how to measure the volume of a irregular & weightless object

To find the volume of an irregular solid, take a measuring cylinder and pour some water in it. Then tie the solid to a thin thread and dip it into the water. When the solid is completely submerged in the water, calculate the rise in the water level from the measuring cylinder. This rise in the water level will give you the volume of the solid.

The object will have some mass since it occupies some volume of space. It can only be weightless when the acceleration due to gravity is cancelled out but the mass of the object will remain the same.