There is no exact formula to calculate the Torsion Constant of a rectangular object.
However there is an approximation given in by the following formula:
Example
Consider a cross arm that is 2m long, 0.1m wide and 0.2m deep. The long side is 0.2m and hence a = 0.2m, b = 0.1m. Solving the torsion constant gives:
This is the same result calculated by clicking on Derive assuming rectangular cross section:
