Inventory Valuation Methods: FIFO, Weighted Average, And Their Real Tax Impact

Inventory valuation sounds like a purely mechanical bookkeeping choice, and for a business with stable input costs, it largely is. Once costs are rising meaningfully year over year, the specific method chosen can shift a real amount of taxable income from one year to another.

The Two Methods Most Businesses Actually Use

Under FIFO, the earliest inventory purchased is treated as the first sold, meaning cost of goods sold reflects older, typically cheaper purchase costs while ending inventory reflects the most recent, typically more expensive costs. Under weighted average cost, all inventory on hand is blended into a single average cost per unit, smoothing out the effect of any single period's purchase price changes rather than assigning a specific purchase order to a specific sale.

Why It Matters More When Costs Are Rising

When input costs are rising, FIFO's older, cheaper costs flowing through cost of goods sold produce a lower expense figure and therefore higher reported taxable income, compared to weighted average, which blends in some of the higher recent costs immediately. The reverse is true when costs are falling. Neither method is inherently more "correct," but the choice has a genuine, quantifiable effect on the timing of taxable income in an environment where purchase costs are not stable.

A Worked Example

Consider a business that purchased 100 units at $10 each early in the year and another 100 units at $14 each later in the year, then sold 120 units. Under FIFO, cost of goods sold reflects the first 100 units at $10 and 20 units at $14, totalling $1,280, leaving 80 units at $14 ($1,120) in ending inventory. Under weighted average, all 200 units blend to an average cost of $12 per unit, so cost of goods sold for 120 units is $1,440, leaving 80 units at $12 ($960) in ending inventory. The $160 difference in cost of goods sold between the two methods flows directly into a corresponding difference in taxable income for the year.

Changing Methods Isn’t Simple

Once a business has adopted an inventory valuation method and used it consistently, changing to a different method generally requires CRA's concurrence, since a change can shift taxable income between years in a way the rules are designed to prevent being used opportunistically. A business considering a change should approach it as a deliberate request with proper justification, not an informal decision made at year-end.

Choosing The Right Method For Your Business

For most small and mid-sized businesses without highly specialized inventory tracking needs, weighted average cost is simpler to administer and produces smoother period-to-period results, while FIFO can better reflect actual physical inventory flow for a business where older stock genuinely does sell before newer stock, such as perishable goods. The right choice depends more on what the method needs to accomplish for the specific business than on any universal preference between the two.