System size, or cooling capacity, is measured in British Thermal Units (BTUs). Determining the right sized HVAC system requires performing a calculation. There are different calculations but basically you want to know the size of the room in square feet, and multiply it by 25 BTU. For example, a 12- by 15-foot room requires an air conditioner of at least 4,500 BTU, but an experienced technician can perform a load calculation to get a more precise value. If you’re A/C is oversized or undersized, it may run constantly, cycle too frequently, or create uneven heating or cooling.