Once you have determined the number of BTUs that will most efficiently cool your rooms, you need to make adjustments based on other factors as well:
If the room is heavily shaded then reduce BTU's by 10%
If the room is in a very sunny location then increase BTU's by 10%
If more than two people are usually in the room(s), then add 600 BTUs for each additional person
If this is to cool a kitchen area then you need to add 4,000 BTUs
How many BTUs to cool a room? Here are some common room sizes and the needed BTUs:
8 X 10 = 80 sq. ft. | 5,000 BTU's Needed
9 X 10 = 90 sq. ft. | 5,000
10 X 12 = 120 sq. ft. | 5,000
11 X 13 = 143 sq. ft. | 5,000
12 X 12 = 144 sq. ft. | 5,000
12 X 14 = 168 sq. ft. | 6,000
13 X 19 = 247 sq. ft. | 6,000-7,000
14 X 18 = 252 sq. ft. | 7,000
16 X 20 = 320 sq. ft. | 8,000