Picture a paint factory where vats of colour pass through three stages — mixing, grinding, and filling. At the end of March, some units are half-done in the grinding stage. That unfinished batch is your Opening Work-in-Progress (WIP). The question is: how do you calculate the cost of finished goods when some of those goods were half-baked from last month? The Weighted Average Method (WAM) answers this by treating ALL costs — old and new — as one common pool, divided over ALL equivalent work done.
Here is the core idea: under WAM, you merge the cost of opening WIP with the current period's costs, then divide by the total equivalent units (units completed + closing WIP equivalent units). You do NOT separately track last month's work like you would in FIFO. This averaging smooths out cost fluctuations between periods, which is why management often prefers it for stable, repetitive production.
The four-step WAM process is your exam blueprint. Step 1 — Physical flow: Account for all units (Opening WIP + Units introduced = Completed + Closing WIP). Step 2 — Equivalent Units (EUs): EUs = Units transferred out + (Closing WIP units × % completion). Critically, opening WIP units do NOT appear separately here. Step 3 — Total Cost: Add Opening WIP cost to Current period cost for each cost element (material, labour, overhead). Step 4 — Cost per EU: Divide total cost by total EUs. Then multiply to get cost of completed units and closing WIP.
One nuance ICAI loves to test: material is often 100% complete at the start of a process (added in one shot), while conversion costs (labour + overhead) accumulate gradually. So you calculate EUs and cost per EU separately for each element. Also, if the question mentions normal loss — units lost due to inherent process reasons — those lost units reduce your expected output and their cost is absorbed by the good output. Abnormal loss is costed separately at cost per EU. This is asked frequently as a 5-to-8-mark question in Paper 4.