Weighted Average Cost Method: Formula & Examples

What Is the Weighted Average Cost Method?

The weighted average cost method values inventory by dividing the total cost of goods available for sale by the total number of units available. Every unit—sold or in stock—carries the same blended cost. It’s one of the three main inventory valuation methods, alongside FIFO and LIFO, and it’s accepted under both GAAP and IFRS.

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The WAC Formula

Weighted Average Cost per Unit = Total Cost of Goods Available for Sale ÷ Total Units Available for Sale

That single per-unit cost is then applied to both the units you sell (COGS) and the units left in ending inventory.

Step-by-Step Example (Periodic)

You have these purchases in a period:

• 100 units at $10 = $1,000
• 100 units at $14 = $1,400

Total = 200 units for $2,400.

Average cost = $2,400 ÷ 200 = $12 per unit.

If you sell 120 units: COGS = 120 × $12 = $1,440. Ending inventory = 80 × $12 = $960. Notice this sits neatly between what FIFO ($1,280) and LIFO ($1,600) would produce.

Perpetual vs. Periodic WAC

• Periodic WAC calculates one average at the end of the accounting period, as above.
• Perpetual WAC (also called the moving average method) recalculates the average cost after every new purchase. If you buy more stock at a new price, the average updates immediately—so COGS on the next sale reflects the latest blend.

Perpetual is more accurate for businesses with frequent purchases and is the standard in modern inventory software.

Pros and Cons of WAC

Pros:

• Simple to calculate and maintain—less recordkeeping than FIFO or LIFO.
• Smooths out price fluctuations, avoiding COGS spikes.
• Ideal for commingled, interchangeable goods (grain, fuel, fasteners).

Cons:

• Less precise than tracking actual costs, so margins on individual sales are approximate.
• Can lag real replacement cost in fast-moving markets.

WAC vs. FIFO vs. LIFO

Choose WAC when your items are indistinguishable and tracking exact costs isn’t practical. Choose FIFO or LIFO when cost flow matters for taxes, perishability, or reporting. WAC’s defining trait is stability—it won’t swing your COGS the way FIFO and LIFO can when prices move sharply.

How Software Calculates WAC

Maintaining a moving average by hand means recalculating after every receipt—tedious and error-prone at scale. A cloud inventory management system updates the weighted average automatically with each purchase and surfaces live COGS and stock value in reporting and analytics. GOIS does this for 2,400+ businesses across 20+ countries.

Keep your average cost accurate automatically. GOIS recalculates WAC on every receipt so your margins are always current. Request a demo

Key Takeaways

• WAC = total cost of goods available ÷ total units available.
• Periodically calculates once per period; perpetual (moving average) updates after each purchase.
• WAC smooths price swings and is simplest for interchangeable goods.
• It’s accepted under both GAAP and IFRS.

Frequently Asked Questions

Is weighted average cost method the same as average cost?
Yes—”weighted average cost” and “average cost method” refer to the same approach.

Is WAC allowed under GAAP and IFRS?
Yes, both accounting standards permit the weighted average cost method.

When should I use WAC instead of FIFO?
Use WAC when your inventory is interchangeable and tracking individual costs isn’t practical and when you want stable, predictable COGS.

Conclusion

The weighted average cost method trades pinpoint accuracy for simplicity and stability—one blended cost applied across the board. For businesses with interchangeable stock and frequent purchases, it’s often the most practical choice, especially when software maintains the moving average for you.