Sq Ft                     Approx BTU's needed

100                                   3500-4500
200                                   5000-5500
300                                   6500-7000
400                                   7500-8000
500                                   8500-9500
600                                   9500-10.5k
700                                   11k-12k
800                                   12k-13k
900                                   13k-14k
1000                                 14k-15k
1500                                 19k-21k
1800                                 22k-24k
2000                                 23k-25.5k

These figures assume an occupied space above the ceiling; add about ½ size if the ceiling is insulated under an attic, and add a full size if the ceiling is non-insulated. Also, add at least one size if the cooled area includes a kitchen.