Differential Costing

Differential cost is the difference in total cost between two alternative courses of action, and differential costing is the practice of comparing alternatives on that difference instead of on their full costs. Differential cost is often used interchangeably with incremental cost, though some texts reserve incremental for an increase in cost specifically. Only the amounts that change between the alternatives enter the comparison, so a cost that is identical under both drops out of it.

The Formula

Differential cost     =  total cost of Alternative A     -  total cost of Alternative B
Differential revenue  =  total revenue of Alternative A  -  total revenue of Alternative B
Differential profit   =  differential revenue            -  differential cost

An alternative here is one complete course of action the business could choose, such as taking a block of work or turning it down. Differential cost is the difference between costs for alternatives and differential revenue is the difference between revenues for alternatives, and looking only at those differences is what the field calls differential analysis (OpenStax, Principles of Accounting, Volume 2: Managerial Accounting, key terms). The revenue line sits beside the cost line because cost alone decides nothing when the alternatives also differ in what they bring in.

Harold Averkamp of AccountingCoach uses the terms differential cost and incremental cost to mean the same thing, the difference in cost between two alternatives (AccountingCoach, Harold Averkamp, CPA, MBA). The cost and management accounting curriculum of the Institute of Chartered Accountants of India (ICAI) keeps the two apart, reading differential cost as the difference between the costs of two production levels, an increase or a decrease, and incremental cost as the increase specifically (ICAI, Cost and Management Accounting, Marginal Costing).

What Drops Out of the Comparison

Any future cost that does not differ between the alternatives is not a relevant cost for the decision, and a sunk cost, one that cannot be avoided because it has already occurred, will not change regardless of the alternative management chooses (OpenStax, Principles of Accounting, Volume 2: Managerial Accounting, relevant information for decision-making). Both leave the comparison for the same reason, since subtracting an identical amount from both sides changes nothing.

Differential is its own axis, separate from whether a cost is traceable and from whether it varies with volume. A cost can be traceable to a single job and still be identical under both alternatives, so the direct and indirect test in direct cost answers a different question. An allocated share of firm overhead that does not move when the choice is made is not differential either, whatever rate carried it there, and that rate and its base belong to costing overheads.

A Worked Example

Say a firm is offered a block of business returns at a fixed fee per return and is deciding whether to take the work or turn it down. The preparer and review hours below are bought only if the block is taken, so they change with the choice. Hours from salaried staff who are paid either way would not differ between the alternatives and would drop out of the comparison the way the rent does.

Alternative A, take the block
  Fee revenue                    150 returns x $420   =  $63,000
  Preparer time         150 returns x 4 hours x $32   =  $19,200
  Review time           150 returns x 1 hour  x $85   =  $12,750
  E-file and software   150 returns x $9              =   $1,350
  Costs that change if the block is taken             =  $33,300

Alternative B, turn the block down
  Fee revenue                                         =       $0
  Costs that change if the block is turned down       =       $0

Identical under both, so excluded from the comparison
  Office rent for the quarter                         =  $30,000
  Allocated share of firm overhead                    =  $12,000

Differential revenue                 63,000 - 0       =  $63,000
Differential cost                    33,300 - 0       =  $33,300
Differential profit             63,000 - 33,300       =  $29,700

One cost that does belong in the comparison is easy to leave out. If taking the block means turning away other work, the profit given up on that other work is an opportunity cost, and it is differential even though nothing is paid out for it. The accounting and economic profit split behind that idea is in explicit vs implicit cost.

Differential Costing and Marginal Costing

Marginal costing is a technique that classifies every element of cost as fixed or variable and treats only the variable costs as costs of the product, while the fixed costs are treated as period costs incurred regardless of the volume of output (ICAI, Cost and Management Accounting, Marginal Costing). Differential costing classifies nothing. It takes two alternatives and subtracts. Averkamp reserves the narrower term marginal cost for a third thing again, the cost of producing the very next unit or operating for just one more hour (AccountingCoach, Harold Averkamp, CPA, MBA).

Where It Sits in a Firm’s Work

Differential costing is decision-support work of the kind managerial accounting covers, and a firm runs it for clients weighing a purchase or a product line and for itself weighing capacity. It produces no journal entry and moves no financial statement, because both alternatives are still projections at the moment the comparison is made. Naming the two alternatives precisely is where the discipline sits, since every figure in the analysis is a difference and a vague alternative puts all of them in doubt.

Accountably places trained offshore accountants and tax preparers inside US CPA and EA firms, and a firm can grade real work on its own files through a trial task before any client file moves.