The inner diameter of the 6-point tube is 19.05 mm. 6 points is based on the diameter of the British pipe, 1 inch = 8 points, 6 points is a three-quarter inch pipe. 1 inch = 25.4 mm, 6 branch diameter = 19.05 mm.
6 branch pipe is also called dn20 pipe, dn20 refers to the nominal diameter of the pipe is 20 mm, it is not the inner diameter of the pipe nor the outer diameter of the pipe, it is the midpoint between the inner diameter and the outer diameter of the pipe. The outer diameter of the 6-point tube is usually 25 mm.