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For inventory accounting, average cost per unit is usually a quantity-weighted average: divide the cost of the inventory available for sale by the number of units available. This guide covers periodic and perpetual moving-average inventory costing. It is not the same as taking a simple arithmetic average of several prices, and it does not cover other meanings of “average cost,” such as investment cost basis or weighted-average cost of capital.
What is the average-cost formula for inventory?
For a periodic weighted-average calculation:
Weighted-average cost per unit = cost of goods available for sale ÷ units available for sale
For a basic inventory pool, cost of goods available generally includes beginning inventory cost plus the costs of purchases included in that pool. Units available are beginning units plus the purchased units. Use consistent units of measure and include only costs that belong to the accounting entity’s defined pool.
Quantity matters because each unit contributes to the average. The equivalent weighted formula is ([Q1 × P1] + [Q2 × P2] + …) ÷ (Q1 + Q2 + …), where Q represents quantity and P represents price. For example, a purchase of 100 units affects the average more than a purchase of 10 units at a different price. This expression is also documented for a particular software inventory model; system-specific settlement behavior should not be assumed to apply to every accounting setup. Microsoft’s weighted-average method documentation explains the quantity-weighted expression.
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How do you calculate average inventory cost step by step?
- Define the pool. Identify the comparable inventory items, period, and costs that belong in the calculation.
- Add the units available. Combine beginning inventory units with units purchased into the pool.
- Add the cost available. Combine beginning inventory cost with the included purchase costs.
- Divide cost by units. The result is the weighted-average cost per unit.
- Apply that rate. Multiply units sold by the rate to calculate cost of goods sold; multiply units remaining by the rate to value ending inventory.
Keep full precision in intermediate calculations, then apply a consistent rounding convention to displayed or posted amounts. Rounding the unit rate before multiplying can cause a small difference between the original available cost and the total assigned to sold and remaining units.
Worked periodic example
In an OpenStax worked example, goods available for sale cost $16,155 and totaled 585 units. The calculation is:
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$16,155 ÷ 585 = $27.6154 per unit, or $27.62 when rounded to cents.
With 285 units remaining, the example reports ending inventory of $7,872 and cost of goods sold of $8,283. These are example results, not industry statistics; the source notes that final values can vary slightly because of rounding. See OpenStax’s periodic inventory example.
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Periodic average versus perpetual moving average
| Approach | When the average is recalculated | How sales or issues are costed | Allocation effect |
|---|---|---|---|
| Periodic weighted average | Once for the period, using the period’s available cost and units. | Apply the period’s average to units sold and units remaining. | Produces one average for that period’s pool. |
| Perpetual moving average | As inventory receipts occur, using the inventory balance and receipt at that point. | Use the resulting current average for subsequent issues or sales. | Transaction sequence can change the amounts assigned to cost of goods sold and ending inventory. |
IAS 2 allows the average to be calculated periodically or as each additional shipment is received, depending on the entity’s circumstances. The Australian Accounting Standards Board’s published IAS 2 text states this in paragraph 26.
OpenStax’s examples show the effect of timing. In its perpetual example, the average is $27 per unit after the first replenishment and $31.24 after a later purchase; 285 ending units are valued at $8,902. Those figures come from that worked example, not a general benchmark. The periodic and perpetual examples allocate available cost differently between ending inventory and cost of goods sold. See OpenStax’s comparison of the two systems.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which inventory can use average cost?
The costing formula is subject to the applicable accounting standard and the nature of the inventory. Under the cited IAS 2 text, items that are not ordinarily interchangeable, and goods or services produced and segregated for specific projects, are assigned using specific identification. Other inventory costs are assigned using FIFO or weighted average.
The same cost formula is used for inventories with a similar nature and use. Different formulas may be justified for inventories with a different nature or use; geographic location alone is not a reason to use different formulas. For a reporting decision, check the current standard and jurisdiction that apply to the entity. The AASB’s IAS 2 text sets out these requirements.
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Common calculation mistakes
- Taking an unweighted mean when quantities differ. Adding purchase prices and dividing by the number of prices ignores how many units each price represents.
- Mixing unlike units or inventory pools. Make sure the included units and costs refer to the same defined pool.
- Using the wrong timing method. A periodic calculation and a moving average use transactions at different points, so they may yield different allocations.
- Rounding too early. Retain precision through the calculation and use a consistent rounding convention for the final amounts.
- Assuming every “average cost” question means inventory. Investment cost basis, process costing, and weighted-average cost of capital are separate topics with different calculations.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
