The density of a material is defined as its mass per unit volume. The symbol of density is ρ (the Greek letter rho).
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Mathematically:
where:
Different materials usually have different densities, so density is an important concept regarding buoyancy, metal purity and packaging.
In some cases density is expressed as the dimensionless quantities specific gravity (SG) or relative density (RD), in which case it is expressed in multiples of the density of some other standard material, usually water or air/gas.
In a wellknown tale, Archimedes was given the task of determining whether King Hiero's goldsmith was embezzling gold during the manufacture of a wreath dedicated to the gods and replacing it with another, cheaper alloy.^{[1]}
Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated easily and compared with the mass; but the king did not approve of this.
Baffled, Archimedes took a relaxing immersion bath and observed from the rise of the warm water upon entering that he could calculate the volume of the gold crown through the displacement of the water. Allegedly, upon this discovery, he went running naked through the streets shouting, "Eureka! Eureka!" (Εύρηκα! Greek "I found it"). As a result, the term "eureka" entered common parlance and is used today to indicate a moment of enlightenment.
The story first appeared in written form in Vitruvius' books of architecture, two centuries after it supposedly took place.^{[2]} Some scholars have doubted the accuracy of this tale, saying among other things that the method would have required precise measurements that would have been difficult to make at the time.^{[3]}^{[4]}
For a homogeneous object, the mass divided by the volume gives the density. The mass is normally measured with an appropriate scale or balance; the volume may be measured directly (from the geometry of the object) or by the displacement of a fluid. Hydrostatic weighing is a method that combines these two.
If the body is not homogeneous, then the density is a function of the position: , where dv is an elementary volume at position . The mass of the body then can be expressed as
The density of a solid material can be ambiguous, depending on exactly how its volume is defined, and this may cause confusion in measurement. A common example is sand: if it is gently poured into a container, the density will be low; if the same sand is compacted into the same container, it will occupy less volume and consequently exhibit a greater density. This is because sand, like all powders and granular solids, contains a lot of air space in between individual grains. The density of the material including the air spaces is the bulk density, which differs significantly from the density of an individual grain of sand with no air included.
Density is defined as mass per unit volume. A concise statement of what this means may be obtained by considering a small box in a Cartesian coordinate system, with dimensions Δx, Δy, Δz. If the mass is represented by a net mass function, then the density at some point (x,y,z) is:
For a homogeneous substance, this derivative is equal to the net mass divided by the net volume. For a nonhomogeneous substance, m is a nonconstant function of position: m = m(x,y,z).
The SI unit for density is:
Densities using the following metric units all have exactly the same numerical value, one thousandth of the value in (kg/m³). Liquid water has a density of about 1 kg/dm³, making any of these SI units numerically convenient to use as most solids and liquids have densities between 0.1 and 20 kg/dm³.
Liters and metric tons are not part of the SI, but are acceptable for use with it. Since 1 L = 1 dm³, we also have these of the same size:
In U.S. customary units density can be stated in:
In principle there are Imperial units different from the above as the Imperial gallon and bushel differ from the U.S. units, but in practice they are no longer used, though found in older documents. The density of precious metals could conceivably be based on Troy ounces and pounds, a possible cause of confusion.
In general, density can be changed by changing either the pressure or the temperature. Increasing the pressure will always increase the density of a material. Increasing the temperature generally decreases the density, but there are notable exceptions to this generalisation. For example, the density of water increases between its melting point at 0 °C and 4 °C; similar behaviour is observed in silicon at low temperatures.
The effect of pressure and temperature on the densities of liquids and solids is small. The compressibility for a typical liquid or solid is 10^{−6} bar^{−1} (1 bar=0.1 MPa) and a typical thermal expansivity is 10^{−5} K^{−1}.
In contrast, the density of gases is strongly affected by pressure. The density of an ideal gas is
where R is the universal gas constant, P is the pressure, M is the molar mass, and T is the absolute temperature. This means that the density of an ideal gas can be doubled by doubling the pressure, or by halving the absolute temperature.
Osmium is the densest known substance at standard conditions for temperature and pressure.
Temp (°C)  Density (kg/m^{3}) 

100  958.4 
80  971.8 
60  983.2 
40  992.2 
30  995.6502 
25  997.0479 
22  997.7735 
20  998.2071 
15  999.1026 
10  999.7026 
4  999.9720 
0  999.8395 
−10  998.117 
−20  993.547 
−30  983.854 
The density of water in kilograms per cubic meter (SI unit) at various temperatures in degrees Celsius. The values below 0 °C refer to supercooled water. 
T in °C  ρ in kg/m^{3} (at 1 atm) 

–25  1.423 
–20  1.395 
–15  1.368 
–10  1.342 
–5  1.316 
0  1.293 
5  1.269 
10  1.247 
15  1.225 
20  1.204 
25  1.184 
30  1.164 
35  1.146 
The density of a solution is the sum of mass (massic) concentrations of the components of that solution.
Mass (massic) concentration of a given component ρ_{i} in a solution can be called partial density of that component.
ASTM specification D79200^{[5]} describes the steps to measure the density of a composite material.
where:
and
Material  ρ in kg/m^{3}  Notes 

Interstellar medium  10^{−25} − 10^{−15}  Assuming 90% H, 10% He; variable T 
Earth's atmosphere  1.2  At sea level 
Aerogel  1 − 2  
Styrofoam  30 − 120  From 
Cork  220 − 260  From 
Water  1000  At STP 
Plastics  850 − 1400  For polypropylene and PETE/PVC 
Glycerol^{[6]}^{[7]}  1261  
The Earth  5515.3  Mean density 
Iron  7874  Near room temperature 
Copper  8920 − 8960  Near room temperature 
Lead  11340  Near room temperature 
The Inner Core of the Earth  ~13000  As listed in Earth 
Uranium  19100  Near room temperature 
Tungsten  19250  Near room temperature 
Gold  19300  Near room temperature 
Platinum  21450  Near room temperature 
Iridium  22500  Near room temperature 
Osmium  22610  Near room temperature 
The core of the Sun  ~150000  
White dwarf star  1 × 10^{9}^{[8]}  
Atomic nuclei  2.3 × 10^{17} ^{[9]}  Does not depend strongly on size of nucleus 
Neutron star  8.4 × 10^{16} — 1 × 10^{18}  
Black hole  4 × 10^{17}  Mean density inside the Schwarzschild radius of an earthmass black hole (theoretical) 
