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For 1,500 square feet of standard insulated space, cooling needs about 37,500 BTU/h, using 25 BTU/h per square foot. That is about 3.1 tons, and 1,500 square feet is typically a three bedroom house. Since central systems come in half ton steps, the rule of thumb points to a 3 ton system for a well insulated home and 3.5 tons for one with a lot of glass or a hot climate. This page loads with 1,500 square feet already entered so you can adjust for ceiling height, sun and space type. Ceiling height makes a bigger difference at house scale than people expect: the rule assumes 8 feet, and 10 foot ceilings throughout raise the figure to about 39,400 BTU/h, enough to tip the size up by half a ton. Two storey homes add another consideration, because heat rises and the upper floor often needs more cooling than its share of the floor area suggests, which is one reason zoned systems are common in two storey houses. For heating, 1,500 square feet needs about 52,500 BTU/h of output, which is central heating sized by an installer. Remember that furnaces are often labelled by input, so an 80 percent efficient furnace needs around 66,000 BTU of input to deliver that. For a larger home see How Many BTU for 2,000 Square Feet, or for a smaller one How Many BTU for 1,200 Square Feet.