Cost allocation refers to the allotment of all the items of cost to cost centers or cost units whereas cost apportionment refers to the allotment of proportions of items of cost to cost centers or cost units.
Thus, the former involves the process of charging direct expenditure to cost centers or cost units whereas the latter involves the process of charging indirect expenditure to cost centers or cost units.
For example, the cost of labor engaged in a service department can be charged wholly and directly but the canteen expenses of the factory cannot be charged directly and wholly. Its proportionate share will have to be found out. Charging of costs in the former case will be termed as “allocation of costs” whereas in the latter, it will be termed as “apportionment of costs.”