What it means
The cost of a manufactured product has three components: direct materials (the steel, fabric or flour), direct labour (the wages of the people who make it) and manufacturing overhead (everything else in the factory that cannot be traced to a specific unit). Accountants group these in two overlapping ways: prime cost is direct materials plus direct labour, while conversion cost is direct labour plus manufacturing overhead.
Direct labour appears in both, materials only in prime cost, and overhead only in conversion cost. The distinction matters because materials and conversion behave differently.
Material cost is largely set by suppliers and by the product's design, whereas conversion cost is set by the efficiency of the factory: labour hours and machine hours per unit, overhead control and capacity use. A manufacturer comparing performance over time or with competitors therefore looks at conversion cost per unit, because a rise in material prices is not a factory problem but a rise in conversion cost per unit is.
In process costing the distinction is operational. Products flow through stages such as mixing, forming and finishing, and materials are usually added at the start of a stage, so a unit that has entered is 100% complete for materials while conversion is applied progressively and the same unit may be only 40% complete for conversion.
Costs are assigned using equivalent units, so 1,000 units that are 40% converted count as 400 equivalent units for conversion cost and 1,000 for materials, which gives a more accurate cost per unit and a more accurate valuation of work in progress. As automation increases, direct labour shrinks as a share of conversion cost and overhead grows, so some manufacturers no longer distinguish them and treat conversion cost as a single pool assigned by machine hours or by activity.
Lean accounting goes further by tracking conversion cost by value stream rather than by product and measuring it against throughput. For inventory valuation, the standards (IAS 2 and ASC 330) require the cost of inventory to include materials, direct labour and a systematic allocation of fixed and variable production overhead based on normal capacity.
Unallocated overhead from abnormal idle capacity is expensed, and non-production costs such as selling, administration and storage of finished goods are excluded. The conversion cost per unit used for valuation must therefore be calculated on normal, not actual, volumes when volumes are abnormally low.
Managing conversion cost means managing the factory: labour productivity, machine utilisation, changeover time, scrap and rework, energy, maintenance and the overhead structure. Reductions come from process improvement, automation, better scheduling and capacity utilisation, and they show up as a lower conversion cost per unit at constant material cost.
In practice
Real-world examples
Example 1
A bakery tracks conversion cost per loaf (labour, ovens, energy) at $0.42, separately from flour and ingredients at $0.38, and targets the conversion figure in its efficiency programme.
Example 2
A brewery's process costing treats malt and hops as added at the start of fermentation and conversion as continuous, valuing the beer in tanks at month end on equivalent units.
Example 3
An electronics assembler with heavy automation reports conversion cost per board by machine hour, with direct labour at only 8% of it.
Think of it
Conversion costs are what it takes to turn materials into products-labor plus overhead.
Formula
Calculation
Conversion Costs = Direct labour + Manufacturing overhead
Prime Costs = Direct materials + Direct labour
Total Manufacturing Cost = Direct materials + Conversion costs
Conversion Cost per Unit = Conversion costs / Units produced (or Equivalent units for conversion)
Equivalent Units (conversion) = Completed units + (Units in progress x Percentage complete for conversion)
Worked example 1, cost structure. A furniture factory's month: direct materials $420,000; direct labour $180,000; manufacturing overhead (supervision $40,000, machine depreciation $55,000, energy $28,000, maintenance $22,000, factory rent $35,000, indirect materials and consumables $15,000, quality control $12,000) $207,000. Units produced: 6,000.
- Conversion costs = $180,000 + $207,000 = $387,000; per unit $64.50
- Prime costs = $420,000 + $180,000 = $600,000; per unit $100.00
- Total manufacturing cost = $420,000 + $387,000 = $807,000; per unit $134.50
- Conversion cost share = 48% of total cost
The following month, timber prices rise 15% (materials $483,000) and the factory produces 6,300 units with the same labour and overhead. Conversion cost per unit = $387,000 / 6,300 = $61.43, an improvement of 4.8%, while total cost per unit = ($483,000 + $387,000) / 6,300 = $138.10, a rise of 2.7%. The conversion measure shows the factory improving; the total cost measure shows the material price effect. Both are true; the conversion measure is the one the factory manager is judged on.
