What size air conditioner do you need? (2024 guide)

Choosing an appropriately sized air conditioner (AC) for your space is the key to keeping cool in summer. The sweet spot is an AC powerful enough to keep your home at the right temperature on the hottest days. A properly sized unit runs long enough to remove humidity without wasting energy from overcooling or cycling on and off constantly.

This guide walks you through the most important considerations when calculating the right AC size, including room dimensions and local climate. We also explain how professionals can give you the most accurate idea of what AC size you need through a Manual J or load calculation. 

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How do you size your home for an air conditioner?

Take the following steps to find the correct size air conditioner for your home.

Step 1: Determine what type of AC you need

First, figure out what kind of AC your home needs. There are five main air conditioner types, each fulfilling different needs and budgets.

• Central ACs: These are the standard choices for most whole-home, centralized cooling needs. The condenser sits outside while vents run cool air throughout your house. This system is great at maintaining a consistent temperature in larger homes. Installation is more complex, but it looks neat and clean.
• Ductless mini-split ACs: One of the most efficient ACs, these use separate indoor units mounted on walls or ceilings in different rooms. They work great in homes without existing ductwork. The only catch is their higher upfront costs.
• Portable ACs: These units use wheels and are ideal for spot cooling any room. Just vent the hot air exhaust through a window or sliding door. However, portables might not be powerful enough for large spaces, so they’re useful in garages or sheds.
• Split ACs: Also designed for whole homes, split ACs have the compressor outside while the evaporator unit is inside. Ductwork runs to different rooms with vents to direct the airflow. Splits offer flexibility but require professional installation.
• Window ACs: These are affordable and easy to install in your window frame, making them popular for cooling down a room or small space. They can sometimes be noisy.

Step 2: Calculate the right AC size for your house

To find out what size AC unit you need, first calculate the square footage of each room that needs cooling. 

Once you’ve crunched the numbers, match your space to the correct British thermal unit (Btu) rating. As a general rule, aim for around 20 Btu per square foot. For example, a 10-by-10-foot room needs about 10,000 Btu. 

The following table gives a general idea of what each AC size looks like for a home’s square footage.

You must also factor in ceiling height, local climate, sun exposure and the number of windows. Rooms with high ceilings or many windows require more powerful AC units. More hot and humid climates need a more powerful cooling system. 

To help you get closer to accurate measurements, Energy Star recommends making the following adjustments:

• Increase the total Btu by 4,000 if the unit is used in a kitchen.
• Increase the total Btu by 10% if a room is very sunny.
• Increase the total Btu by 600 for each additional person occupying a common space, such as a living room.
• Reduce the total Btu by 10% if a room is heavily shaded.

Measuring for window-mounted ACs

Window-mounted air conditioners are measured similarly to large central air conditioners. For the room you’re placing the window AC, calculate the room’s square footage to determine what size you need. 

Multiply the room’s length by width to get its square footage, and do the same for adjoining rooms not separated by a door. Add the square footage up. The total is the square footage your window AC will need to cool. 

Most window ACs have a cooling capacity between 5,000 and 12,500 Btu. 

Converging Btu to tons

When it comes to central air conditioning systems, the capacity is measured in tonnage. A “ton” refers to how much heat the AC can remove from your home per hour. It’s defined as 12,000 Btu per hour.

Step 3: Request a Manual J calculation

While the square footage and heat gain/loss provide a good estimate, a Manual J calculation or load calculation is the most accurate way to determine the perfect AC size. This more formal process accounts for all possible inputs to recommend a properly sized unit. 

Though it requires additional effort and money, a Manual J calculation ensures your new AC system will keep your home cool and comfortable without wasting energy. Contact a local heating, ventilation and air conditioning technician to receive a proper Manual J calculation.

Why does your AC’s size matter?

Choosing the right size air conditioner is important because it cools your home the most efficiently. An undersized AC will run constantly, driving up your energy costs without making your home comfortable. Alternatively, an oversized unit will cool your rooms too quickly and turn off before removing enough moisture, leaving your home clammy and humid. 

What’s next?

While the steps we outlined can give you a general idea of what size AC your home needs, the best way to ensure you buy the right air conditioner is to obtain professional help. 

Look at the seasonal energy efficiency ratio rating and Energy Star certification when shopping for an AC, and contact a pro for a Manual J calculation.