Purchasing a new air conditioner requires careful thought, especially when it comes to selecting the right size for your space. It's time to remove the guesswork and learn how to pick the best air conditioner for your home.

Step 1: Measure Your Space

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Understanding how those air conditioner sizes affect your home's comfort is important. An air conditioner that is too small will have to run constantly, using extra electricity while remaining unable to evenly cool the space, while a unit that’s too large will cool the room too quickly and will be turned on and off more frequently, which can cause unnecessary wear and tear on your air conditioner. Choosing the right air conditioner size will facilitate more energy-efficient cooling throughout your space.

To find what size air conditioner you need, you’ll start by measuring your space. Measure the length and width of the room, multiplying these to determine the square footage of the area. Add the square footage of any rooms not separated by doors, as the air conditioner will be used to cool both spaces. You should also measure the height of your ceilings – tall ceilings may require a higher cooling capacity to keep your entire space cooled.

Step 2: Calculate Cooling Capacity Needed

Rather than measuring the literal size of the cooling device, air conditioner sizes are rated by their cooling capacity. Air conditioners use British thermal units (Btus) to measure the energy required to remove heat from your home in an hour. The higher Btu rating, the quicker an air conditioner can effectively cool your space.

How to Determine Btus

As a rule of thumb, an air conditioner requires 20 Btus to properly cool each square foot of space in your home. A simple mathematical calculation can help you determine the minimum number of Btus you should choose. From there, you can take the other factors into account.

Once you’ve calculated the square footage of your space, you can multiply this number by 20 Btus to get the cooling size for that room. For example, if a room is 15 feet long by 12 feet wide, it will be 180 square feet. Multiply this number by 20 to get 3,600 Btus, which is the minimum cooling capacity of the air conditioner you should get.

Tip!

If your Btu requirement is between sizes, choose the larger unit as that extra power will come in handy on extremely hot days, though you should go no more than 15% over the recommended amount of Btus to achieve the most effective cooling.

Btus for Common Room Sizes

Small Room: A small bedroom or home office of between 100 and 300 square feet will need between 5,000 and 6,000 Btus for cooling.

Medium Room: A primary bedroom or playroom of between 250 and 400 square feet requires an air conditioner with between 7,000 and 8,500 Btus for cooling.

Large Room: For living rooms, kitchens, and open plan spaces of between 350 and 650 square feet, you would benefit from between 9,800 and 12,500 Btus.

Step 3: Think About Additional Factors

Though multiplying your square footage can give you a good place to start, other factors can increase or decrease the Btus needed for your space.

• Ceiling Height: A ceiling of 8 feet or higher will need 10% added to the Btu for each additional foot.
• Natural Sunlight: South- or west-facing rooms often receive more sunlight. If the room is very sunny, add 10% – 15% capacity. Alternatively, rooms that are very shady require less Btus, so you can subtract 10% from your calculation.
• Climate: Homes in warmer climates often require higher Btus to maintain a comfortable indoor temperature.
• Insulation: If your home has proper insulation, it’s more likely to retain cool air and won’t have to work as hard to maintain a steady temperature.
• Kitchen: If the air conditioner is placed in the kitchen, increase the capacity by 4,000 Btu. Large appliances like ranges and refrigerators tend to make the space warmer and harder to cool.
• Occupancy: If more than two people often occupy the room, then add 600 Btu per additional person.
• Windows: The size and type of window can impact the efficiency of air conditioners. For example, large single-pane windows are much thinner, which provides less insulation and will require more Btus to keep cool.

Step 4: Consider the Most Compatible Type

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Once you know what capacity you'll need, you can finally move onto selecting the type of air conditioner. Depending on the cooling capacity required for your space, you might benefit from one type over another. If you would like a more in-depth look at these air conditioners, read our No Sweat Guide to Air Conditioner Types.

Portable: Portable air conditioners are great for cooling small spaces and individual rooms. They have Btu ratings of up to 14,000 and can cool rooms of up to 500 square feet. These units are smaller in size and can be moved to wherever there is a window through which to vent. To cool more than one room at a time, you will need to buy additional units.

Window: Window air conditioners are mounted within a window frame and use fans to
bring in cool air while removing hot air. They can have up to 12,500 Btus and can cool rooms up to 650 square feet. It is possible to place multiple window air conditioners throughout a room or house to increase their cooling capacity.

Through-the-Wall: Through-the-wall air conditioners are similar to window models, but are permanently installed in an exterior wall using a wall sleeve for weight support. These air conditioners have up to 18,00 Btus and can cool rooms of up to 700 square feet. They are generally used to cool only one room at a time and can also be used as supplemental cooling to other air conditioners.

Ductless Mini Split: All ductless mini split air conditioners are permanently installed and connected to an exterior air compressor to connect to up to four units which can be placed throughout your home. These units have Btu ratings of up to 56,000 and can cool a space of 3,100 square feet.

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