How many square feet does an 18000 BTU air conditioner cool?

The general rule is 600 sq.ft. per ton of refrigeration, so an 18000 BTU A/C will cool approx. 900 sq.ft. . There are many variables, such as insulation R value, normal summer climate in your area, age of the structure, etc., so this is just a guide. To know for sure, have a heat load calculation done.