Plastic modulus is a particular concept in structural engineering, while the moment of inertia is a concept in physics. Plastic modulus is mostly used as a formula to solve the problems of irreversible plastic behavior, which can easily form a kind of deformation. The plastic moment or the total cross-section of a material can be determined using plastic modulus. Moment of Inertia, on the other hand, simply refers to what measures the body's resistance to a change in its acceleration. Simply put, it is the force needed by an object to alter its speed.

If an object has a low moment of inertia, it means little effort is needed to change its speed, vice versa. While both plastic modulus and moment of inertia are calculated with different formulae, both are also represented with different letters. Plastic modulus is denoted by the letter Z, while the moment of inertia is represented with the letter 'I.'