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18x18 what? 18 feet x 18 feet? 18 microns x 18 microns?
18x18 is 324
18 x 18 feet = 324 square feet
18 x 18 inches = 2.25 square feet
18 ft X 18 ft =324 sq ft
The answer is 833 (for 180 mm x 180 mm).