What it means
When a business buys the same product at different prices over time, it has to decide which cost to match against each sale. The average cost method sidesteps the question by blending all purchases into one average cost per unit and using that single figure for both the cost of sales and the stock still on hand.
It matters because the choice changes reported profit and the value of inventory on the balance sheet, sometimes materially, even though the physical goods are identical. In a period of rising prices the average method reports a profit between what first in first out and last in first out would show, which makes it the least volatile of the three.
There are two versions in practice. A weighted average is recalculated at the end of each period, while a moving average is recalculated after every purchase, and most inventory software uses the moving version because it can cost each sale as it happens.
The method suits businesses where units are genuinely interchangeable: fuel, grain, screws, chemicals, raw plastic. It is a poor fit where items carry serial numbers or individual values, such as vehicles or jewellery, because the specific cost of each item can and should be tracked.
Accounting rules permit the average method and require consistency once it is chosen, so switching methods to improve the look of a result is not acceptable. Note that last in first out is allowed in some jurisdictions and prohibited in others, whereas the average method is broadly accepted, which is one reason international groups favour it.
One nuance catches people out at year end. Average cost does not override the rule that stock must be carried at the lower of cost and net realisable value, so if the market price of the goods falls below the computed average, the stock has to be written down anyway.
In practice
Real-world examples.
Example
A diesel distributor buys fuel weekly at fluctuating prices and cannot tell which litres in the tank came from which delivery. It values the tank at a moving average cost, so every sale is costed at the blended rate and the margin reported each week is smooth rather than jumping with delivery prices.
Example
A coffee roaster buying green beans across a season uses weighted average costing and finds its reported gross margin is steadier than a competitor using first in first out. When bean prices spike, its cost of sales rises gradually as the average drifts upwards, which gives the sales team time to reprice.
Example
An electronics assembler holds 40,000 identical capacitors bought in six batches at prices from $0.18 to $0.26. Rather than tracking batches, it recalculates one average cost after each purchase and applies that figure to every unit issued to production, cutting the bookkeeping to a single calculation per delivery.
Formula
Calculation
Weighted average cost per unit = total cost of goods available for sale / total units available for sale. Cost of sales = units sold x weighted average cost per unit.
A hardware wholesaler starts the month with 2,000 boxes of fixings costing $6.00 each, a total of $12,000. It then buys 3,000 boxes at $7.00 each for $21,000 and 5,000 boxes at $8.00 each for $40,000. Total units available are 2,000 + 3,000 + 5,000 = 10,000 and the total cost is $12,000 + $21,000 + $40,000 = $73,000.
The weighted average cost per unit is $73,000 / 10,000 = $7.30. If 6,500 boxes are sold during the month, cost of sales is 6,500 x $7.30 = $47,450, and the closing stock of 3,500 boxes is valued at 3,500 x $7.30 = $25,550. Those two add back to $73,000, which is the check that the calculation is right.Case study
Seen in the real world.
Ridgeway Polymers is an illustrative, fictional plastics compounder used to show the effect of the method. It buys resin in bulk and, over one volatile year, paid between $1,100 and $1,700 a tonne for the same grade of material.
Under first in first out its reported gross margin swung between 19% and 31% from quarter to quarter, which made its sales commission scheme almost random and unsettled its bank. Moving to a weighted average cost pooled the purchases, and exactly the same trading produced a margin that stayed between 23% and 26%.
Nothing about the business itself changed: the cash paid for resin and received from customers was identical. In this illustrative case the method simply stopped the accounts telling a misleading story about which quarter had been a good one, which is the main argument for average costing in a volatile input market.
Watch out
Common mistakes.
- Believing the average method changes how much money the business really makes. Over the life of the stock the total cost charged is the same; the method only changes which period the cost lands in.
- Recalculating the average after a sale rather than after a purchase. Sales do not change the average cost per unit, only purchases at a different price do.
- Using one average across different products or grades. The pool must contain genuinely interchangeable units, or the resulting cost per unit means nothing.
Questions
People also ask.
Is the average cost method allowed under the main accounting frameworks?
Yes, both the international standards and the American rules permit it for interchangeable inventory, provided it is applied consistently from year to year.
Which method gives the highest profit when prices are rising?
First in first out does, because it charges the oldest and cheapest costs to sales, while average cost falls between first in first out and last in first out.
Can we switch methods?
Only with good reason and proper disclosure, because a change in inventory costing is a change in accounting policy and usually requires prior figures to be restated for comparison.
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