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{{Short description|Weight per unit volume of a material}}
{{Distinguish|Specific gravity|Specific density|Specific force}}

The '''specific weight''', also known as the '''unit weight'''   (symbol ''{{mvar|γ}}'', the Greek letter [[gamma]]), is a [[Specific quantity|volume-specific quantity]] defined as the [[weight]] per unit [[volume]] of a material.

A commonly used value is the specific weight of [[water]] on [[Earth]] at {{convert|4|C|abbr=on}}, which is {{convert|9.807|kN/m3|disp=or|lk=on}}.<ref name="FE">National Council of Examiners for Engineering and Surveying (2005). ''Fundamentals of Engineering Supplied-Reference Handbook'' (7th ed.). {{ISBN|1-932613-00-5}}.
</ref>

==Definition==
The specific weight, {{mvar|γ}}, of a material is defined as the product of its [[density]], {{mvar|ρ}}, and the [[standard gravity]], {{mvar|g}}: 
<math display=block>\gamma = \rho \, g</math>

The [[density]] of the material is defined as [[mass]] per unit volume, typically measured in kg/m<sup>3</sup>. The standard gravity is acceleration due to gravity, usually given in m/s<sup>2</sup>, and on Earth usually taken as {{val|9.81|u=m|up=s2}}.

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.<ref name="fluids" />

==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<sup>3</sup> or lbf/ft<sup>3</sup>).  Specific weight can be used as a characteristic [[List of materials properties|property]] of a fluid.<ref name="fluids" />

===Soil mechanics===
Specific weight is often used as a property of soil to solve [[Earthworks (engineering)|earthwork]] problems.

In soil mechanics, specific weight may refer to:

{{glossary}}
{{term|Moist unit weight}}
{{defn|The unit weight of a soil when void spaces of the soil contain both water and air.
<math display=block>\gamma = \frac{(1+w)G_s\gamma_w}{1+e}</math>
where
* {{mvar|γ}} is the moist unit weight of the material 
* {{math|''γ<sub>w</sub>''}} is the unit weight of water
* {{mvar|w}} is the [[moisture content]] of the material 
* {{math|''G<sub>s</sub>''}} is the [[specific gravity]] of the solid
* {{mvar|e}} is the [[void ratio]]
}}

{{term|Dry unit weight}}
{{defn|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:
<math display=block>\gamma_d = \frac{G_s\gamma_w}{1+e} = \frac{\gamma}{1+w}</math>
where
* {{mvar|γ}} is the moist unit weight of the material 
* {{math|''γ<sub>d</sub>''}} is the dry unit weight of the material 
* {{math|''γ<sub>w</sub>''}} is the unit weight of water
* {{mvar|w}} is the [[moisture content]] of the material 
* {{math|''G<sub>s</sub>''}} is the [[specific gravity]] of the solid
* {{mvar|e}} is the [[void ratio]]
}}

{{term|Saturated unit weight}}
{{defn|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:
<math display=block>\gamma_s = \frac{(G_s+e)\gamma_w}{1+e}</math>
where
* {{math|''γ<sub>s</sub>''}} is the saturated unit weight of the material 
* {{math|''γ<sub>w</sub>''}} is the unit weight of water
* {{math|''G<sub>s</sub>''}} is the [[specific gravity]] of the solid
* {{mvar|e}} is the [[void ratio]]<ref name="soils">Das, Braja M. (2007). ''Principles of Geotechnical Engineering''. Canada: Chris Carson.  {{ISBN|0-495-07316-4}}.</ref>
}}

{{term|Submerged unit weight}}
{{defn|The difference between the saturated unit weight and the unit weight of water.<ref name="Intelligent Compaction">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</ref> It is often used in the calculation of the [[effective stress]] in a soil.

The formula for submerged unit weight is:
<math display=block>\gamma' = \gamma_s - \gamma_w</math>
where
* {{math|''γ''&prime;}} is the submerged unit weight of the material
* {{math|''γ<sub>s</sub>''}} is the saturated unit weight of the material 
* {{math|''γ<sub>w</sub>''}} is the unit weight of water
}}
{{glossary end}}

===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 (engineering)|deformation]].

==Specific weight of water==
{{See also|Water (molecule)#Density of water and ice|l1=Water density}}
{| class="wikitable" style="text-align:center" 
|+ Specific weight of water at standard sea-level atmospheric pressure (Metric units) <ref name="fluids" />
|-
! scope="col" | Temperature(°C)
! scope="col" | Specific weight (kN/m<sup>3</sup>)
|-
| 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
|}

{| class="wikitable" style="text-align:center"
|+ Specific weight of water at standard sea-level atmospheric pressure (English units) <ref name="fluids">Finnemore, J. E. (2002). ''Fluid Mechanics with Engineering Applications''. New York: McGraw-Hill. {{ISBN|0-07-243202-0}}.</ref>
|-
! scope="col" | Temperature(°F)
! scope="col" | Specific weight (lbf/ft<sup>3</sup>)
|-
| 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 air==
{{Main|Density of air}}
{| class="wikitable" style="text-align:center"
|+ Specific weight of air at standard sea-level atmospheric pressure (Metric units) <ref name="fluids" />
|-
! scope="col" | Temperature(°C)
! scope="col" | Specific weight (N/m<sup>3</sup>)
|-
| −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
|-100 ii 89
|}

{| class="wikitable" style="text-align:center"
|+ Specific weight of air at standard sea-level atmospheric pressure (English units) <ref name="fluids" />
|-
! scope="col" | Temperature(°F)
! scope="col" | Specific Weight (lbf/ft<sup>3</sup>)
|-
| −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
|}

==References==
{{Reflist}}

==External links==
* [https://www.ajdesigner.com/phpspecificgravity/specific_gravity_equation_submerged_water_weight_loss.php Submerged weight calculator]
* [http://www.fxsolver.com/solve/share/qtmx1x0eEynB9VGZDGMBLA==/ Specific weight calculator]
* http://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.html
* http://www.themeter.net/pesi-spec_e.htm

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[[Category:Soil mechanics]]
[[Category:Fluid mechanics]]
[[Category:Physical chemistry]]
[[Category:Physical quantities]]
[[Category:Density]]
[[Category:Volume-specific quantities]]