Method :

  1. The room is 10 feet wide by 15 feet long and height is 20 feet (double height).

  2. That means 10×15 x 20 = 3000 cu.feet.

  3. (3000)/1000

  4. We get 3 T

  5. Choose an AC which is 3 Tonnes.

So basically for a room which is 3000 cu. feet (double height), we need to get 3 ton AC.​


  • Smaller ton will continuously run in a larger area as it will take time to cool. This will increase your electric bill with minimal cooling effectiveness.

  • A larger ton AC can cool quickly but causes a sticky and clammy feeling. This is because an air conditioner also removes moisture from the air. This takes time. If the AC doesn't run long enough, the moisture does not get extracted properly and causes the sogginess.

  • Larger tonnage is better than lower tonnage.

  • For larger spaces, an age-old rule of thumb is to have 1 Ton AC for 400 sqft. of conditioned area.

  • Buy the best AC you can. More stars = better AC energy efficiency.

  • If your AC usage is 8 to 10 Hrs a day for 3 to 4 months, it is recommended to buy at least 3 Star rated AC. In case AC usage is extended 5 to 7 months, you should opt for 5-Star rated AC. For usage of 10 to 12 months one should opt for cooling only or Hot & Cold Inverter AC.

  • You can increase the tonnage by half ton or more if :

  • the rooms are exposed to direct sunlight

  • there are more people in the room than normal

  • if the room is located on the topmost floor

  • if room is located near kitchen

  • outside temperature is more than normal

  • Outdoor units must be installed

  • in areas not exposed to rain or sunlight

  • proper ventilated areas

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