How much crowded a city is? How much mass a lift accumulates? What's the mass of 1 cubic centimeter of gold? These all decides the Density! Its often confused by many that mass and density are the same. If it is so how come density of a cubic centimeter of gold varies from aluminium? Mass is the matter in a given object. Density is the amount of mass in a certain volume. Thus volume is that what makes the mass different from density.

Density ($\rho$) tells about how much mass present in a given volume. It is given by
$\rho$ = $\frac{Mass}{Volume}$
There is a easy way to find the density of any body or object:
 Take the given object. Note down its mass and volume then convert the given mass in to kg.
 Measure the volume of the given object and convert it into meter cube (m^{3})
 Divide the mass by volume. You get the density in kilogram per meter cube (kg/m^{3}).
The SI unit of density is kilograms per cubic meter (kg/m^{3}). Liters and metric tons are also used as other types of units.
Some other units are: Kilograms per liter (kg/L)
 Grams per milliliter (g/mL)
 Metric tons per cubic meter (t/m^{3})
 Kilograms per cubic decimeter (kg/dm^{3})
 Grams per cubic centimeter (g/cc, gm/cc or g/cm^{3})
 Mega grams per cubic meter (Mg/m^{3})
In U.S. customary unitâ€™s density can be stated in:
 Avoirdupois ounces per cubic inch (oz/cu) in Slugs per cubic foot
 Avoirdupois pounds per cubic inch (lb/cu)
 Pounds per cubic foot (lb/cu ft)
 Pounds per cubic yard (lb/cu yd)
 Pounds per U.S. liquid gallon (lb/gal)
 Pounds per U.S. bushel (lb/bu)
The density can be measured using the weighing instrument called Hydrometer. It is measured using the displacement of the fluid.
Hydrometer is an instrument to determine the density of the liquid. It consists of glass float with a long thin stem which is graduated.
Relative density is that what tells about comparison of the density of any object with that of standard substance. It is the ratio of the density of a substance to the density of the reference material. It is given by
The transmission of light of any wavelength in an optical medium for a given medium decides its optical density. The higher the value of optical density lower is the transmittance.
The energy density is that tells how much energy is stored in a given system or a region. It is denoted by U and expressed in Joule per meter cube (J/m^{3}). It is given byHere E is the total energy of the system,
V is the volume of the system.
Air density is is nothing but the mass of the air for the given volume and tells about how much air is accumulated in a given volume.It gives relation between the temperature, pressure and density as a function of altitude. It decreases with increasing altitude and is given as
Here $\rho_{dry air}$ is the density of dry air (kg/m^{3}),
p is the air pressure (Pa),
R is the specific gas constant,
T is the temperature.
The density of water is the mass of water for a given volume. It varies based on the temperature. Here is a list of densities of water based on temperature.
Temperature (^{o} C)  Density (kg/m^{3}) 
100  958.4 
80  971.8 
60  983.2 
40  992.2 
30  995.65 
25  997.04 
22  997.77 
20  998.20 
Water is frozen state is what we call as ice. The compactness of molecular arrangement of ice to that of water is different. Here as temperature increases density increases as each molecule in water is hydrogen bonded $\frac{3}{4}$ other molecules while in ice each hydrogen is bonded to 4 other molecules.This makes the ice to float on water and has less density.