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The Steel Tube Institute of North America specifies the measurement of square tubing by its outside width.
For example, a tube with 2-inch wide sides calls out as 2 by 2 inches. Sizes range from 1-1/4 by 1-1/4 inch to 32 by 32 inch. From 1-1/4 inches to 2-1/2 inches, the sizing varies by a quarter-inch.