How do you calculate volume in an object?

If you know his Mass and his Density, you can calculate like this:

V = M / P (M - mass per unit; P - density)

If you don't know anything, you can do an experiment:

Put your object in a bowl of water - - | but not full | - - (you have to calculete the water volume), and after you put that object, calcule the new volume in the bowl.

You will obtain the volume if you want when you decrease that two values.