The quantifi cation of the total amount of all materials into and out of a process is referred to as a mass balance . This type of analysis provides confi rmation that all materials have been fully accounted for, and no streams are missing. The fundamental concept is that the total mass fl owing into a process should equal total mass fl owing out. [See Figure 17-1. ] In practice, it is a useful way to identify previously hidden waste streams: if the mass coming out of a process is less than the combined mass of the inputs, then some other stream-most likely waste-must be leaving the process, too. This is a particularly important tool in light of the maxim that “waste is lost profi t.”