As the level of business activities changes, some costs change while others do not. The response of a cost to a change in business activity is known as cost behavior. Managers should be able to predict the behavior of a particular cost in response to a change in particular business activity. For this purpose, costs are classified as variable, fixed and mixed costs. This article explains these three types of costs as well as their response to business activities.

Variable cost:

A cost that changes, in total dollar amount, with the change in the level of activity is called variable cost. A common example of variable cost is direct materials cost. Consider the following example to understand how variable cost behaves in a manufacturing company.

A mobile phone manufacturing company purchases speakers from another company at a cost of $2 per speaker. The speaker is a direct materials cost for mobile phone manufacturing company. One speaker is used to complete a mobile phone. The total and per unit cost of speakers at various levels of activity is given below:

No. of mobile
phones produced
Cost of 1
speaker
Total cost
of speakers
1 $2.00 $2
50 $2.00 $100
100 $2.00 $200
150  $2.00  $300
200 $2.00 $400
250 $2.00 $500
300 $2.00 $600

variable-cost-behavior

Notice that the total cost of speakers increases as the mobile phones produced are increased but per unit cost remains constant.

Other examples of variable cost include lubricants, sales commission and shipping costs etc.

Fixed cost:

A cost that does not change, in total, with the change in activity is called fixed cost. A common example of fixed cost is rent. In above example, if mobile phone manufacturing company rents a building for its factory for $5,000 per month, it will have to pay $5,000 for every month even no mobile phone is produced during the month. The behavior of fixed is shown in the following figure:

fixed-cost-behavior

Total fixed cost does not change with the change in activity but per unit fixed cost changes with the rise and fall in the level of activity. There is an inverse relationship between per unit fixed cost and activity. If production increases, per unit fixed cost decreases and if production decreases, per unit fixed cost increases. To understand this point, we can use the data from the above example of mobile manufacturing company. Consider the following table:

Monthly rent
of the building
Number of mobile
phones manufactured
Average cost per
mobile phone
$5,000 1 $5,000/1 = $5,000
$5,000 50 $5,000/50 = $100
$5,000 100 $5,000/100 = $50
$5,000 150 $5,000/150 = $33.33
$5,000 200 $5,000/200 = $25
$5,000 250 $5,000/250 = $20
$5,000 300 $5,000/300 = $16.67

Notice that average fixed cost (computed in the last column) decreases as the production of mobile phones increases. It is an interesting property of fixed cost.

Mixed or semi-variable cost:

A cost that has the characteristics of both variable and fixed cost is called mixed or semi-variable cost. For example, the rental charges of a machine might include $500 per month plus $5 per hour of use. The $500 per month is a fixed cost and $5 per hour is a variable cost. Another example of mixed or semi-variable cost is electricity bill. The electricity bill can be divided into two parts – (1) line rent and (2) cost of units consumed. The line rent is not affected by the consumption of electricity whereas the cost of units consumed varies with the change in units consumed.