BTU Calculator

BTU sizing is one of the few areas of home improvement where bigger is emphatically worse. An air conditioner does two jobs — it lowers temperature and it removes moisture — and the second requires sustained runtime. An oversized unit satisfies the thermostat quickly, shuts off, and never runs long enough to dehumidify. The result is a room that is cold and damp rather than cool and dry, and equipment that wears out early from constant short-cycling.

The rough starting point is twenty BTU per square foot, then adjusted for the factors that actually drive load. Climate matters most, followed by insulation quality and how much sun the room takes. Windows are treated separately because glass is a far worse insulator than wall, and a large west-facing window can add more load than the rest of the room combined. People, appliances and especially cooking add heat that has to be removed too.

Everything here is a rule of thumb suited to sizing a window unit or a mini-split for one room, and it is genuinely useful at that scale. Sizing a whole-house system this way is not appropriate. A proper Manual J calculation models each window's specification and orientation, measured air infiltration, duct location and losses, and internal gains room by room. Contractors who size from square footage alone routinely oversize by thirty percent or more, which is why so many houses have systems that never seem to dehumidify properly.