This is calculated for a material balance area (MBA) over a material balance period using the material balance equation, commonly written as:

MUF = (PB + X - Y) - PE

where

PB is the beginning physical inventory,

X is the sum of increases to inventory,

Y is the sum of decreases from inventory,

PE is the ending physical inventory.

Because book inventory is the algebraic sum of PB, X and Y, MUF can be described as the difference between the book inventory and the physical inventory. For item MBAs, MUF should be zero, and a non-zero MUF is an indication of a problem (e.g. accounting mistakes) which should be investigated. For bulk handling MBAs, a non-zero MUF is expected because of measurement uncertainty and the nature of processing. The operator's measurement uncertainties associated with each of the four material balance components are combined with the material quantities to determine the uncertainty of the material balance σMUF.