There is no recognised word for it. Although it would be possible to construct one using Greek (or Latin) prefices, and that would be good for obfuscation or being pretentious, it would not be much use if you wanted to communicate. I would suggest "eighteen-fold".
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The diagonal of an 18 inch square is about 25.46 inches. (18 times the square root of 2).
18 / (9*3) = 0.66666.... (18/9) * 3 = 6.
18 ft Diagonal of a square is the square root of 2, times the side.
3 times the square root of 2
3 sqrt(6) x sqrt(6) = 18