When a business purchases items of inventory, they may pay different prices due to diversity in the types of inventory stock or the same stock items purchased at different times.
In the weighted average cost method, the cost of goods available for sale is divided by the number of units available for sale and is commonly used when inventory items are so melded or identical to each other that it is impossible to assign specific costs to single units.
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Calculating the weighted average cost
When using the weighted average method, you divide the cost of goods available for sale by the number of units available for sale, which yields the weighted-average cost per unit. In this calculation, the cost of goods available for sale is the sum of opening inventory and net purchases. This weighted average figure is then used to assign a cost to both ending inventory and the COGS.
Weighted Average Cost Formula
Formula:
Weighted average cost per unit = Cost of Goods Available for Sale / Number of Units Available for Sale
Weighted average cost example
Corporation A uses the weighted average method, and during the month of June, it records the following transactions:
| Transactions for June | Quantity Change | Actual Unit Cost | Actual Total Cost |
| Beginning inventory (1 June) | +150 | $220 | $33,000 |
| Sale | -125 | n/a | n/a |
| Purchase | +200 | $270 | $54,000 |
| Sale | -150 | n/a | n/a |
| Purchase | +100 | $290 | $29,000 |
| Ending inventory (30 June) | Units 175 |
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Actual total cost of all inventory is $116,000 ($33,000 beginning inventory + $83,000 purchased), and total units of inventory are 450 (150 beginning inventory + 300 purchased)
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The weighted average cost per unit, therefore, is $257.78 ($116,000 ÷ 450 units)
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Ending inventory valuation is $45,112 (175 units × $257.78 weighted average cost), and COGS valuation is $70,890 (275 units × $257.78 weighted average cost)
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The total of these two amounts equals the $116,002 total actual cost of all purchases and beginning inventory
The result of using the weighted average cost method is that the recorded amount of on-hand inventory will represent a value somewhere between the oldest and most recent stock units purchased.
Equally, the COGS will reflect a cost somewhere between that of the oldest and newest units that were sold during the period.
Periodic Vs Perpetual Inventory Systems
Understanding whether your business uses a periodic or perpetual inventory system is essential when applying the average inventory formula and the weighted average cost methods. The timing of updates in each system affects both the accuracy and volatility of the resulting weighted average costs.
Periodic Inventory System
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Inventory quantities and valuations are updated only at the end of an accounting period.
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The weighted average cost is calculated after all purchases and adjustments have been made.
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This means the average cost doesn’t update with each purchase, and inventory valuation remains static until the next cycle.
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Periodic systems are simpler, but can distort cost accuracy when purchase prices fluctuate significantly.
Perpetual inventory systems
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Inventory and weighted average costs are updated continuously, after every purchase.
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Each new purchase recalculates the moving average cost in real-time.
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This provides a more accurate match between current costs and your on-hand inventory.
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Perpetual systems are widely used in modern cloud-based platforms due to their precision.
Because the weighted average costs formula depends on when inventory data is updated, periodic systems produce a single averaged figure for the entire period, while perpetual systems produce a constantly updated moving average – an important nuance for accurate COGS and inventory valuations.
Benefits of The Weighted Average Cost Method
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Simple to administer – No need to track costs by batch; all items are valued at a single average cost.
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Easy Calculation – Unlike FIFO or LIFO, weighted average uses a straightforward, single calculation.
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Reduced paperwork – Requires only one calculation record, not detailed cost layers for each purchase.
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Consistent costing – The same cost valuation is applied to all units in ending inventory and COGS.
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Lower labour costs – Minimises the time required to track or assign costs to individual units.
Disadvantages of the Weighted Average Cost Method
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Reduced pricing accuracy when costs fluctuate – High-cost units may never be fully received if selling prices remain stable.
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Assumes all units are identical – Problematic when newer versions include upgrades or added features.
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Can mask cost changes- The smoothing effect of averaging may hide the margin's shrinkage.
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Retrospective valuation – Looks backwards over purchases and may not reflect current replacement cost.
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Risk of losses when products are discontinued – If lower-cost units aren’t sold later, initial losses may never balance out.
Struggling to Keep Costs Accurate as Prices Fluctuate?
Unleashed automatically updates weighted average costs in real-time, giving you total confidence in your margins.
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Frequently Asked Questions
Is weighted average cost GAAP compliant?
Yes. The weighted average costs method is accepted under GAAP for inventory valuation, provided businesses apply the method consistently across reporting periods.
What is the difference between FIFO and WAC?
FIFO (First-In, First-Out) values inventory by assuming the oldest stock is sold first. WAC (Weighted Average Cost) averages all inventory costs together, diving total cost of goods available by the total units available.
When to use WAC?
WAC is best used when:
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Inventory items are identical or so closely merged that you can’t assign individual costs
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Prices fluctuate, and you want to avoid dramatic changes in COGS.
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You use a perpetual inventory system and need real-time recalculation of average costs
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You want simpler administration, with no cost layers to track
