Specific weight

The specific weight, also known as the unit weight (symbol γ, the Greek letter gamma), is a volume-specific quantity defined as the weight W divided by the volume V of a material: $${\displaystyle \gamma =W/V.}$$ Equivalently, it may also be formulated as the product of density, ρ, and gravity acceleration, g: $${\displaystyle \gamma =\rho \,g.}$$ Its unit of measurement in the International System of Units (SI) is the newton per cubic metre (N/m³), expressed in terms of base units as kg⋅m⁻²⋅s⁻². 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 pounds-force per cubic foot.^[1]

The density of a material is defined as mass divided by volume, typically expressed with the unit kg/m³. 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 9.80665 m/s².

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]

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/m³ or lbf/ft³). Specific weight can be used as a characteristic property of a fluid.^[2]

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

The unit weight of a soil when void spaces of the soil contain both water and air. $${\displaystyle \gamma ={\frac {(1+w)G_{\text{s}}\gamma _{\text{w}}}{1+e}}}$$ where

• 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

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: $${\displaystyle \gamma _{\text{d}}={\frac {G_{\text{s}}\gamma _{\text{w}}}{1+e}}={\frac {\gamma }{1+w}}}$$ where

• 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

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: $${\displaystyle \gamma _{\text{s}}={\frac {(G_{\text{s}}+e)\gamma _{\text{w}}}{1+e}}}$$ where

• 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

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: $${\displaystyle \gamma '=\gamma _{\text{s}}-\gamma _{\text{w}}}$$ where

• 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

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.

• Submerged weight calculator
• Specific weight calculator
• http://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.html
• http://www.themeter.net/pesi-spec_e.htm