Thank you for your question. In addition to square footage, the size of system you will require is dependent upon several other factors. These additional factors include the climate zone the space is located, the number of windows and the quality of insulation. For instance, in the warmest U.S. climates a 4 ton system will cool about 2101 to 2400 square feet. In the colder U.S. climates, this same 4 ton system will cool about 2301 to 2700 square feet. For heating, different parameters apply. In the warmer climates, you will need about 30 to 35 BTU per square foot for adequate heating; In colder climates you will need about 50 to 60 BTUs per square foot for adequate heating. But again, the quality of insulation and number of windows also impacts this range. For more in depth information on sizing, please reference our sizing guide. If you have any further questions, please give us a call.