Recognizing the Limitations of Marginal Costing Techniques

Marginal costing is one of the cost accounting techniques students learn early and use often, mainly because it makes decisions like pricing, product-mix selection, and break-even analysis so much simpler. But that simplicity comes at a cost of its own. The technique rests on a set of assumptions that don’t always hold up once you step outside the textbook and into a real factory, a real balance sheet, or a real pricing meeting. Knowing where marginal costing struggles is just as important as knowing where it excels, especially if you want to apply it correctly in exams and in practice.

Separating fixed and variable costs is trickier than it looks 🔗

The entire marginal costing framework depends on one foundational step: splitting every cost into a purely fixed portion and a purely variable portion. In practice, very few costs are that obliging. Electricity bills, maintenance charges, supervisory salaries with overtime components, and telephone expenses are all semi-variable, carrying a fixed base charge plus a variable element that moves with activity. The Institute of Cost Accountants of India’s classification standard itself acknowledges semi-variable costs as a distinct category precisely because they refuse to sit neatly on either side of the fixed-variable line.

Why the high-low method falls short 🔗

One popular way to separate semi-variable costs is the high-low method, which uses only the highest and lowest activity levels from historical data to estimate the variable rate. It’s quick, but it also throws away every data point in between. If either the highest or lowest month happens to be an unusual one, say a festival-season spike in a retail business or a month lost to a machine breakdown, the entire cost split gets skewed. As accounting educators point out, the method’s convenience comes directly at the expense of accuracy, since it ignores the bulk of the available data in favour of two extreme observations.

The analytical method’s hidden subjectivity 🔗

The alternative, the analytical or judgement-based method, asks an experienced accountant to estimate the fixed and variable components based on knowledge of the business. This avoids the extreme-point problem but introduces a different one: two analysts looking at the same cost data can reasonably arrive at different splits. When the classification itself is debatable, every contribution figure, break-even point, and pricing decision built on top of it inherits that uncertainty. This is not a minor technical quibble. If the fixed-variable split is wrong, the marginal cost per unit is wrong, and every decision that flows from it, from special-order pricing to make-or-buy calls, is built on shaky ground.

Marginal cost pricing can leave fixed costs unrecovered 🔗

Marginal costing is genuinely useful for one-off decisions, such as accepting a special export order at a price below the normal selling price, as long as the order covers its variable cost and adds something to contribution. The danger appears when this logic gets applied too broadly. If a sales team starts quoting several regular orders near variable cost because each one looks “profitable” in isolation, the business can end up with healthy total contribution that still falls short of covering fixed overheads for the period.

A simplified illustration makes this clearer. Suppose a company has monthly fixed costs of ₹5,00,000 and a variable cost of ₹200 per unit.

Each order individually clears its variable cost and adds a positive contribution, so none of them looks like a bad decision on its own. But total contribution here is only ₹1,35,000 against fixed costs of ₹5,00,000. Marginal costing tells you whether an individual order is worth accepting; it does not automatically stop you from accepting too many low-margin orders and under-recovering fixed costs across the period as a whole. That check has to come from separate monitoring of aggregate contribution against total fixed overheads.

Marginal costing undervalues inventory for statutory reporting 🔗

Because marginal costing charges the entire fixed production overhead to the period in which it is incurred, closing stock is valued at variable cost alone, with no share of fixed manufacturing overhead attached to it. This produces a lower inventory value than absorption costing, where a portion of fixed overhead is carried forward in the value of unsold units.

This isn’t just a theoretical difference. Ind AS 2, the accounting standard on inventories, requires that the cost of conversion include a systematic allocation of fixed production overheads, based on the normal capacity of the production facility, before inventory can be reported in financial statements. Marginal costing simply does not meet this requirement, which is why companies preparing statutory accounts, tax computations, or reports for lenders cannot use it as the sole basis for inventory valuation. Guidance on this allocation also clarifies that fixed overhead absorbed per unit should be based on normal capacity rather than actual output, a nuance marginal costing sidesteps entirely by excluding fixed overhead from product cost altogether.

There’s a knock-on effect on reported profit too. When production exceeds sales in a period, absorption costing carries some fixed overhead forward in closing stock, which tends to show a higher profit than marginal costing for that period. When sales exceed production, the relationship reverses. Businesses that rely purely on marginal costing figures for internal reviews can therefore see profit numbers that diverge, sometimes significantly, from what the audited financials eventually show, as explained in comparisons of the two costing approaches. Most Indian manufacturers handle this by running marginal costing internally for decisions while maintaining absorption costing separately for statutory and cost-audit compliance.

The constant per-unit cost assumption doesn’t always hold 🔗

Marginal costing assumes that variable cost per unit stays fixed regardless of how many units are produced, and that the relationship between cost and output is a straight line. Real production rarely behaves this way. Bulk purchase discounts can push the variable material cost per unit down as order size grows. Push production past normal capacity, and overtime premiums, extra machine wear, or rush-order freight charges can push the variable cost per unit up. Labour also doesn’t stay uniformly efficient. Workers typically get faster with practice, which can lower variable labour cost per unit over a production run, and then plateau or reverse if fatigue or quality issues set in at very high volumes.

In other words, the cost-volume relationship marginal costing assumes is linear is, in most factories, more like a curve with several bends in it. The technique’s break-even and contribution calculations remain reasonably reliable within a “relevant range” of output near current activity levels, but stretch that range too far in either direction, and the constant-marginal-cost assumption starts to mislead rather than inform.

A few more constraints worth keeping in mind 🔗

Beyond the four issues above, marginal costing has a handful of narrower but still important limitations:

Weak fit for long-term decisions. Capital investment appraisal, plant expansion, and other long-horizon choices depend heavily on fixed costs and on the time value of money. Marginal costing, built for short-term operating decisions, is not designed to handle either well on its own.

Complications in multi-product businesses. When a company sells several products that share the same production facility or the same scarce resource, ranking products purely by contribution per unit of the limiting factor can get complicated once you factor in different fixed costs attributable to specific product lines, something plain marginal costing tends to gloss over.

Understates the full cost of a product. Because fixed manufacturing costs never enter the product cost, marginal costing can understate what it genuinely costs a business to bring a unit to market, which matters when setting a floor price for the long run rather than for a single incremental order.

None of this makes marginal costing a poor technique. It remains genuinely useful for short-term decisions precisely because it isolates the costs that actually change with a decision. Cost accounting literature consistently frames these limitations not as reasons to discard the method, but as reasons to pair it with absorption costing and other techniques rather than rely on it exclusively.

What do you think? If your college project or internship involved pricing a product purely on variable cost, would you now check the aggregate contribution against total fixed costs before finalising that price? And in a business with highly seasonal or fluctuating output, how would you decide when marginal costing’s “constant cost per unit” assumption is safe to use, and when it isn’t?

References

1. https://icmai.in/upload/CASB/2017/CAS1-Revised.pdf
2. https://www.accountingverse.com/managerial-accounting/cost-behavior/high-low-method.html
3. https://icai.org/resource/23698IndAS-2.pdf
4. https://www.taxmann.com/post/blog/allocation-of-fixed-production-overheads-to-determine-the-cost-of-inventory-under-ind-as-2/
5. https://www.geeksforgeeks.org/accountancy/difference-between-marginal-costing-and-absorption-costing/
6. https://www.economicsdiscussion.net/cost-accounting/advantages-and-disadvantages-of-marginal-costing/31773