Mich. Comp. Laws § 125.569

Sec. 19. The metal used in all tanks shall be of a minimum gauge, U.S. standard, depending upon the capacity or size as given in the following table: Capacity Thickness (Gallons) of material 1 to 500 14 gauge 501 to 1100 12 gauge 1101 to 4000 7 gauge 4001 to 10500 1/4 inch 10501 to 20000 5/16 inch 20001 to 30000 3/8 inch