How To Size A Window Air Conditioner

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Updated September 16, 2022

The best air conditioners operate at a low sound level, come packed with cooling power, and even offer remote control. Before you buy an ac unit, however, you will want to ask yourself, “how many BTUs do I need for an air conditioner?” This can be a tricky question for people with central air conditioners. Finding the correct BTU level ensures an ac unit provides a large enough cooling area and will not require hours of operation time to keep your family cool.


  • The rule of thumb is 20 BTU (British Thermal Unit) for every square foot of room.
  • Always check a unit’s electrical requirements to ensure your home can accommodate its use.
  • Always take your room’s ceiling height into account when measuring the square footage of the space.

How to Properly Size a Window Air Conditioner

You might be asking yourself “how many tons of air conditioning do I need” but it’s important to note that BTU and tonnage are different. And it is important to find the BTU as well because, put simply, size matters. If a room is too large, the window unit cannot cool it. You can avoid this by comparing your room size to BTUs put out by the unit. It is a four-step process where you find the square foot size of your room and then find a product with the right cooling area.

Insider Tip

Make sure your AC has a clean filter. Dirty filters can reduce efficiency and affect the room’s air.


Measure the length, width, and height of the room you want to cool. Multiple the three numbers together to find the square footage of the room. This is easiest to do if two people work together, but if you are working alone, use the tape to hold down one end of the tape measure to make it easier.


Once you have the room’s measurement, multiply it by 20. The EPA suggests ACs have a cooling area of about 20 BTU per square foot. By multiplying your room’s size by 20, you will determine how many BTU you need, which will cause your window A/C to blow colder air. This is primarily for Energy Star products, but it is also an excellent general rule.


Measure the window frame where you intend to put the unit. Then measure from the edge to the nearest outlet. AC units require a lot of power and need a committed outlet. Write these measurements down along with the required BTU rate.


Shop for units that offer a detailed product description. This includes product tips, whether they include a convenient installation kit, the cooling capacity, and the product dimension. Moreover, get the supplies you need to clean your window air conditioner correctly and keep it maintained.


AC shipping weight is quite high. Look for special offers before buying a unit and try to avoid standard shipping. Also, be sure to check shipping dimensions as the units can be very heavy.


How many watts does a window AC use?

An AC’s energy usage varies from model to model. Some focus on energy efficiency while others do not. If you want to increase your energy savings, look for an Energy Star model. They often come with an efficiency rating and still offer exceptional cooling power.

How do I calculate the right size air conditioner?

Window units are much like portable air conditioners. They have a specific cooling area described as BTU per square foot. This is found by following the above formula to find a room’s square footage, then multiplying it by 20.

How do I make my window air conditioner airtight?

A unit’s efficient installation usually relies on padding and other materials provided by the manufacturer, especially for their featured products. In addition, a good sales team will know what products you need to create air deflection. They can also offer answers about product setup and find a window installation kit if one is required.

What to consider before buying?

If you want an energy-efficient machine to reduce operating costs, consider an Energy Star product. You should also consider if it has a washable filter and variable fan speed, or if you’ll need extension cords to reach the committed outlet. There are also special features like voice control or response to voice commands, sleep mode, a full-featured remote, or an LED display.

STAT: If your central air conditioning unit is more than 12 years old, replacing it with an ENERGY STAR certified model could cut your cooling costs by 30%. (source)

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