Full Costing vs Variable Costing: Profit, Inventory, and Reconciliation

Learning objectives

By the end of this chapter, you should be able to:

• Explain how full costing and variable costing differ, and why reported profit can change when production and sales volumes are not equal.
• Prepare profit statements using full costing and variable costing.
• Reconcile profit differences between the two methods by analysing changes in inventory.
• Analyse how rising or falling inventory levels affect profit under each method.

Overview & key concepts

Two costing approaches are commonly used to measure product cost and report profit:

• Full costing (absorption costing): each unit produced carriesvariable production cost plus an allocated share of fixed production overhead. These costs are held in inventory until the units are sold.
• Variable costing (marginal costing): each unit produced carriesonly variable production cost.Fixed production overhead is treated as a period expenseand charged in full to the period.

When inventory levels change, absorption costing can defer or release fixed production overhead through inventory valuation, creating a profit difference versus marginal costing. Marginal costing profit will still move with inventory through variable production costs, but the gap between the two methods is driven only by the fixed production overhead timing under absorption costing.

For external financial reporting, inventory is valued using production cost, which includes a systematic share of production overheads, rather than variable cost alone.

Absorption (full) costing

Under absorption costing, product cost includes:

• direct materials
• direct labour
• variable production overhead
• fixed production overhead (allocated to units produced)

Because fixed production overhead is included in unit cost, it affects:

• Statement of financial position: inventory includes a portion of fixed production overhead.
• Statement of profit or loss: fixed production overhead is expensed throughcost of saleswhen goods are sold.

Marginal (variable) costing

Under marginal costing, product cost includes only variable production costs. Fixed production overhead is not attached to units and is expensed in the period.

This method supports analysis by separating:

• variable costs (linked to volume)
• fixed costs (linked to time/capacity)

Contribution

Contribution is the first margin you earn once all costs that rise with activity have been covered. It shows how much each period’s sales generate towards paying fixed capacity costs and then delivering profit.

A useful “bridge” view:

• sales bring value in
• variable costs consume value in proportion to volume
• what remains (contribution) must fund fixed costs; anything beyond that is profit

Mini-check: if you sell 1 unit for £50 and variable costs per unit are £24, the contribution per unit is £26. Selling an extra unit adds £26 towards fixed costs and profit (assuming fixed costs do not change).

Fixed production overhead

Fixed production overhead is the cost of running production capacity that does not vary with short-term output (e.g., factory rent, production supervisors’ salaries). The key difference is where it is charged:

• Absorption costing: allocated to units and included in inventory/cost of sales.
• Marginal costing: expensed in full in the period.

Inventory valuation

Inventory valuation differs because absorption costing includes a fixed overhead element in unit cost and marginal costing does not.

• If inventory increases, absorption costing holds some fixed production overhead in closing inventory (deferring expense).
• If inventory decreases, absorption costing releases fixed production overhead from opening inventory into cost of sales (accelerating expense).

Profit reconciliation

The profit difference is caused by only one item: fixed production overhead attached to units under absorption costing.

• When inventory rises, some fixed production overhead is stored in closing inventory instead of being charged this period.
• When inventory falls, fixed production overhead stored in opening inventory is released into this period’s cost of sales.

So the adjustment is simply:

Adjustment = (Closing units − Opening units) × fixed production overhead absorbed per unit

Absorption profit − Marginal profit
= (Closing units − Opening units) × fixed production overhead absorbed per unit

Direction check (sign):

• inventory up → absorption profit higher
• inventory down → absorption profit lower

Core theory and frameworks

Recognition of production costs

• Absorption costingtreats fixed production overhead as part of the cost of making inventory. It becomes an expense when inventory is sold (via cost of sales).
• Marginal costingtreats fixed production overhead as a period cost and charges it in full to the period.

Measuring unit costs under absorption costing

Fixed production overhead must be allocated to units on a systematic basis. For exam purposes, the denominator is often stated explicitly (frequently actual production as a simplifying assumption). Conceptually, the anchor is allocation based on normal capacity to avoid distorting unit costs when output is unusually low or high.

Practical rule for answers:

• Use the basis stated in the question.
• If no basis is stated,prefer normal capacity if it is provided.
• If normal capacity is not provided, use the most defensible capacity/activity measure implied by the data and apply it consistently.

Fixed production overhead absorbed per unit = Fixed production overhead for the period ÷ Activity level used for absorption

Presentation in profit statements

• Absorption costing profit statement: shows gross profit (sales less cost of sales), then deductsselling and administration.
• Marginal costing profit statement: shows contribution, then deducts fixed costs.

