A fixed cost is one that does not change in total within a reasonable range of activity. For example, the rent for a production facility is a fixed cost if the rent will not change when there are reasonable changes in the amount of output or input. (Of course, if there is a need to double the output the rent will change when the company occupies additional work space.)
While a fixed cost remains constant in total, the fixed cost per unit of output or input will change inversely with the change in the quantity of output or input. For instance, if the rent of the production facility is fixed at $120,000 per year and there are 30,000 machine hours of good output during the year, the rent will be $4 ($120,000/30,000) per machine hour. If there are 40,000 machine hours during the year, the rent will be $3 ($120,000/40,000) per machine hour.
Many manufacturing overhead costs are fixed and the amounts occur in large increments. Some examples include depreciation on a company-owned factory, depreciation on machinery and equipment, salaries and benefits of manufacturing supervisors, factory administration costs, etc. One challenge for accountants is the allocation or assigning of the large fixed costs to the individual units of product (which likely vary in size and complexity). This allocation (or assigning or absorbing) is required by the accounting and income tax rules for valuing inventories and the cost of goods sold. If the fixed overhead is assigned using machine hours, one must keep in mind that the cost rate per machine hour is not how the fixed costs behave or occur. In our example, the cost of the rent might be assigned to the products at the rate of $3 or $4 per machine hour but the rent actually occurs at the rate of $10,000 per month.