properties of fluids

Properties of Fluid-Density or Mass Density, Specific weight, Specific Volume, Gravity, PDF

Amrit Kumar Fluid Mechanics Hydraulic Machines Leave a Comment

Time to Read: 5 minutes

Before going to study properties of fluid let’s have an overview of What is fluid and Types of Fluid.

What is Fluid:

Generally, a Fluid definition is,

A fluid is any substance that flows or deforms under applied shear stress. or

The substances which have the tendency to flow is also called as fluid.

Different types of fluids are:

Basically, there are 5 different types of fluid has been classified:

1. Ideal fluid
2. Real fluid
3. Newtonian fluid
4. Non-Newtonian fluid, and
5. Ideal plastic fluid

1. Ideal Fluid:

The fluid which cannot be compressed and have no viscosity falls in the category of an ideal fluid. Ideal fluid is not found in actual practice but

It is an imaginary fluid because all the fluid that exists in the environment have some viscosity. So, there is no ideal fluid in reality.

2. Real Fluid:

A fluid which is having some viscosity is called real fluid. Actually, all the fluids existing or present in the environment are called real fluids.

3. Newtonian Fluid:

The Fluid which obeys Newton’s law of viscosity (i.e the shear stress is directly proportional to the shear strain) then it is known as the Newtonian fluid.

4. Non-Newtonian Fluid:

The Fluid which does not obey Newton’s law of viscosity then it is called Non-Newtonian fluid.

5. Ideal Plastic Fluid:

A fluid having the value of shear stress more than the yield value and shear stress is proportional to the shear strain (velocity gradient) is known as ideal plastic fluid.

Properties of Fluids:

  1. Density or Mass Density
  2. Weight Density or Specific Weight:
  3. Specific Volume and 
  4. Specific Gravity
properties of fluids
properties of fluids

1. Density or Mass Density:

The mass density or Density of a fluid is defined as the ratio of a mass of fluid to its a volume of the fluid.

Density is called a Mass per unit volume of a fluid. This is denoted by symbol ρ (rho) and the unit of mass density is (kg/m3). 

The density of liquid may be constant but the density of gases changes with the variation of temperature and pressure.

(The Density of water is 1000 g/m3) or (we can say 1 g/cm3).

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ρ (rho) = (Mass of Fluid) / (Volume of Fluid)

2. Weight Density or Specific Weight:

Weight density or specific density of a fluid is defined as the ratio of the weight of fluid to its volume of the fluid.

Weight density is called Weight per unit volume of a fluid. This is denoted by symbol ‘w‘ and the unit of mass density is (N/m3). 


w = (Weight of Fluid) / (Volume of fluid)

=(Mass of fluid) * (Acceleration due to gravity) / (Volume of fluid)

And we know from the previous formula of Density. So this becomes,

w = ρ * g

The value of weight density or specific weight  for water is 9.81*1000 N/m3

3. Specific Volume:

The specific volume of a fluid is defined as the ratio of the volume of fluid to the mass of fluid.


The volume of a fluid occupied by a unit mass or volume per unit mass of a fluid is called Specific volume.

The unit of Specific volume is m³/kg and This is commonly applied to Gases.


Specific Volume = (Volume of fluid) / (Mass of fluid)

When we divide by volume of fluid to the Numerator and Denominator the fraction we get is,

= (1) / (ρ)

Specific volume is the reciprocal of Mass Density as seen from the formula we got.

4. Specific Gravity:

This is defined as the ratio of the Density or weight density of a fluid to density or weight density of a standard fluid.

We know the Standard fluid is water so for liquid the water is standard fluid and 

For gases, the Standard fluid is taken as Air.

Most important The specific gravity is called Relative Density. This is denoted by the symbol ‘S’ and this is dimensionless because the upper unit and lower units get canceled.


(liquid) = ( Weight Density of liquid)/(Weight Density of water)

Here liquid cannot be the same as water it can be some oil and other.

(Gases) = ( Weight Density of Gases / (Weight Density of Air)


              Weight density of liquid  = S * Weight Density of water

                                               =  S * 9.81 * 1000 N/m3

Density of liquid = S * Density of water = S * 1000 kg/m3

I have covered the topics Types of Fluid Flow in details.

Steady, Unsteady, Uniform, Non-uniform, Laminar, Turbulent, Compressible Flow, PDF.

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About the Author

Amrit Kumar

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Amrit is a Co-founder of Learn Mechanical Portal (Previously owned ""). He completed his diploma in Mechanical Engineering from Acharya Polytechnic Bangalore and also completed his B-Tech from Delhi. Amrit is now preparing for Govt. exams. If you need help, you can ask him regarding any suggestions in the mechanical engineering field. He loves to meet new people.

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