Moving Average Cost
Moving Average Cost (MAC) is an inventory valuation method that recalculates the average unit cost after each purchase receipt by dividing total inventory value on hand by total units on hand. In perpetual systems, all units carry the same current average cost until the next receipt changes it. Formula: New MAC = (Previous MAC x Previous Quantity + New Purchase Cost) / Total Quantity.
On a shop floor or in a warehouse, MAC gives inventory controllers a single rolling unit cost for raw materials, WIP receipts, and stocked components when purchase prices fluctuate. Each time receiving posts a purchase receipt, the ERP or WMS blends the new receipt cost with existing on-hand value, so subsequent picks, kit issues, and production consumption are valued immediately at the updated average. This is especially useful for raw material tracking because suppliers, freight charges, and production returns can create wide price swings, yet planners still need one live cost for issuing materials and valuing remaining stock. Systems such as Dynamics 365 and Infor LN apply moving-average/MAUC perpetually, including purchase, production, transfer, and adjustment transactions. The operational consequence is clear: receiving clerks and cost accountants must post receipts promptly and accurately, because any delay or mistake changes the cost basis used for downstream transactions and month-end inventory valuation.
- Late or incorrect receipt posting: Entering a receipt after issues have already been valued leaves material moving at the old cost, then retroactively changes the basis and creates variances that distort WIP and COGS figures.
- Incomplete landed-cost capture: When freight and duties are excluded from the purchase cost, MAC is understated and inventory margin reporting becomes misleading.
- Transaction discipline failures: Unposted adjustments, transfers, or production receipts keep quantities correct while value drifts, breaking the synchronization MAC needs and leaving valuation wrong.
Is MAC the same as weighted average cost?
In many ERP and accounting contexts, MAC is treated as a perpetual weighted-average method that updates after each purchase rather than at period end.
Does MAC change after a sale?
Under perpetual moving average, the unit cost used for issues does not change because of the sale itself; it changes when new inventory is received and the average is recalculated.
Why do ERP systems use MAC for manufacturing inventory?
It provides a single current cost for materials that fluctuate in price, simplifying issue costing, inventory valuation, and financial posting across continuous receipts and consumptions.