The specific weight, also known as the unit weight, is the weight per unit volume of a material.
A commonly used value is the specific weight of water on Earth at 4 °C (39 °F), which is 9.807 kilonewtons per cubic metre or 62.43 poundsforce per cubic foot.^{[1]}
The terms specific gravity, and less often specific weight, are also used for relative density. A common symbol for specific weight is γ, the Greek letter Gamma.
Definition
The specific weight, γ, of a material is defined as the product of its density, ρ, and the standard gravity, g:
The density of the material is defined as mass per unit volume, typically measured in kg/m^{3}. The standard gravity is acceleration due to gravity, usually given in m/s^{2}, and on Earth usually taken as 9.81 m/s^{2}.
Unlike density, specific weight is not a fixed property of a material. It depends on the value of the gravitational acceleration, which varies with location. Pressure may also affect values, depending upon the bulk modulus of the material, but generally, at moderate pressures, has a less significant effect than the other factors.^{[2]}
Applications
Fluid mechanics
In fluid mechanics, specific weight represents the force exerted by gravity on a unit volume of a fluid. For this reason, units are expressed as force per unit volume (e.g., N/m^{3} or lbf/ft^{3}). Specific weight can be used as a characteristic property of a fluid.^{[2]}
Soil mechanics
Specific weight is often used as a property of soil to solve earthwork problems.
In soil mechanics, specific weight may refer to:

Moist unit weight, which is the unit weight of a soil when void spaces of the soil contain both water and air.
 γ is the moist unit weight of the material
 γ_{w} is the unit weight of water
 w is the moisture content of the material
 G_{s} is the specific gravity of the solid
 e is the void ratio

Dry unit weight, which is the unit weight of a soil when all void spaces of the soil are completely filled with air, with no water.
The formula for dry unit weight is:
 γ is the moist unit weight of the material
 γ_{d} is the dry unit weight of the material
 γ_{w} is the unit weight of water
 w is the moisture content of the material
 G_{s} is the specific gravity of the solid
 e is the void ratio

Saturated unit weight, which is the unit weight of a soil when all void spaces of the soil are completely filled with water, with no air.
The formula for saturated unit weight is:
 γ_{s} is the saturated unit weight of the material
 γ_{w} is the unit weight of water
 G_{s} is the specific gravity of the solid
 e is the void ratio^{[3]}

Submerged unit weight, which is defined as the difference between the saturated unit weight and the unit weight of water.^{[4]} It is often used in the calculation of the effective stress in a soil.
The formula for submerged unit weight is:
 γ′ is the submerged unit weight of the material
 γ_{s} is the saturated unit weight of the material
 γ_{w} is the unit weight of water
Civil and mechanical engineering
Specific weight can be used in civil engineering and mechanical engineering to determine the weight of a structure designed to carry certain loads while remaining intact and remaining within limits regarding deformation.
Specific weight of water
Temperature(°C)  Specific weight (kN/m^{3}) 

0  9.805 
5  9.807 
10  9.804 
15  9.798 
20  9.789 
25  9.777 
30  9.765 
40  9.731 
50  9.690 
60  9.642 
70  9.589 
80  9.530 
90  9.467 
100  9.399 
Specific weight of water at standard sealevel atmospheric pressure (Metric units) ^{[2]} 
Temperature(°F)  Specific weight (lbf/ft^{3}) 

32  62.42 
40  62.43 
50  62.41 
60  62.37 
70  62.30 
80  62.22 
90  62.11 
100  62.00 
110  61.86 
120  61.71 
130  61.55 
140  61.38 
150  61.20 
160  61.00 
170  60.80 
180  60.58 
190  60.36 
200  60.12 
212  59.83 
Specific weight of water at standard sealevel atmospheric pressure (English units) ^{[2]} 
Specific weight of air
Temperature(°C)  Specific weight (N/m^{3})  

−40  14.86  
−20  13.86  
0  12.68  
10  12.24  
20  11.82  
30  11.43  
40  11.06  
60  10.4  
80  9.81  
100  9.28  
200  7.33  
Specific weight of air at standard sealevel atmospheric pressure (Metric units) ^{[2]} 
Temperature(°F)  Specific Weight (lbf/ft^{3})  

−40  
−20  0.0903  
0  0.08637  
10  0.08453  
20  0.08277  
30  0.08108  
40  0.07945  
50  0.0779  
60  0.0764  
70  0.07495  
80  0.07357  
90  0.07223  
100  0.07094  
120  0.06849  
140  0.0662  
160  0.06407  
180  0.06206  
200  0.06018  
250  0.05594  
Specific weight of air at standard sealevel atmospheric pressure (English units) ^{[2]} 
References
 ^ National Council of Examiners for Engineering and Surveying (2005). Fundamentals of Engineering SuppliedReference Handbook (7th ed.). ISBN 1932613005.
 ^ ^{a} ^{b} ^{c} ^{d} ^{e} ^{f} Finnemore, J. E. (2002). Fluid Mechanics with Engineering Applications. New York: McGrawHill. ISBN 0072432020.
 ^ Das, Braja M. (2007). Principles of Geotechnical Engineering. Canada: Chris Carson. ISBN 0495073164.
 ^ The Transtec Group, Inc. (2012). Basic Definitions and Terminology of Soils. http://www.intelligentcompaction.com/downloads/IC_RelatedDocs/SoilCmpct_Basic%20definitions%20of%20Soils.pdf (Page viewed December 7, 2012
External links
 Water Weights & Weight Belts (Arrow Weights Manufacturing)
 Specific weight calculator
 http://www.engineeringtoolbox.com/densityspecificweightgravityd_290.html
 http://www.themeter.net/pesispec_e.htm