Fixed cost

Fixed costs are expenses that do not change in proportion to the activity of a business, within the relevant period or scale of production. For example, a retailer must pay rent and utility bills irrespective of sales. Unit fixed costs, called average fixed costs (AFC), decline with volume, following a rectangular hyperbola as the inverse of the volume of production.

Variable costs by contrast change in relation to the activity of a business such as sales or production volume. In the example of the retailer, variable costs may primarily be composed of inventory (goods purchased for sale), and the cost of goods is therefore almost entirely variable. In manufacturing, direct material costs are an example of a variable cost. An example of variable costs are the prices of the supplies needed to produce a product.

In accounting terminology, fixed costs will broadly include all costs (expenses) which are not included in cost of goods sold, and variable costs are those captured in costs of goods sold. The implicit assumption required to make the equivalence between the accounting and economics terminology is that the accounting period is equal to the period in which fixed costs do not vary in relation to production. In practice, this equivalence does not always hold, and depending on the period under consideration by management, some overhead expenses (such as sales, general and administrative expenses) can be adjusted by management, and the specific allocation of each expense to each category will be decided under cost accounting.