Boiler capacity is measured in BTUs, or British Thermal Units. Every building has a unique BTU requirement based on its geographical location and climate, the number of windows and doors in the home, and the quality and amount of insulation in the walls and ceilings. An easy rule-of-thumb for BTU requirements is to figure that you need about 50 BTU per square foot of interior space in a cold climate — 35 BTU per square foot in a moderate climate, and 20 BTU per square foot in a hot climate.