manufacturing overhead also needs to be recorded on the job cost sheet. However, assigning manufacturing overhead to a specific job involves some difficulties. There are three reasons for this:
Manufacturing overhead is an indirect cost. This means that it is either impossible or difficult to trace these costs to a particular product or job.
Manufacturing overhead consists of many different types of cost ranging from the grease used in machines to the annual salary of the production manager. Some of these costs are variable overhead costs because they vary in direct proportion to changes in the level of production (e.g., indirect materials, supplies, and power) and some are fixed overhead costs because they remain constant as the level of production fluctuates (e.g., heat and light, property taxes, and insurance).Page 123
Because of the fixed costs in manufacturing overhead, total manufacturing overhead costs tend to remain relatively constant from one period to the next even though the number of units produced can fluctuate widely. Consequently, the average cost per unit will vary from one period to the next.
Given these problems, allocation is used to assign overhead costs to products. Allocation is accomplished by selecting an allocation base that is common to all of the company’s products and services. An allocation base is a measure such as direct labor-hours (DLH) or machine-hours (MH) that is used to assign overhead costs to products and services. The most widely used allocation bases in manufacturing are direct labor-hours, direct labor cost, machine-hours and (where a company has only a single product) units of product.
Manufacturing overhead is commonly assigned to products using a predetermined overhead rate. The predetermined overhead rate is computed by dividing the total estimated manufacturing overhead cost for the period by the estimated total amount of the allocation base for the period as follows:
The predetermined overhead rate is computed before the period begins using a four-step process. The first step is to estimate the total amount of the allocation base (the denominator) that will be required for next period’s estimated level of production. The second step is to estimate the total fixed manufacturing overhead cost for the coming period and the variable manufacturing overhead cost per unit of the allocation base. The third step is to use the cost formula shown below to estimate the total manufacturing overhead cost (the numerator) for the coming period:
Y = a + bX
Y = The estimated total manufacturing overhead cost
a = The estimated total fixed manufacturing overhead cost
b = The estimated variable manufacturing overhead cost per unit of the allocation base
X = The estimated total amount of the allocation base
The fourth step is to compute the predetermined overhead rate. Notice, the estimated amount of the allocation base is determined before estimating the total manufacturing overhead cost. This needs to be done because total manufacturing overhead cost includes variable overhead costs that depend on the amount of the allocation base.
The Need for a Predetermined Rate
Instead of using a predetermined rate based on estimates, why not base the overhead rate on the actual total manufacturing overhead cost and the actual total amount of the allocation base incurred on a monthly, quarterly, or annual basis? If an actual rate is computed monthly or quarterly, seasonal factors in overhead costs or in the allocation base can produce fluctuations in the overhead rate. For example, the costs of heating and cooling a factory in Illinois will be highest in the winter and summer months and lowest in the spring and fall. If the overhead rate is recomputed at the end of each month or each quarter based on actual costs and activity, the overhead rate would go up in the winter and summer and down in the spring and fall. As a result, two identical jobs, one completed in the winter and one completed in the spring, would be assigned different manufacturing overhead costs. Many managers believe that such fluctuations in product costs serve no useful purpose. To avoid such fluctuations, actual overhead rates could be computed on an annual or less-frequent basis. However, if the overhead rate is computed annually based on the actual costs and activity for the year, the manufacturing overhead assigned to any particular job would not be known until the end of the year.
predetermined overhead rate, cost driver, fixed overhead, variable overhead
Raw materials, work in process, finished goods, cost of goods manufactured, manufactured overhead cost
Cost of goods manufactured
Job order costing, Direct cost, indirect cost, common cost, manufacturing overhead cost, indirect material, job cost sheet, job number, subsidiary ledger, material requisition form, bill of materials, time ticket, allocation base