A 5,000 Btu air conditioner can effectively cool a room that measures 100 to 150 square feet, according to a basic industry standard. But while using the room size is a good starting point when sizing an air conditioner, there are other factors to consider, such as the room's location, how much sun it gets and how many people use it. Therefore, the best way to choose an air conditioner capacity is to start with the room's square footage, and then make adjustments as needed for the specific space.
Determine Btu capacity for an air conditioner based on the size of the room and other important criteria.
The Importance of Proper Sizing
Air conditioners cool interior spaces by removing heat and moisture (humidity) from the air, and they have to do both to work effectively. If an air conditioner is too big for a given area, it will turn on for short periods, a condition called "short-cycling." This cools the air quickly, but the air conditioner doesn't run long enough to remove the moisture, leaving the air feeling damp or clammy and sometimes uncomfortably chilly. The flip side of this is an air conditioner that's too small. It will run for long periods, using a lot of electricity and possibly never cooling the room adequately.
Room Size Calculations
Start your room size calculation by measuring the width and length of the entire space you intend to cool. Multiply the numbers to find the total square footage, or area; for example, a room that's 12 feet wide and 20 feet long has an area of 240 square feet. The chart below lists the standard recommended range of air conditioner capacity (in Btu) for various room sizes. These values assume that you're cooling a single, relatively open area and that the ceiling is a standard height of about 8 feet.
Btu ratings are typically given for small- to medium-size air conditioners, including window (or "room") units, mini split heat pumps and portable air conditioners. Central air conditioner systems typically are rated in tons, or refrigeration tons (RT). One ton of refrigeration capacity is equal to 12,000 Btu.
Adjusting Your Calculation
Use the chart shown to determine a base Btu capacity for the room you intend to cool. Add or subtract to this number using any of the following factors that apply. For example, if you want to cool a bedroom that is 200 square feet, your base Btu capacity is about 6,000 Btu. If the room is south-facing and receives a lot of afternoon light, increase the capacity by 10 percent, for a total of 6,600 Btu.
- High ceiling: If the room has a 10-foot-high ceiling, add 10 percent to your Btu total.
- Climate: Add 10 percent if you live in a climate that is consistently hot and humid.
- Occupants: If the room is typically used by more than one person, add 10 percent for each additional person; for example, if the room is used by three people, increase the Btu total by 20 percent.
- Sun exposure: Reduce the Btu total by 10 percent if the area has no direct sun exposure throughout the day; increase the total by 10 percent if the room is commonly warmed by the sun
- Kitchen: Add 600 Btu to your total for a kitchen.
Philip Schmidt is author of Install Your Own Solar Panels, The Complete Guide to Treehouses, and 18 other home-related how-to books. A former carpenter, he has been a full-time writer and editor for over two decades, teaching DIYers about houses and everything we do with them.