Start by calculating the total square footage of the room (feet wide by feet long = square footage). You must include the square footage of any other unconditioned areas that will normally be open to the room (walk-in closets, bathrooms, etc.).
One (1) ton (12,000 Btu) of cooling capacity is required for every 400 -500 sq. ft. of floor area depending on how well your walls, ceilings and windows are insulated. Simply stated, the base cooling load required (in BTU/Hr) will be 22 to 35 btu/ hr/ per sq ft of floor space. For example, a well insulated room with 750 total square feet of floor area will require a mnimum of 18,750 Btu/Hr of air conditioning (750 x 25).
Next, add the following if applicable:
1. For a room with South-facing windows, add to the 50 to 200 Btu per sq. ft. of glass area in the window.
(The btu is based on the quality of window and amount of shade or tint blocking direct sunlight from radiating heat into space)
2. A human at rest will add 450 Btu/hr of heat to the room, so you need to add this amount times the maximum number of occupants that you estimate will be in the room on a design cooling day (the hottest day of the summer).
3. A human exercising (e.g. Gymnasium or exercise room) will add 2,000 Btu/hr of heat to the room so you need to add this amount times the maximum number of occupants that you estimate will be exercising in the room on a design cooling day
(the hottest day of the summer).
4. Cooking appliances if present (in your Kitchen for example) must be included in your calculation. You will need to know the Kilowatt (kw) rating of each cooking appliance and then add 3,500 Btu per kw
(the manufacturer of your appliance should be able to provide you this information).
Example Load Calculation:
You would like install a mini-split air conditioner in a 20ft. by 30ft. room that has a 4 ft by 6 ft window facing North, and with a normal occupancy of two persons. The room is well insulated and will be used for reading and watching TV.
The floor area = 20 ft x 30 ft = 600 sq. ft.
Base Cooling load = 25 Btu per square foot x 600 = 15,000 Btu
For two occupants at rest add 450 x 2 or 900 Btu to the base cooling load
15,000 + 900 = 15,900 TOTAL DESIGN COOLING LOAD