Calculate the room's area in square feet. If the room is rectangular, find its length and width and multiply the measurements. Otherwise, divide it into smaller rectangles and find the area of each.
Find the room's cooling requirement, in British thermal units (BTUs). Begin with a set value of 5,000 BTUs. Add 1,000 BTUs for each 100 square feet over 150.
If more than two people will live in the room, add 600 BTUs for each. If the room gets notable sun, multiply by 1.1. If it gets notably little sun, multiply by 0.9. Add 4,000 BTUs if you are installing the AC in the kitchen.
Divide by 12,000. The answer is your AC's necessary size, in tons.