Step 1
Measure the room's area in square feet. For a square or rectangular room, multiply length by width. If the shape is more complex, divide the room into rectangles where possible, multiply the length of each by its width, and add the square footage of the sections together. If the room has a triangular section, multiply that section's length by its width, divide by 2, and add that to the total.
Step 2
Determine the area's necessary cooling requirement in British Thermal Units (BTUs). Start with a base of 5,000 BTUs for the first 150 square feet. Add 1,000 for the first hundred square feet after that, then 1,000 for each additional 50 square feet for the next several hundred square feet. For a more detailed conversion table, see the link in Resources.
Step 3
Add 4,000 BTUs if you are calculating for a kitchen. If more than two people will occupy a room regularly, add 600 for each of them. Multiply by 1.1 if the room is especially sunny and by 0.9 if it is especially shaded.
Step 4
Divide by 12,000 to convert BTUs to tonnage.