Joint and By-Products Costing: Practical Illustrations and Examples

Theory tells you that joint costs must be split among joint products. It rarely tells you exactly how the numbers move when a factory floor produces four different outputs from one process. That gap is exactly what comprehensive illustrations are meant to close. Once you work through a few fully solved problems, apportioning joint costs and pricing by-products stops feeling abstract and starts feeling like basic arithmetic with a logical story behind it.

This post walks through worked examples for the three most commonly tested joint cost apportionment methods, and then shows how to calculate the cost and profit of a by-product once it goes through further processing. Keep a pen handy, because the numbers matter more than the definitions here.

Why illustrations matter more than definitions 🔗

Joint products are two or more products that emerge from a single process and a single set of inputs, up to a certain stage called the split-off point. Beyond that point, each product may need further processing before it is fit for sale. By-products are the minor, lower-value outputs of the same process. The accounting challenge is always the same: a single joint cost has to be divided among outputs that did not individually “cause” that cost.

Exam questions and real costing decisions rarely stop at explaining a method. They give you production quantities, selling prices, and further processing costs, and expect you to compute the exact rupee amount each product should absorb. That is the skill these illustrations build.

Apportioning joint costs: three practical approaches 🔗

There is no single “correct” method for apportioning joint costs. The choice depends on whether the products are sellable immediately at split-off, whether they need further processing, and whether reliable market values are available at that stage. The three approaches below cover almost every scenario you will encounter.

Physical units method 🔗

This is the most intuitive method. Joint costs are divided in proportion to the physical quantity, weight, or volume of each product. It works well when products can be measured in comparable units and is simple to apply because it needs no market data, though that same simplicity is its weakness: it ignores the fact that a litre of cream and a litre of skimmed milk are not equally valuable.

Consider a dairy that processes 10,000 litres of raw milk into three joint products at the split-off point:

Each product simply carries the same proportion of joint cost as its share of total output. The math is clean, but notice that cream, which sells for far more per litre than skimmed milk, ends up bearing the same per-litre cost as the cheapest product. That mismatch is exactly why the market value method exists.

Market value (sales value at split-off) method 🔗

Here, joint costs are apportioned according to each product’s relative sales value at the split-off point, on the reasoning that a higher selling price usually reflects a higher share of the resources consumed. Using the same dairy example, assume cream sells at ₹60/litre, toned milk at ₹30/litre, and skimmed milk at ₹15/litre.

Cream now absorbs a much larger share of the joint cost than it did under the physical units method, which better reflects its higher market value. This approach is widely used precisely because it links cost allocation to economic value rather than pure volume, and it forms the basis for several worked textbook illustrations on joint cost apportionment.

Net realisable value or reverse cost method 🔗

Products often cannot be sold at the split-off point at all. They need further processing before they have any market value. In such cases, you cannot use the split-off sales value directly, so you work backward from the final selling price, deducting the further processing cost to arrive at the net realisable value (NRV), and apportion the joint cost in that ratio.

Suppose a chemical unit incurs a joint cost of ₹2,00,000 to produce two products, P and Q, neither of which is sellable until processed further.

The full cost of each product is then the joint cost apportioned plus its own further processing cost: Product P costs ₹1,50,000 (₹1,20,000 + ₹30,000) and Product Q costs ₹1,30,000 (₹80,000 + ₹50,000). This method is recommended by the Institute of Chartered Accountants of India’s cost accounting study material precisely for situations where products cannot be valued fairly at the split-off stage.

Costing by-products after further processing 🔗

By-products get simpler treatment than joint products because their value is usually small relative to the main product. When the amount realised from a by-product is minor, it is common practice to simply deduct its net realisable value from the cost of the main product rather than apportion any joint cost to it at all, an approach reflected in ICMAI’s guidance on by-product accounting.

Where the by-product needs further processing before sale, the calculation proceeds as follows:

1. Sales value of the by-product – quantity produced multiplied by final selling price.
2. Less: further processing cost – any cost incurred after split-off to make the by-product saleable.
3. Less: selling and distribution expenses, if separately identifiable.
4. The result is the net realisable value of the by-product, which is credited against the cost of the main product.

Take a saw mill that produces furniture-grade timber as its main product and sawdust as a by-product. The joint process cost is ₹5,00,000, producing 8,000 units of timber. The sawdust output is 2,000 kg, but it needs bagging and drying before it can be sold, at a cost of ₹8,000, and it sells at ₹12/kg. Selling expenses are estimated at ₹1,000.

This ₹15,000 is credited to reduce the cost of the main product: ₹5,00,000 − ₹15,000 = ₹4,85,000, which spread over 8,000 units gives a cost of ₹60.63 per unit of timber, instead of ₹62.50 per unit if the by-product’s value had been ignored entirely. This is essentially the reversal cost method of costing by-products, and it shows how even a “minor” output can meaningfully lower the cost of your main product once you account for it properly.

Putting it together: a combined illustration 🔗

Real problems in exams and in practice often combine both ideas: apportion joint costs among the main products, and separately credit the value of a by-product. Consider an edible oil unit that crushes oilseed at a joint cost of ₹3,60,000, yielding two joint products, Oil and Cake, and a by-product, Husk.

• Oil: 3,000 kg, sells at ₹150/kg after refining, refining cost ₹90,000.
• Cake: 6,000 kg, sells at ₹20/kg at split-off with no further processing needed.
• Husk (by-product): 1,000 kg, sells at ₹8/kg after drying, drying cost ₹1,000.

Step one: value the by-product and credit it against the joint cost. Husk’s NRV is (1,000 × ₹8) − ₹1,000 = ₹7,000. Net joint cost to apportion between Oil and Cake becomes ₹3,60,000 − ₹7,000 = ₹3,53,000.

Step two: apportion the net joint cost using NRV, since Oil needs further processing while Cake does not. Oil’s NRV = (3,000 × ₹150) − ₹90,000 = ₹3,60,000. Cake’s NRV = 6,000 × ₹20 = ₹1,20,000. Total NRV = ₹4,80,000, giving a ratio of 3:1. Oil is apportioned ₹2,64,750 and Cake ₹88,250 of the net joint cost.

Step three: add each product’s own further costs. Oil’s full cost is ₹2,64,750 + ₹90,000 = ₹3,54,750, giving a cost of ₹118.25/kg. Cake’s full cost is simply ₹88,250, or ₹14.71/kg, since it needed no further processing. This kind of layered illustration is exactly what shows up in comprehensive numerical problems, and working through it step by step is the fastest way to internalise the logic rather than memorise formulas.

What do you think? 🔗

What do you think? If you were running the saw mill in the example above, would you rather sell sawdust as-is at a lower price or spend more on processing it into a higher-value product? And when a joint product’s market value is uncertain at split-off, do you think the net realisable value method gives a fairer picture than simply guessing at a split-off price?

References

1. https://www.financestrategists.com/accounting/cost-accounting/joint-cost-allocation-methods/
2. https://www.accountingnotes.net/cost-accounting/joint-costs/apportionment-of-joint-costs-with-illustrations-cost-accounting/5817
3. https://live.icai.org/bos/vcc/pdf/08032022_CA__Vipin_Bohra_Joint_by_product_1646721363.pdf
4. https://icmai.in/upload/Students/Syllabus-2012/RTP/Dec14/Paper19.pdf
5. https://www.accountingformanagement.org/market-value-or-reversal-cost-method-of-costing-by-products/