The second moment of inertia indicates the resistance to deflection of a particular section of a profile or beam. Other (more) correct names are moment of inertia of plane area, area moment of inertia, or second area moment. The unit used for the 2^{nd} moment of inertia is length to the fourth power (m^{4}).

The second moment of inertia is independent of material and environment and is purely determined by geometric values ??of the element. That is, shape, length and width. The outcome of the calculation can be used to determine the response of an element to a particular load. This is usually a form of deflection. The calculators on this website assume that the element always bends around its neutral line which runs across the gravitational center of the cross section.

Closely related is the section modulus which is used to determine stress at a certain load. For this, the second moment of inertia is divided by the outer fiber distance seen from the neutral line. The polar moment of inertia is determined from the same geometric input and is used in torque calculations.

It is common for the second moment of inertia to be confused with the mass moment of inertia that indicates the resistance to acceleration of an object.