Multiply the torque applied to the beam by the length of the beam. Ensure that the length of the beam is in meters.
Divide the value from Step One by the angle of twist of the beam. Ensure that the angle is in radians. If the angle is in degrees, divide the value by 360 and multiply it by 2 * pi to convert it to radians.
Divide the value from Step Two by the the shear modulus of the material. A table of shear modulus values for common materials is provided in the resources section. The resulting value is the torsion constant. The unit of the value is in meters^4.