WEBMar 15, 2022 · A BTU rating indicates the amount of heat an A/C can remove from a room. Look for a unit with a higher BTU rating to cool a larger room. Here are some general guidelines for choosing the appropriate BTU for your room size: 150 to 350 square feet: 5,000 to 8,000 BTU. 350 to 550 square feet: 8,000 to 12,000 BTU.