The density of any substance is defined as its mass per unit volume.
Here, m is the mass of the substance and V is the volume of the substance.
Assume that the standard of density in this problem is that of water. In such a situation, we have to be accurate with the values of the standard temperature, standard pressure, and quality of the water as density varies with temperature and pressure.
In addition, it is not possible to measure density as a single unit like length and mass, as it depends on the values of mass and volume. So, density is a derived quantity. So, the values of mass and volume of water must be accurate in order to consider the density of water as a standard unit.