Specific Gravity Formula: Equations & Examples

A specific gravity formula indicates the ratio of the density of a given substance to the density of water (H₂O). This is a special case of relative density, which gives information about the properties of fluids in terms of their density or weight.

• Specific gravity is a dimensionless quantity that depends upon the mass of an object and the density of the fluid.
• If you are wondering why an object sinks or floats in water, specific gravity is one reason. 
• In case the value of specific gravity is less than one then the object will float.
• On the other hand, if the specific gravity is greater than one, then the object will sink.
• Mathematically, it can be represented as:

Specific Gravity Formula : \(RD= \frac{\rho_{substance}}{\rho_{reference}}\) 

• where RD = relative density, \(\rho_{substance}\) = density of the substance being measured, \(\rho_{reference}\) = density of the reference.

Key Terms: Gravity, Specific Gravity, Density, Mass, Weight, Volume, Relative Density, Pressure, Temperature, Specific Gravity Formula

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What is Specific Gravity?

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Specific gravity may also be defined as the ratio of the density of a substance to the density of the water at a specified temperature. It is mainly used to determine the properties of the fluid in relation to the concerned objects.

• Science represents specific gravity with the symbol ‘S’. 
• It is the most important topic in Physics, where the concept of specific density is implied to calculate whether any object or body placed in water, taken as fluid, will sink or float in it. 
• The specific gravity of the fluid takes into account the density, mass or weight of the object and the fluid. 

However, there are multiple factors affecting the tendency of the object or body to sink or float, such as,

• Weight/ mass of the object
• Volume of water
• Buoyant Force
• Density of water
• Substance placed in it

Specific Gravity

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Specific Gravity Formula

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In general, specific gravity formula (S) is calculated in terms of the density, mass and weight of the concerned object or body in relation with the given fluid or say water.

• The temperature for calculating the specific gravity remains approximately 4^o Celsius.
• We can have three different formulas for calculating the specific gravity as mentioned below:

Specific Gravity = Mass of unit volume of the substance / Mass of unit volume of water

Specific Gravity = Weight of the substance / Weight of the equal amount of water

Specific Gravity = Density of substance / Density of equal amount of water

• Here, all the three formulas have used different elements for calculating specific gravity, which states that the specific gravity is not a confined concept, rather, it depends upon multiple factors, such as, mass, density and weight of both water and the object, as already mentioned above.

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Derivation of Specific Gravity Formula

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We know that, Density = Mass (m) / Volume (v)

• Here, m = mass of the object
• v = volume of the object,
• Therefore, with the above formula of density, it can be analyzed that the density and mass of an object are directly proportional.
• So, the specific gravity can be calculated by dividing the mass of the object with that of water, such as,

Specific Gravity = Mass of the substance / Mass of unit volume of water

SG = M_object / M_Water

• Now, it can also be seen that the weight of the object and the water are affecting the density, such as,

Specific Gravity = Weight of the substance / Weight of the equal amount of water

SG = W_object / W_Water

• However, in all the formulas, the units of different elements shall remain the same, so that they cancel each other because specific gravity is a unitless concept. 

Note: We generally consider the density of water at a temperature 4 degree C as a reference point because water at this point of temperature has the highest density of 1000 kg/m3.

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Specific Gravity of Gas

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The ratio of the density of gas to that of the density of air at a specific temperature and pressure resembles the specific gravity of the gas. Therefore, it can be said that the specific gravity of gas is mainly understood in terms of the air.

• However, the air is recognized in its constant terms. Mathematically, the specific gravity could be represented as,

SG = ρgas / ρair

Specific Gravity of Different Elements

The specific gravity of different materials or substances are mentioned below for better understanding.

• SG of water = 1
• SG of mercury = 13. 6
• SG of aluminium = 2.72
• SG of gold = 19.3

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Things to Remember

• Specific gravity mainly determines if the object will sink or float after being placed in the fluid,.
• The density of air at room temperature, that is, 1.20 kg /m3 , also impacts the calculations of the specific gravity, mainly of gas.
• It has been found that the density of water is 1000 kilogram / cubic meters.
• For the same density, the specific gravity of the water always remains one.
• Specific gravity is of extreme importance for multiple industries.
• The specific gravity formula can be utilized either to test the purity levels in the gems or to identify the amount of solvent in urine concentration.
• It also helps recognize the mineral contents in rocks, which are majorly used by geologists.
• One of its major uses is in the medical sector, where urinalysis is an important part of usual treatments.

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