Decision-making implications

Marginal costing is particularly useful when assessing short-term decisions because it highlights the incremental effect of volume. Absorption costing is used where inventory valuation must include a share of production overhead and where a gross profit figure is required.

Impact on profit when inventory changes

• If production equals sales, profits are equal under both methods (ignoring any over/under absorption adjustments and assuming no change in unit variable production cost and a consistent inventory valuation basis).
• If inventory increases (production > sales), absorption profit is higher because some fixed production overhead is held in closing inventory.
• If inventory decreases (sales > production), absorption profit is lower because fixed production overhead held in opening inventory is released into cost of sales.

Reconciliation process

1. Compute fixed production overhead absorbed per unit.
2. Compute change in inventory units (closing − opening).
3. Multiply units change by the absorbed fixed production overhead per unit.
4. Adjust profit to move between methods (watch the sign).

Quick method (both directions):

• Absorption profit = Marginal profit + adjustment (if inventory rises)
• Marginal profit = Absorption profit − adjustment (if inventory rises)

Reverse the sign if inventory falls.

Over/under absorption

If the absorption rate is set using normal capacity (or budgeted activity), fixed overhead absorbed into production may not match fixed overhead actually incurred. The difference is:

• over-absorption: absorbed more than incurred
• under-absorption: absorbed less than incurred

Example: if fixed overhead incurred is £120,000 but fixed overhead absorbed is £110,000, the period has under-absorbed £10,000.

How it is treated in an answer depends on the policy stated in the scenario (and often on materiality). In many management accounting questions, the adjustment is taken to profit (often via cost of sales), unless instructed otherwise. Some questions require a split between inventory and cost of sales. Follow the instruction given and apply it consistently.

Worked example

Narrative scenario

A manufacturing company produces a single product with the following information for one period:

• Selling price:£50 per unit
• Variable production cost:£20 per unit
• Fixed production overhead:£120,000 per period
• Variable selling cost:£4 per unit sold
• Fixed selling and administration:£30,000 per period
• Opening inventory:2,000 units
• Production:18,000 units
• Sales:17,000 units

Required

1. Prepare a marginal costing profit statement.
2. Prepare an absorption costing profit statement.
3. Reconcile the profit difference between the two methods.
4. Explain the impact of inventory changes on profit.

Solution

1) Marginal costing profit statement (showing inventory flow)

Sales revenue
17,000 units × £50 = £850,000

Variable production cost of sales (inventory reconciliation)

• Opening inventory: 2,000 units × £20 =£40,000
• Variable production cost of production: 18,000 units × £20 =£360,000
• Goods available (variable):£400,000
• Closing inventory: 3,000 units × £20 =£60,000
• Variable production cost of sales:£340,000

Variable selling cost
17,000 units × £4 = £68,000

Contribution
£850,000 − £340,000 − £68,000 = £442,000

Fixed costs

• Fixed production overhead:£120,000
• Fixed selling and administration:£30,000
• Total fixed costs:£150,000

Profit (marginal costing)
£442,000 − £150,000 = £292,000

Closing inventory valuation (marginal costing)
3,000 units × £20 = £60,000

2) Absorption costing profit statement

Fixed production overhead absorbed per unit (as required by this example)
£120,000 ÷ 18,000 units = £6.6667 per unit

Absorption production cost per unit
£20.0000 + £6.6667 = £26.6667

Inventory units
Closing inventory units = 2,000 + 18,000 − 17,000 = 3,000 units

Cost of sales (absorption costing)

• Opening inventory: 2,000 × £26.6667 =£53,333.33
• Production: 18,000 × £26.6667 =£480,000.00
• Goods available for sale:£533,333.33
• Closing inventory: 3,000 × £26.6667 =£80,000.00
• Cost of sales:£453,333.33

Gross profit
£850,000 − £453,333.33 = £396,666.67

Selling and administration

• Variable selling cost:£68,000
• Fixed selling and administration:£30,000

Profit (absorption costing)
£396,666.67 − £68,000 − £30,000 = £298,666.67

Closing inventory valuation (absorption costing)
3,000 units × £26.6667 = £80,000

3) Reconciliation of profit difference

Inventory rose by 1,000 units (3,000 − 2,000). Under absorption costing, each unit carried £6.6667 of fixed production overhead.

