Cost Behavior

Cost behavior describes how a cost's total amount changes in response to a change in some activity level, such as units produced, miles driven, or machine hours used. Understanding cost behavior is the foundation of cost-volume-profit analysis, break-even calculations, and most short-term management decisions, since knowing how a cost actually moves with activity is what lets a manager predict how profit will change under a different scenario.

A variable cost changes in total in rough proportion to activity — gasoline and tire wear on a vehicle rise with the number of miles driven, and direct materials rise with the number of units produced. A fixed cost stays the same in total within a reasonable range of activity, regardless of whether activity rises or falls somewhat — vehicle insurance and parking fees do not change with the number of miles driven, and factory rent does not change with a modest change in production volume. A mixed cost, sometimes called a semivariable cost, contains both a fixed and a variable component — a cell phone plan with a fixed monthly base fee plus a per-minute charge for usage beyond an included allowance is a familiar everyday example.

Cost behavior classifications hold only within a relevant range of activity, the span of volume over which the assumed behavior is actually expected to apply. A cost treated as fixed, such as factory rent, eventually becomes variable if volume grows enough to require a second factory, and a cost treated as purely variable can flatten out at very high volumes if a supplier offers bulk discounts. Analysts and managers using cost behavior assumptions for planning purposes are implicitly relying on activity staying within the range over which those assumptions were originally estimated, and results outside that range should be treated with caution.

Businesses determine cost behavior using several techniques, including the high-low method, which estimates the fixed and variable components using only the highest and lowest activity levels observed, and regression analysis, a more statistically rigorous technique that uses many data points to estimate the same relationship. Account analysis, in which an experienced manager or accountant classifies each cost based on judgment and knowledge of the business, is a simpler, more commonly used approach in practice, particularly for costs where a clear mathematical pattern is not obvious from the data alone.

Because fixed costs do not change with volume in the short run, a business with largely fixed costs faces greater risk from a sales downturn, since it cannot shed much cost quickly if revenue falls, but also benefits disproportionately from rising volume once fixed costs are covered, since each additional unit sold then contributes almost entirely to profit. Understanding which costs are fixed and which are variable is essential before attempting any contribution margin calculation, break-even analysis, or special-order pricing decision, since misclassifying a cost's behavior can lead directly to the wrong business decision even if every other number used in the analysis is otherwise correct.