Parallel axis theorem: Learn how to calculate

Learn how to calculate the moment of inertia or the second moment of area of a rigid body about any axis , given the body's moment of inertia about a parallel axis through the center of gravity. See the derivation, the tensor generalization and the applications of the parallel axis theorem . Parallel axis theorem states that the moment of inertia of a rigid body about any axis parallel to its centroidal axis is equal to the sum of the body's moment of inertia about its centroidal axis and the product of its mass and the square of the distance between the two axes. These theorems help students calculate the moment of inertia of various bodies about different axes with ease. They are especially useful when direct computation is complex or not practical. Understanding these theorems allows you to solve many numerical and conceptual problems in rotational dynamics confidently. This article explores the theorem ’s theory, its application in structural engineering, and a worked example involving a steel truss.