Seismic magnitude is the quantitative measurement of amount of energy released by an earthquake. The size of the earthquakes is independent of the density of population and type of construction. Earthquakes may be characterized as minor or great, depending on their magnitude. Different magnitude scales used to measure the magnitude of an earthquake are Richter scale, Body-wave magnitude scale, Surface-wave magnitude scale and Moment-magnitude scale. Seismograms recorded at different epicentral distances are employed to determine origin time, epicenter focal depth and type of faulting as well as to estimate the energy released during an earthquake. Selection of the scale depends upon the earthquake size. In this paper, we discuss the different magnitude scales, the relevance of each scale and their conversion equations.