Calculating Moment of Inertia (2D rectangles)

Hi everyone. i'm presently improving a 2D physics engine i wrote, with angular dynamics. Moment of inertia is becoming a big sticking point for me.

However, given that my applications are fairly simple, i'm hoping someone can help me come up with a simple way of calculating the moment of inertia for stuff.

All of my objects for now will (as far as physics is concerned) be rectangular, or made up of several rectangles. And i know there's a table of common moments of inertia that i might use, however that is mostly concerned with axes of rotation that pass through the centre of the object, or one of it's ends. i can't always guarantee that. I need to figure out a way of calculating for objects with an arbitrary centre of mass, which may not always be anywhere near the centre of geometry


Also another related question:

Take for example, a hammer.
if i divide it into two rectangles (the head, and the shaft) each of which has an evenly distributed mass over its area (but a different mass each) resulting in the centre of mass of the total object being closer to the head,

would that help to simplify the calculations involved, as opposed to:

making the whole hammer a single rectangle (with an unevenly distributed mass)


I'd like to get this system working for rectangles, and then later i can add support for circles (and ovals). Rectangular and rounded shapes would probably cover all of my physics needs for the forseable future on this project. I don't have any need of calculating I for wierd and irregular shapes, which i hope would make this simpler.

Any thoughts and references on this subject would be appreciated.