Density vs. Specific Weight and Specific Gravity
An introduction to density, specific weight and specific gravity.
Density is defined as mass per unit volume. Mass is a property and the SI unit for density is [kg/m^{3}].
Density can be expressed as
ρ = m / V = 1 / ν_{ }[1]
where
ρ = density [kg/m^{3}], [slugs/ft^{3}]
m = mass [kg], [slugs]
V = volume [m^{3}], [ft^{3}]
ν = specific volume [m^{3}/kg], [ft^{3}/slug]
 What is weight and what is mass?  the difference between weight and mass
The Imperial (U.S.) units for density are slugs/ft^{3} but poundmass per cubic foot  lb_{m}/ft^{3}  is often used. Note that there is a difference between poundforce (lb_{f}) and poundmass (lb_{m}). Slugs can be multiplied with 32.2 for a rough value in poundmass (lb_{m}).
 1 slug = 32.174 lb_{m} = 14.594 kg _{}
 1 kg = 2.2046 lb_{m} = 6.8521x10^{2} slugs
 density of water: 1000 kg/m^{3}, 1.938 slugs/ft^{3}
See also Unit converter  mass and Unit converter  density
On atomic level  particles are packed tighter inside a substance with higher density. Density is a physical property  constant at a given temperature and pressure  and may be helpful for identification of substances.
Below on this page: Specific gravity (relative density), Specific gravity for gases, Specific weight, Calculation examples
Se also: Densities for some common materials
Water  Density, Specific Weight and Thermal Expantion Coefficient  variation with temperature at 1, 68 and 680 atm, SI and Imperial units
Air  Density, Specific Weight and Thermal Expantion Coefficient  variation with temperature and pressure, SI and Imperial units
How to measure density of liquid petroleum products
Example 1: Density of a Golf ball
Example 2: Using Density to Identify a Material
Example 3: Density to Calculate Volume Mass
Specific Gravity (Relative Density)  SG  is a dimensionless unit defined as the ratio of the density of a substance to the density of water  at a specified temperature and can be expressed as
SG = ρ_{substance} / ρ_{H2O} [2]
where
SG = Specific Gravity of the substance
ρ_{substance} = density of the fluid or substance [kg/m^{3}]
ρ_{H2O} = density of water  normally at temperature 4 ^{o}C [kg/m^{3}]
It is common to use the density of water at 4 ^{o}C (39^{o}F) as a reference since water at this point has its highest density of 1000 kg/m^{3 }or 1.940 slugs/ft^{3}.
Since Specific Gravity  SG  is dimensionless, it has the same value in the SI system and the imperial English system (BG). SG of a fluid has the same numerical value as its density expressed in g/mL or Mg/m^{3}. Water is normally also used as reference when calculating the specific gravity for solids.
See also Thermophysical Properties of Water  Density, Freezing temperature, Boiling temperature, Latent heat of melting, Latent heat of evaporation, Critical temperature ...
Example 4: Specific Gravity of Iron
Specific Gravity for some common Materials
Substance  Specific Gravity  SG  

Acetylene  0.0017 
Air, dry  0.0013 
Alcohol  0.82 
Aluminum  2.72 
Brass  8.48 
Cadmium  8.57 
Chromium  7.03 
Copper  8.79 
Carbon dioxide  0.00198 
Carbon monoxide  0.00126 
Cast iron  7.20 
Hydrogen  0.00009 
Lead  11.35 
Mercury  13.59 
Nickel  8.73 
Nitrogen  0.00125 
Nylon  1.12 
Oxygen  0.00143 
Paraffin  0.80 
Petrol  0.72 
PVC  1.36 
Rubber  0.96 
Steel  7.82 
Tin  7.28 
Zinc  7.12 
Water (4^{o}C)  1.00 
Water, sea  1.027 
Wood, Oak  0.77 
Specific Gravity of gases is normally calculated with reference to air  and defined as the ratio of the density of the gas to the density of the air  at a specified temperature and pressure.
