The specific weight, also known as the unit weight (symbol , the Greek letter gamma), is a volume-specific quantity defined as the weight divided by the volume of a material:
<math display=block>\gamma = \frac{W}{V} \ .</math>
Equivalently, it may also be formulated as the product of density, , and gravity acceleration, :
<math display=block>\gamma = \rho \, g .</math>
Its unit of measurement in the International System of Units (SI) is the newton per cubic metre (N/m<sup>3</sup>), expressed in terms of base units as kg⋅m<sup>−2</sup>⋅s<sup>−2</sup>.
A commonly used value is the specific weight of water on Earth at , which is .
Discussion
The density of a material is defined as mass divided by volume, typically expressed with the unit kg/m<sup>3</sup>.
Unlike density, specific weight is not a fixed property of a material, as it depends on the value of the gravitational acceleration, which varies with location (e.g., Earth's gravity).
In practice, the standard gravity (a constant) is often assumed, usually taken as .
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.
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
{| class="wikitable" style="text-align:center"
|+ Specific weight of water at standard sea-level atmospheric pressure (metric units)
|-
! 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
{| class="wikitable" style="text-align:center"
|+ Specific weight of air at standard sea-level atmospheric pressure (metric units)