I am trying to calculate the diameter of a drive shaft. I would have thought that all I would need in order to determine the diameter of a shaft (concerning torsion) would be how much torque is going to be applied to the shaft and the shear modulus rating of the drive shaft material. According to this web page http://www.engineeringtoolbox.com/torsion-shafts-d_947.html in order to determine shaft diameter, first I must know the diameter of the shaft! – Which is not at all helpful. That site says: And it also says: So in order to determine the diameter of a shaft I need to know the Tmax of the shaft. But in order to know what the Tmax is I need to know the radius of the shaft!! Am I missing something here?