Adjustment = (Closing units − Opening units) × fixed production overhead absorbed per unit
= 1,000 × £6.6667
= £6,666.67

Absorption profit − Marginal profit = £6,666.67
Check: £298,666.67 − £292,000.00 = £6,666.67

Direction check (sign):

• inventory up → add to marginal to get absorption
• inventory down → subtract from marginal to get absorption

4) Impact of inventory changes on profit

Because inventory increased, absorption costing carried £6,666.67 of fixed production overhead in closing inventory instead of charging it through cost of sales this period. This is why absorption profit is higher by £6,666.67.

Exam lens (quick rules):

• production = sales → profits equal (subject to assumptions noted earlier)
• inventory up → absorption profit higher
• inventory down → absorption profit lower
• adjustment = (closing units − opening units) × fixed production overhead absorbed per unit (watch the sign)

Common pitfalls and misunderstandings

• Not showing the marginal costing inventory flow: it prevents errors when costs change and earns method marks.
• Wrong absorption denominator: use the basis stated; do not invent a “rule” from the numbers.
• Double-counting fixed production overhead under absorption costing: it is already in unit cost and therefore in cost of sales.
• Reconciling with the wrong sign: inventory up increases absorption profit; inventory down decreases it.
• Inconsistent labels/layout: keep contribution format for marginal and gross profit format for absorption.
• Rounding drift: keep the absorption rate to at least 4 decimal places in workings and round final answers appropriately.

Summary and further reading

Absorption costing and marginal costing differ in the treatment of fixed production overhead:

• Absorption costing allocates fixed production overhead to units, so inventory and cost of sales include that fixed element.
• Marginal costing charges fixed production overhead in full to the period, while inventory includes only variable production costs.

When production and sales volumes differ, the profit gap between the methods is explained by fixed production overhead being carried in or released from inventory under absorption costing.

FAQ

Why does absorption costing often show higher profit than marginal costing?

When inventory increases, absorption costing stores some fixed production overhead in closing inventory rather than charging it through cost of sales this period. Marginal costing charges the full fixed production overhead immediately, so profit is lower by comparison.

How does inventory level affect profit under each costing method?

Both methods carry production costs in inventory, so inventory movements affect profit. The difference between methods arises because absorption costing includes a fixed overhead element in inventory. Inventory up increases absorption profit relative to marginal; inventory down decreases it.

What is the role of contribution in decision-making?

Contribution is sales less all variable costs. It shows how much sales generate towards fixed costs and profit and is particularly useful for short-term decisions where fixed costs are unchanged.

Why is profit reconciliation important?

It shows that the profit difference is a timing issue caused by fixed production overhead moving into or out of inventory under absorption costing. This helps interpret performance when production and sales volumes diverge.

What are common assessment traps in this area?

Common traps include: incorrect fixed overhead absorption rates, missing inventory flows, double-counting fixed overhead under absorption costing, and applying the wrong sign in reconciliation when inventory rises or falls.

Summary (Recap)

This chapter compared absorption costing and marginal costing and explained why they can report different profits when inventory levels change. Absorption costing allocates fixed production overhead to units and therefore carries part of that fixed cost in inventory when inventory increases. Marginal costing charges fixed production overhead to the period in full. The worked example demonstrated preparation of both profit statements and a reconciliation using inventory unit movement multiplied by the fixed production overhead absorbed per unit.

Glossary

Absorption costing (full costing)
A method where product cost includes variable production costs plus an allocated share of fixed production overhead, so inventory carries fixed overhead until goods are sold.

Marginal costing (variable costing)
A method where product cost includes only variable production costs; fixed production overhead is charged as a period expense.

Contribution
Sales less all variable costs. It is the amount available to cover fixed costs and then generate profit.

Fixed production overhead
Production-related costs that do not change with short-term output. Under absorption costing they are allocated to units; under marginal costing they are charged to the period.

Variable production cost
Production costs that vary with output (e.g., materials and output-based labour). Included in inventory under both methods.

Inventory valuation
The cost assigned to closing inventory. Under absorption costing it includes fixed overhead absorbed; under marginal costing it does not.

Profit reconciliation
An explanation of the difference between absorption and marginal costing profits, based on the fixed production overhead carried in or released from inventory under absorption costing.

Over-absorption / under-absorption
The difference between fixed production overhead absorbed into production and fixed production overhead incurred, arising when the absorption rate is set using normal capacity or budgeted activity. Treatment depends on the policy stated in the scenario (often influenced by materiality).