Torsional moment of inertia?

Hi people, i am in the process of designing a timber beam to eurocode 5, for university. One of the formulas used to calculate a critical factor, requires the value of torsional moment of inertia, and i have looked everywhere and cant find it. The shape of the beam is rectangular and as i say, the formula i need requires this particular value, i dont know if it is called something else but can anyone help???