Worked example 2, process costing. A chemical plant's blending stage for the month: opening work in progress nil; 50,000 litres started; 42,000 litres completed and transferred; 8,000 litres in progress at month end, complete for materials (added at the start) and 60% complete for conversion. Costs: materials $250,000; conversion $187,200.
- Equivalent units, materials = 42,000 + 8,000 x 100% = 50,000; cost per equivalent unit = $250,000 / 50,000 = $5.00
- Equivalent units, conversion = 42,000 + 8,000 x 60% = 46,800; cost per equivalent unit = $187,200 / 46,800 = $4.00
- Cost of completed units = 42,000 x ($5.00 + $4.00) = $378,000
- Cost of closing work in progress = 8,000 x $5.00 + 4,800 x $4.00 = $40,000 + $19,200 = $59,200
- Check: $378,000 + $59,200 = $437,200 = $250,000 + $187,200. Agreed.
Had conversion been treated as 100% complete for the work in progress, the conversion cost per equivalent unit would have been $187,200 / 50,000 = $3.744 and closing work in progress would have been valued at 8,000 x ($5.00 + $3.744) = $69,952. That overstates work in progress by $10,752 and understates the cost of completed units by the same amount, which overstates the month's gross profit.
Worked example 3, inventory valuation at abnormal volume. The furniture factory's normal capacity is 6,000 units a month. In a month of weak demand it produces only 3,600 units, while fixed overhead of $150,000 (within the $207,000) is unchanged. Fixed overhead per unit at normal capacity = $150,000 / 6,000 = $25.00, whereas at actual volume it would be $150,000 / 3,600 = $41.67. Under the inventory standards the allocation is made at normal capacity: 3,600 x $25.00 = $90,000 of fixed overhead is included in inventory cost, and the unabsorbed $60,000 is expensed in the period as the cost of idle capacity. Had it been allocated at actual volume, inventory would have carried an extra $16.67 per unit, about $60,000 in total, and profit for the period would have been overstated by that amount.Case study
Seen in the real world
This is a fictional, illustrative scenario. A plastics moulder measured its performance on total cost per unit, which had risen about 19% over two years, and its owner concluded the factory was becoming inefficient and considered outsourcing to a lower-cost country. The finance manager separated the figures. Resin prices had risen 35% over the period and materials were 60% of total cost at the start, so materials alone added about 21 points to the cost per unit; conversion cost per unit had fallen 6% as the factory had reduced cycle times and scrap, taking away about 2.4 points. The factory was more efficient than it had ever been; the product was more expensive because its raw material was.
Outsourcing would have moved production to a supplier facing the same resin prices with a conversion cost only marginally lower and freight and lead-time costs far higher. The owner instead renegotiated resin supply with a second source, introduced a resin surcharge clause into customer contracts (which competitors had already done), and reinvested in the factory's automation. Conversion cost per unit fell a further 9% the following year. The finance manager's note said the company had nearly outsourced the only part of its cost it had been getting right.
Watch out
Common mistakes
- Judging factory efficiency on total cost per unit, which moves with material prices the factory does not control. Conversion cost per unit isolates the factory's performance.
- Treating partly finished units as fully complete for conversion in process costing, which overstates work in progress and understates cost of sales.
- Valuing inventory at actual conversion cost per unit when volumes are abnormally low, which carries the cost of idle capacity in inventory instead of expensing it.
Questions
People also ask
What is the difference between conversion costs and prime costs?
Prime costs are direct materials plus direct labour (traceable costs). Conversion costs are direct labour plus manufacturing overhead (transformation costs). Direct labour is in both.
Are conversion costs included in inventory?
Yes. Inventory is valued at materials plus direct labour plus a systematic allocation of production overhead at normal capacity, so full conversion cost is included; abnormal idle capacity cost is not.
How can conversion costs be reduced?
Through labour productivity, machine utilisation, shorter changeovers, less scrap and rework, energy and maintenance efficiency, and overhead control: the levers of manufacturing improvement.