How to Calculate AC Size

Larger rooms need larger air conditioners.
Choose an AC that suits your room's size.Choose an AC that suits your room's size.
Undersized ACs may run continuously, though they aren't designed to, while still not managing to cool your room sufficiently. Oversized ACs, which some contractors recommend, waste energy and money. They're also too loud, cycle on and off too often, and dehumidify inefficiently. Manufacturers measure AC size in tons. The air conditioner's tonnage relates directly to the room's size.

Step 1

Calculate the room's area in square feet. If the room is rectangular, find its length and width and multiply the measurements. Otherwise, divide it into smaller rectangles and find the area of each.

Step 2

Find the room's cooling requirement, in British thermal units (BTUs). Begin with a set value of 5,000 BTUs. Add 1,000 BTUs for each 100 square feet over 150.

Step 3

If more than two people will live in the room, add 600 BTUs for each. If the room gets notable sun, multiply by 1.1. If it gets notably little sun, multiply by 0.9. Add 4,000 BTUs if you are installing the AC in the kitchen.

Step 4

Divide by 12,000. The answer is your AC's necessary size, in tons.

Things You Will Need

  • Ruler or measuring tape

About the Author

Ryan Menezes is a professional writer and blogger. He has a Bachelor of Science in journalism from Boston University and has written for the American Civil Liberties Union, the marketing firm InSegment and the project management service Assembla. He is also a member of Mensa and the American Parliamentary Debate Association.