A shaft fixed at one end and twisted at the other end due to the action of torque T. The radius of shaft is R and the length is L. Imagine a horizontal radial line drawn on the end face. When the end is twisted, the line rotates through an angle θ. G is one of the elastic constants of the material. The equation is only true so long as the material remains elastic.

T/J = Gϴ/L

Where
T = Torque applied
J = polar moment of inertia of the shaft = πd⁴/32
C = rigidity modulus of the material
ϴ = relative angle of twist in radians
L = gauge length (length of the shaft over which the relative angle of twist is measured)