You have to multiply each small unit of mass with the square of the distance from the axis of rotation. This requires an integration, but you can find tables which have this integral
http://wiki.answers.com/Q/What_is_the_formula_for_...

( ′säl·əd ′mō·mənt əv i′nər·shə ) (physics) The integral of the products of the mass of each of the infinitesimal
http://www.answers.com/topic/solidmomentofinert...

Multiply moment of inertia (lb.inch2) by 0.0002926397 to obtain
http://www.chacha.com/question/whatistheunitsf...

1. Multiply the mass and square of distance to the rotation axis to calculate the moment of inertia of the point mass. For example, the mass is 2 kg and the distance is 2 m. Hence
http://www.ehow.com/how_5161917_calculatemomenti...
