What is the formula for Moment of Inertia of a rigid body?

Inertia of a body is its inability to change by itself its state of rest or of uniform motion in a straight line. Similarly Moment of inertia of a body is its inability to change by itself its state of rest or of uniform rotatory motion about an axis.
The formula for Moment of Inertia of a rigid body is given as,
The moment of inertia of the rigid body (l) = Sum of moments of inertia of
all the particles present in the body.
Let us consider,

m₁, m₂, m₃ = masses of large particles in a rigid body

r₁, r₂, r₃ = distance of the masses from the axis
The formula for Moment of Inertia of a rigid body will be,

 l= m₁r₁² + m₂r₂² + m₃r₃² + …

The above formula can also be expressed as,

m_i = mass of i^th particle

r_i = distance between i^th particle and axis o rotation