Density is defined as mass per unit volume. This is commonly given in the units of kilograms (or grams for smaller amounts) per milliliter. So the two components that are needed to measure the density of any object are its mass and the volume of space that it occupies. The mass can be measured on a scale or balance. The volume that it occupies can be measured by the displacement of water. If a container with volume markings such as a graduated cylinder is filled with a measured amount of water, then the object can be immersed in the water and the resulting change in the water level will give you the volume of the object. Divide the mass by the volume and you have the density.