Why average cost method matters to a first-time founder
The average cost method spreads purchase-price differences across inventory units that can be substituted for one another. It can simplify costing when individual units are difficult to trace and affects the cost assigned to sold goods, the value of remaining inventory and profit before other expenses. Founders must apply the selected method consistently under the applicable accounting rules.
Examples of average cost method
A retailer buys 100 units at £8 and 200 units at £11. Before sales, weighted-average cost is £10 per unit: total cost of £3,000 divided by 300 units. If 120 units are sold under a periodic average, £1,200 is assigned to cost of sales.
Common misunderstanding
It is not calculated by adding purchase prices and dividing by the number of purchases. A large purchase affects the average more than a small one because quantity matters. Calculating one average for the period and recalculating the average after each purchase can produce different results. Products also cannot be grouped together unless the units are sufficiently similar.
When to use it
Use it for interchangeable inventory where the accounting rules permit it and tracing each unit’s actual cost is impractical. Define which items are grouped, whether the average is calculated for the period or after each purchase, and whether freight or other costs are included. Keep quantity and cost records accurate, then apply the method consistently between periods.