Franciscan University of Steubenville defines a conversion factor as a ratio written in fraction form that can express the same quantity or value in two different units. Conversion factors are used to change units while maintaining the original measurements. When using conversion factors, Education Portal states that the original measurement is multiplied using the conversion factor to get the same measurement, but it is expressed in new units.
SparkNotes further defines a conversion factor as a fraction or ratio that represents the ratio between two distinct units and is always equal to one. The numerator and the denominator are equal with the only difference being that they are expressed differently. Since it is always equal to one, multiplying this ratio using the original value only changes its unit but not its value. According to Education.com, conversion factors come in handy when comparing two measurements that are not expressed in the same units. For instance, it's possible to use conversion factors to solve a measurement in meters when a map reads only in kilometers. To sum it up, SparksNotes states that cancelling units using conversion factors makes it easy to work on and check work in chemistry. The only requirement is that the units wanted in a final answer are the ones retained throughout.