Base BTU from room area
Energy Star and the DOE recommend a base cooling capacity of about 20 BTU per square foot of living space. A 300-square-foot room needs roughly 6,000 BTU as a starting point. Adjustments are then made for ceiling height, sun exposure, number of occupants, and whether the room is a kitchen.
Add 10 percent for sunny rooms, subtract 10 percent for heavily shaded rooms. Add 600 BTU for each occupant beyond two. Add 4,000 BTU if the unit will cool a kitchen.
Base BTU = Room area x 20
Starting capacity before adjustments, following DOE guidelines.