This chapter describes the centroid and moment of inertia can be determined for irregular sections using calculus. The centroid of an object is the point where the masses of all grains of the object orients. It is the point about which the area could be balanced if it were supported by that point. The term ‘centroid’ is derived from the word ‘center’. The study of centroid is important in engineering to locate the point of load application in a structure, determining the neutral plane in a structural member that deform, analyzing structure in an efficient way, etc. The procedure to calculate the centroid of composite areas that can be divided into several regular sections such as squares, rectangles, triangles, circles, etc. are discussed in this chapter.