Category - Air Conditioning
Guide author photo

Will Scholfield, Engineer

31 Jan : Updated 24 May ● 4 min read

What size air conditioner do I need? How to calculate


Although the UK doesn’t experience hot weather year-round, it becomes unbearable when we get a heatwave, as many households don’t have the necessary cooling systems. Instead of investing in an air conditioning unit, many rely on fans to maintain a comfortable temperature, which only moves hot air around the room.

Using air conditioning will ensure your home is always prepared for the unexpected hot spells, making you wonder why you put it off for so long. However, it’s important to choose a model that’s a suitable size for your space. Otherwise, it can be expensive to run and won’t achieve the intended results.

To help you find an air conditioner that will effectively and efficiently cool down the room, BOXT has put together a guide on determining which sized unit you need.

Why does an air conditioner’s size matter?

Opting for a smaller air conditioner than needed may save you money initially, but it’ll likely cost more to run long-term. For a small air conditioner to cool down a large room, it would need to use more energy, causing your electricity bills to surge, without achieving the desired temperature.

In contrast, an air conditioner that’s too large won’t keep your room colder than if you installed the recommended sized unit. Instead, it’ll cool the room down quicker and then turn off without having enough time to remove the moisture from the air.

As a result, you’ll be left with a cold but humid space. It’ll also waste a lot of energy as the system will have to constantly turn on and off to regulate the temperature, which can cause premature wear and tear. 

What sized air conditioner do you need?

Now that you’re familiar with the importance of installing a suitably sized air conditioner, you’ll need to follow the below formula to find a suitable unit for your home.


Measure the room

Before researching home air conditioning units, you’ll need to know the dimensions of the space you want to cool down. If the area includes two rooms that a door doesn’t separate, combine the sizes.

To find the floor square footage, measure the room in feet and then multiply the room’s length times the width. If the space is triangular, divide the total amount by two.

If your room’s shape is complex, you may need to consult with a professional to determine the accurate metrics.

Find the BTU (British Thermal Unit)

The BTU is the rating used to measure the cooling capacity of an air conditioner. One BTU is the amount of heat needed to increase the room’s temperature. In the case of air conditioning, it’s the amount of heat it needs to remove. So, the higher the BTU of a unit is, the more powerful it is.

To match the air conditioner’s size to the room’s size, multiply the area’s square footage by 25, which will give you the required BTU. 

Incorporate other factors

You should incorporate certain aspects of the room into the BTU calculation, to help you find an appropriate air conditioner. Here are some elements that would require adjusting the calculation:

  • Lower the BTU by 10% if the room is extremely shaded.
  • Add 10% to the BTU if the room has high sun exposure.
  • Due to oven use, increase the BTU by 4,000 if you’re placing the unit in a kitchen.
  • If more than two people frequently inhabit the area, add 600 BTU for every additional person.

Other aspects that can impact which sized air conditioner is most appropriate for a room are the property’s location and insulation.

For further advice on finding an appropriately sized air conditioning unit for your house, seek instant advice from a qualified BOXT engineer using our live chat.

Need help finding a suitably sized AC unit?

If you’ve decided that your home would benefit from air conditioning but need help choosing a suitable unit, use the BOXT ‘find an AC’ online tool. After answering a few simple questions about your home, we’ll find the best air conditioner for your property.

Getting the unit installed couldn’t be easier either. Simply choose from a wide range of installation dates, including the next day, and get to known the engineer through their profile. The installer can also offer advice on the best placement for your air conditioner.

To keep your home at a comfortable temperature through heatwaves, get a fixed price quote for an installation today.

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