How to Choose Right Size Air Conditioner for a Room?

No, we are not HVAC Engineer.

But if someone asks us to recommend how much tonne AC is required for their room? What do we do?

As a Civil Engineer, we need to know each and every component that makes a complete home.

Let’s dive into our topic.

What is Air Conditioning?

According to Google, Air Conditioning is a machine system which is used to control the humidity, ventilation and temperature of a building.

Air Conditioning

In History, people used ice cubes to make the surrounding cooler.

When Ice Cubes are in its melting point 32° Fahrenheit (0° celsius), it absorbs the surrounding heat and starts to melt and make the surrounding cooler as long as it completely melts down.

The idea of Air Conditioning starts from here. 

However, the functioning of an Air Conditioner is a whole lot different story. We stick to our topic.

Why do we measure Air Conditioning Capacity in Tonnes?

We know that A/C comes in Ton capacity which is not a weight notation.

It is Ton in refrigeration normally denoted in RT (Refrigeration Tonne).

In simple terms, We hope you have studied something as BTU in physics which is British Thermal Unit used to denote heat energy. We need 143 BTU’s to melt 1 pound of an ice cube.

So if we have 1 tonne of ice, it takes 286000 BTU’s to completely melt it down at 24 Hours time interval.

1 hour = 286000/24 = 12000 BTU’s.

So if we say 1 Refrigeration tonne (RT) AC, it will remove 12000 BTU heat from the room per hour.

How many tonnes of Air Conditioner is required for a room?

Here we are calculating how much amount of heat (BTU’s) we need to absorb from a room and then select the right size of AC.

Selecting Proper Size A/C depends on many factors.

  • Height of the ceiling
  • Number of people in the room
  • How well the room is insulated such as flooring, glass and other openings
  • Where is the room located? Is it directed against East (sun direction)?
  • How much heat will be emitted in the room? – depends on the type of the rooms such as Kitchen or bedroom
  • At which floor the room is located? Is there any floor above the room?


Below is a rough calculation. It is not accurate.

Consult with an A/C expert before finalising the size of the Air Conditioner.

AC Size Example Calculation

Assuming we need to install A/C for a bedroom of size 10 feet X 15 feet and 9 feet height where 3 people are going to stay.

Room 10X15

Step 1 Find the Area of the Room,

Total Area of the Room = 10 feet X 15 feet = 150 Square Feet

Step 2 Multiply the value with constants

Area by 30 (Constant) = 150 X 30 = 4500 BTUs.

If it is a kitchen, multiply the area by 40 ( 150 X 40 = 6000 BTUs – taking account of the extra heat emitted by cooking).

We stick to that 30 (Constant) = 4500 BTU’s

Step 3 – Height of Ceiling

For Every one foot above 8 feet, we need to add +1000 BTUs

Therefore, Total BTUs = 4500+1000 = 5500

Step 4 – Number of People

For each person above 2 people, we need to add 600 BTUs. If it is 4 people then we need to add 1200.

Here we are calculating only for 3 people. ( extra 1 person )

Therefore, Total BTUs = 4600 + 600 = 6100 BTUs.

Step 5 – Convert BTU’s to RT (Refrigeration Ton)

We know that 1 RT = 12000 BTUs

Therefore 6100 BTUs = 0.5 – 0.75 Ton AC.

So we need 0.75 Refrigeration Tonne AC for this room.

For Quick Reference, you can make use of this Blue Star AC Calculator.

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Happy Learning 🙂