Variable costs are expenses that change in proportion to the activity of a business. In other words, variable cost is the sum of marginal costs. Along with fixed costs, variable costs make up the two components of total cost. Direct Costs, however, are costs that can be associated with a particular cost object. Not all variable costs are direct costs, however; for example, variable manufacturing overhead costs are variable costs that are not a direct costs, but indirect costs.Variable costs are sometimes called unit-level costs as they vary with the number of units produced.
[edit] Explanation
For example, a manufacturing firm pays for raw materials. When activity is decreased, less raw material is used, and so the spending for raw materials falls. When activity is increased, more raw material is used and spending therefore rises. Note that the changes in expenses happen with little or no need for managerial intervention.
A company will pay for line rental and maintenance fees each period regardless of how much power gets used. And some electrical equipment (air conditioning or lighting) may be kept running even in periods of low activity. These expenses can be regarded as fixed. But beyond this, the company will use electricity to run plant and machinery as required. The busier the company, the more the plant will be run, and so the more electricity gets used. This extra spending can therefore be regarded as variable.
In retail the cost of goods is almost entirely a variable cost; this is not true of manufacturing where many fixed costs, such as depreciation, are included in the cost of goods.
Although taxation usually varies with profit, which in turn varies with sales volume, it is not normally considered a variable cost.
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