The Specific Gravity can be calculated asSG = ρ_{gas} / ρ_{air} [3]
where
SG = specific gravity of gas
ρ_{gas} = density of gas [kg/m^{3}]
ρ_{air} = density of air (normally at NTP  1.204 [kg/m^{3}])
 NTP  Normal Temperature and Pressure  defined as 20^{o}C (293.15 K, 68^{o}F) and 1 atm ( 101.325 kN/m2, 101.325 kPa, 14.7 psia, 0 psig, 30 in Hg, 760 torr)
Molecular weights can be used to calculate Specific Gravity if the densities of the gas and the air are evaluated at the same pressure and temperature.
See also Thermophysical Properties of Air  density, viscosity, critical temperature and pressure, triple point, enthalpi and entropi, thermal conductivity and diffusicity,......
Specific Weight is defined as weight per unit volume. Weight is a force. The SI unit for specific weight is [N/m^{3}]. The imperial unit is [lb/ft^{3}].
Specific Weight (or force per unit volume) can be expressed as
γ = ρ a_{g} [4]
where
γ = specific weight (N/m^{3}], [lb/ft^{3}]
ρ = density [kg/m^{3}], [slugs/ft^{3}]
a_{g} = acceleration of gravity (9.807 [m/s^{2}], 32.174 [ft/s^{2}] under normal conditions)
 What is weight and what is mass?  the difference between weight and mass
Example 5: Specific Weight of Water
Specific Weight for Some common Materials
Product  Specific Weight  γ   

Imperial Units (lb/ft^{3})  SI Units (kN/m^{3})  
Aluminum  172  27 
Brass  540  84.5 
Carbon tetrachloride  99.4  15.6 
Copper  570  89 
Ethyl Alcohol  49.3  7.74 
Gasoline  42.5  6.67 
Glycerin  78.6  12.4 
Kerosene  50  7.9 
Mercury  847  133.7 
SAE 20 Motor Oil  57  8.95 
Seawater  63.9  10.03 
Stainless Steel  499  512  78  80 
Water  62.4  9.81 
Wrought Iron  474  499  74  78 
Examples
Example 1: Density of a Golf ball
A golf ball has a diameter of 42 mm and a mass of 45 g. The volume of the golf ball can be calculated as
V = (4/3) π (42 [mm] * 0.001 [m/mm]/2)^{3 } = 3.8 10^{5} [m^{3}]
The density of the golf ball can then be calculated as
ρ = 45 [g] * 0.001 [kg/g] / 3.8 10^{5 }[m^{3}] = 1184 [kg/m^{3}]
Example 2: Using Density to Identify a Material
An unknown liquid substance has a mass of 18.5 g and occupies a volume of 23.4 ml (milliliter).
The density of the substance can be calculated as
ρ = (18.5 [g] /1000 [g/kg]) / (23.4 [ml] /(1000 [ml/l] * 1000[l/m^{3}]))
= 18.5 10^{3} [kg] /23.4 10^{6} [m^{3}] = 790 [kg/m^{3}]
If we look up the densities of some common liquids we find that ethyl alcohol  or ethanol  has a density of 789 kg/m^{3}. The liquid may be ethyl alcohol!
Example 3: Density to Calculate Volume Mass
The density of titanium is 4507 kg/m^{3}. The mass of 0.17 m^{3} volume titanium can be calculated as
m = 0.17 [m^{3}] * 4507 [kg/m^{3}] = 766.2 [kg]
Note!  be aware that there is a difference between "bulk density" and actual "solid or material density". This may not be clear in the description of products. Always double check values with other sources before important calculations.
Example 4: Specific Gravity of Iron
The density of iron is 7850 kg/m^{3}. The specific gravity of iron related to water with density 1000 kg/m^{3} is
SG(iron) = 7850 [kg/m^{3}] / 1000 [kg/m^{3}] = 7.85
Example 5: Specific Weight of Water
The density of water is 1000 kg/m3 at 4 °C (39 °F).
The specific weight in SI units is
γ = 1000 [kg/m^{3}] * 9.81 [m/s^{2}] = 9810 [N/m^{3}] = 9.81 [kN/m^{3}]
The density of water is 1.940 slugs/ft3 at 39 °F (4 °C).
The specific weight in Imperial units is
γ = 1.940 [slugs/ft^{3}] * 32.174 [ft/s^{2}] = 62.4 [lb/ft^{3}]
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