Moment of Inertia of mass

Hello,

I am trying to find moment of inertia of various random 3D shapes and I have few general questions.

I take very thin 'plates' of the shape, calculate moment of inertia of that area, multiply it by thickness of one 'plate unit', also I add a square distance it is from the origin multiplied by a mass of that unit plate. Since I need to find out moment of inertia around z axis and the shapes are symmetrical by x and y axis I get the general formula like this:

I = ƩdI = 2*(dI_area * density * t + distance^2 * dm);

where dI_area is a moment of inertia of unit plate
t is a thickness of unit plate
distance is a distance from origin
dm = Area*t*density; where Area is area of thin plate (or unit plate);

I would like someone to comment on my actions because I do not obtain right results using this method.

Thank you in advance