You don't actually 'convert' them. The 'density' of a substance is the same whether you have a speck of it or a carload of it. But the mass depends on how much of it you have. If you have a piece of...
Well, you can find the volume. If it is a basic, straight shape, measure the length times width times height. You have its volume. If you cannot measure it like that, and if it is a small object, you...