I have experienced this in my professional life. My client once asked me about what size of AC he would need for his bedroom (newly constructed). I told him politely that he could contact the showroom person to pick the right size.


After some time, it made me realize that the client did not need an accurate answer. He wants to get some professional recommendations. It also made me look like a naive person in the field in front of the client.

If it’s a high storey building, obviously there will be a separate HVAC team to make decisions on this. But that doesn’t mean we don’t need to know these details. At least it will be useful personally.

Let’s begin the topic.

What is Air Conditioning?

Air Conditioning is nothing but the mechanical act of absorbing heat from the air. It is a machine that is used to control the humidity and temperature of the air.

Every air conditioner has a tonnage value. It is not the weight unit; it is Refrigeration Tonne.


What is Refrigeration Tonne (RT)? 

In history, people used ice cubes to cool their houses. When ice cubes are at 32° Fahrenheit at their melting point, it starts to absorb the heat from the surrounding air and melts. Eventually makes the atmosphere cool.

We have studied something called BTU (British Thermal Unit). To completely melt down the 1-tonne ice cubes we need 286000 BTU heat energy at a 24 hours time interval.

So we need 286000/24 = 12000 BTU per hour to melt the ice gradually. Therefore 1 RT becomes 12000 BTU.

We need 12000 BTU heat per hour to melt 1 tonne of an ice cube in 24 hours. That’s how the refrigeration tonne denomination was born.

Air conditioner size calculation

Air Conditioner

So to choose the right size AC unit, we need to know the total temperature (BTU) that will be emitted by the room. It includes

  • Size of the room
  • Height of the ceiling
  • The humidity of the location
  • Purpose of the room (Kitchen, living room). Since each room has some appliances which release significant heat energy, we need to calculate them separately.
  • Total number of people residing inside the room
  • Number of appliances
  • How effectively has the place been insulated? (number of doors, windows and gaps)
  • Is it directly under the terrace? The slab transfers an enormous amount of heat from the sunlight.

It is much more complicated to calculate all these things accurately. You can use this calculator for quick reference.


Example Calculation – Assume we are going to calculate the ac unit needed for a master bedroom size of 12 x 15 feet for three people which is located on the first floor.

  • Step 1 – Calculate the area of the room = 12 x 15 = 180 sq.ft
  • Step 2 – Multiply the area by 30 (constant value to calculate the heat emission) = 180 X 30 = 5400 BTU
  • Step 3 – Add 1000 BTU for every feet above 8 Feet Ceiling = 5400+1000 = 6400 BTU
  • Step 4 – Add 600 BTU for each person above 2 people = 6400+600 = 7000 BTU
  • Step 5 – Convert BTU into Refrigeration Tonne (RT); We know 12000 BTU = 1 RT
                          • Therefore, 7000 BTU = 1/12000 x 7000 = 0.58 RT

Since a 0.5 tonne AC unit is not enough, We need at least 0.7 – 1 RT AC unit to make the room cool efficiently.

The above method is a rough calculation. Before buying the AC unit, get recommendations from experts.

Happy Learning 🙂


Bala is a Planning Engineer & he is the author and editor of Civil Planets.

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