Properties of Fluid [Notes with PDF]

The fluid is a substance which is having a tendency to flow. The properties of fluid include Density or Mass Density, Weight Density or Specific Weight, Specific Volume, Specific Gravity, and Viscosity.

Note: You can download PDF at the end of the article.

So let’s start with definition first.

Definition of Fluid:

The fluid is a substance which deforms continuously (change in shape due to relative motion) under the action of shear force, no matter how small.

For examples, liquid and gases.

Fluid is also called as zero memory substance.

Properties of Fluids:

There are five properties of fluids and those are:

  1. Density or Mass Density
  2. Weight Density or Specific Weight
  3. Specific Volume
  4. Specific Gravity
  5. Viscosity
Image of Five Properties of Fluid

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 kg/m3) or (we can say 1 g/cm3).


Formula of Density

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

Some Additional Information:

Value of Density of water, Glycerin. air and Mercury
  • At 40 density of H2O is 1000 kg/m3 and This is maximum.
  • As T increases the molecule will move away and density increases.

2. Weight Density or Specific Weight:

Weight density or specific density of a fluid is defined as the ratio of the weight of the 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). 


formula of Specific Weight
Specific Weight

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

  • w depends on ρ and g and ρ is depended on T and P, So
  • w=f(T, P, location(g))
Comparison of specific gravity between earth and moon

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. or 

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.


S (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

5. Viscosity:

Viscosity is defined as resistance to flow deformation. It is an internal resistance between two particles.

The reciprocal of the viscosity is called the fluidity.

In Mathematically the viscosity is the measure of internal resistance between two adjacent fluid particles in motion. Unit is 1/s

Viscosity curve

dθ is angular deformation.

dθ/dt, It is rate of angular deformation to the rate of shear strain or strain rate.

du/dy is the velocity gradient.

So this is all about the Properties of Fluid, I hope you are able to understand the lesson completely, if you have any doubts don’t forget to comment down with your queries.

References [External Links]:

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Anup Kumar Dey

A Mechanical Engineer with 19 years of working experience in various renowned MNCs.

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