Inertia and Inertial mass is found out using Newton’s Second law. Newtons second law states: “A larger resultant force acting upon an object, the greater acceleration that object will have, Meaning force and acceleration are directly proportional to each other (f∝a), and acceleration is inversely proportionate to the mass of an object. This is shown through the formula: f = ma, where
f = Force in Newtons, m = Mass in Kilograms, Kg and a = Acceleration in m/s^2
This, therefore, can be arranged to find Mass, or Acceleration, using that acceleration is inversely proportionate to the mass of an object.
Inertia is 'The tendency for motion to remain unchanged', basically the measurement that objects move at the same velocity, or not at all. This links to Newton's first law that until acted upon a resultant force, objects moving and not moving will stay moving at that velocity.
Inertial mass is the measurement for how difficult it is to change the velocity for the object. Using the previous Newtons second law formula: F =ma