FIFO vs Weighted Average: Why Inventory Margin Can Change

Two businesses can buy and sell the same quantities yet report different cost of goods sold when they use different permitted inventory cost formulas. FIFO and weighted average allocate purchase costs differently. They do not change the cash already paid to suppliers or the physical number of items remaining.

The IFRS Foundation’s IAS 2 summary describes cost formulas for interchangeable inventories and measurement at the lower of cost and net realisable value. This guide explains a simplified example; the appropriate policy and tax treatment depend on the entity’s applicable reporting framework and professional advice.

Use one small purchase history

Imagine a hardware startup buying identical adapters in one currency:

Assume the company then sells 120 units at $20 each. Revenue is $2,400. Eighty units remain. Ignore freight, taxes, discounts, returns, and other costs for this example so the allocation mechanism stays visible.

Real inventory cost may include more than the supplier’s headline unit price. Define the included costs consistently before comparing formulas. Otherwise, a difference attributed to FIFO might actually come from inconsistent freight allocation.

Calculate FIFO

FIFO assigns the earliest costs to the units sold first. The 120 sold units take the first 100 units at $10 and another 20 units at $14.

Cost of goods sold is $1,000 + $280 = $1,280. The remaining 80 units are valued at $14 each, giving ending inventory of $1,120. The reconciliation is $1,280 + $1,120 = $2,400, equal to the total available cost.

Gross profit before other expenses is revenue of $2,400 minus $1,280, or $1,120. Gross margin is $1,120 / $2,400, approximately 46.67%. That is an accounting result under the example’s assumptions, not a forecast of cash profit.

Calculate periodic weighted average

For a periodic weighted-average calculation, divide the period’s available cost by its available units: $2,400 / 200 = $12 per unit.

The 120 units sold receive cost of $1,440. The remaining 80 units receive cost of $960. Again, $1,440 + $960 = $2,400. Gross profit is $960 and gross margin is 40%.

The same purchases and sales therefore produce a $160 difference in gross profit between these two calculations. No extra cash appeared under FIFO. The difference is where the existing $2,400 cost is allocated: expense now or inventory remaining.

Do not mix periodic and moving averages

A perpetual moving-average system recalculates average cost as receipts occur. If sales happen between purchases, its result can differ from an average computed once for the whole period.

Our example places both purchases before the sale, so that timing complication is absent. For a real comparison, preserve event dates and order. A month-end spreadsheet that ignores the sequence may not reproduce the accounting system’s moving-average ledger.

Label the calculation method precisely. “Average cost” without explaining periodic or moving treatment can lead finance and engineering teams to implement different algorithms while believing they agreed.

Separate cost flow from physical flow

Warehouse picking rules and accounting cost formulas are related operational concerns but not identical. A warehouse might physically ship the oldest items to reduce ageing while the accounting system uses an approved weighted-average policy.

For serialized or non-interchangeable equipment, specific identification may be relevant instead. Do not force the interchangeable-adapter example onto unique machines, customized assemblies, or items requiring individual tracking.

Returns also need a defined treatment. A returned adapter must reconnect to the appropriate cost records under the adopted policy; assigning today’s purchase price automatically can distort both inventory and margins.

Add the checks a real ledger needs

Reconcile opening units plus receipts minus shipments and adjustments to closing units. Separately reconcile opening cost plus cost additions minus cost of goods sold and other cost adjustments to closing inventory value.

Investigate negative quantities, backdated receipts, duplicate shipments, and missing costs. These data defects can overwhelm the difference between formulas. Changing the formula will not repair an incomplete stock ledger.

Review damaged, obsolete, or unsaleable items under the applicable valuation rules. A calculated historical cost is not proof that inventory can still be sold for that amount. Keep write-down decisions separate from the mechanical FIFO-versus-average comparison.

Keep quantities and currency rounding separate in the implementation. Test partial shipments from a cost layer and verify that the remaining layer value plus allocated expense equals its original cost. Small per-line rounding differences should have an explicit reconciliation policy rather than disappearing into an unexplained margin adjustment.

Conclusion

FIFO and weighted average allocate the same available costs differently. Reconcile units and values, state the timing method, and distinguish accounting margin from cash generation before using inventory results to judge the business.