Air Conditioner Sizing Calculator

About this calculator

Air conditioner sizing calculates the cooling capacity, measured in BTU/hr (British Thermal Units per hour), that a room needs to reach and hold a comfortable temperature. The baseline is roughly 20 BTU per square foot of floor area, then adjusted upward for higher ceilings (more air volume to cool), significant sun exposure (rooms facing south or west typically need 10% more capacity), and occupancy, since HVAC guidelines add roughly 600 BTU for each additional person regularly in the room beyond the first two, because bodies generate heat.

Homeowners and HVAC installers use this sizing before buying a window unit, split system or portable AC, because an undersized unit runs constantly without ever reaching the target temperature, while an oversized unit cycles on and off too quickly (short-cycling), which wastes energy and fails to properly dehumidify the room — bigger is not automatically better with air conditioning.

This calculator takes your room's floor area, ceiling height, sun exposure and occupant count and returns the BTU capacity you should look for when buying an air conditioner.