What size of Air Conditioner do I need?

How to determine the appropriate size of an Air Conditioner.

A good air conditioner will not only cool your house in the summer; it can also effectively heat your house in the winter. But, in order for your air conditioner to be effective you need to size it correctly to your house or the space you’re trying to cool / heat.

Before you get to grips with sizing it is important to remember that maintaining your air conditioner is vital. A good system is an investment; you need a reliable service such as Ace Electrix to install and maintain the unit or system for you.

It’s essential that you don’t underestimate this; if you do you’ll be replacing your air conditioner far more often than you need to.

Sizing Your Room

Air conditioners generally give you a British Thermal Unit (BTU) rating. This is a universal measurement which refers to the amount of heat that it takes to raise the temperature of one pound of water by one degree Fahrenheit.

Of course your room isn’t measured in BTU’s; you’ve probably measured it in square feet.

If you’re not sure how to do this simply take the length of your room and multiply it by the width. For example, a 15ft wide room by 20ft long has a size of 15 x 20 = 300 square foot.

You can then assess the size of the air conditioner you need by the size of the room and the BTU needed to heat it:

Sq Feet To Be Cooled BTU’s Needed
100 – 150 5,000
150 – 250 6,000
250- 300 7,000
300 – 350 8,000
350 – 400 9,000
400 – 450 10,000
450 – 550 12,000
550 – 700 14,000
700 – 1000 18,000
1,000 – 1,200 21,000
1,200 – 1,400 23,000
1,400 – 1,500 24,000
1,500 – 2,000 30,000
2,000 – 2,500 34,000
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However, there are other factors that must be considered before you purchase the right size air conditioner:


The more sunshine the room has the more effort it will take to cool. It’s a good idea to increase your BTU rating by at least 10% to cover the additional effort your air conditioner unit will need to cool effectively.

Kitchen Heat

The sun is powerful but so are your ovens, if you’re cooking in the heat of summer. You’ll need to add at least 4,000 BTU’s to your air conditioner to ensure it is capable of keeping your kitchen space cool.


You should also consider the amount of people that will be in the room for any length of time. If 2 people are spending most of their tie in one room then the BTU requirements should be put up by another 600. This increases by another 300 for every additional person in the room.


It is also important to consider how often the doors are open to the room; whether there are funny corners and features that will affect the size of the room and the likelihood of heat getting into the building from another means.

All of these can affect your calculations.

However, don’t be tempted just to opt for the biggest air conditioner you can find. You will be wasting your money and the unit may not work efficiently if it is trying to cool a space much smaller than its capabilities.

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