Inventory Costing Methods

Why Inventory Costing Methods Matter

Every e-commerce business holds inventory purchased at different costs over time. Your supplier raised prices. Freight rates changed between shipments. Currency exchange rates shifted between purchase orders. The result is inventory in your warehouse with a range of different unit costs, all for the same SKU. When you sell a unit of that SKU, which cost do you assign to the sale? The answer to that question is your inventory costing method, and it directly determines your reported cost of goods sold, your gross margin, the value of your remaining inventory on the balance sheet, and your taxable income.

This is not an abstract accounting exercise. The inventory costing method you choose can shift your reported gross margin by 3 to 8 percentage points in periods of rising or falling costs. It affects how much tax you owe, how healthy your business appears to lenders and investors, and how you make pricing and purchasing decisions. Yet many e-commerce operators never deliberately choose a method. They either use whatever their accounting software defaults to or they do not apply any consistent method at all, assigning costs ad hoc. Both approaches create financial reporting problems that grow more severe as the business scales.

FIFO: First In, First Out

FIFO assumes that the oldest inventory is sold first. When you sell a unit, the cost assigned to that sale is the cost from the earliest purchase order that still has remaining inventory. As older cost layers are depleted, the next-oldest layer begins to apply.

Consider a simple example. You purchase 100 units at $5.00 each in January and another 100 units at $6.00 each in March. In April, you sell 120 units. Under FIFO, the first 100 units sold carry the January cost of $5.00, and the remaining 20 units carry the March cost of $6.00. Your total COGS for those 120 units is $620 (100 x $5.00 + 20 x $6.00), and your remaining 80 units of inventory are valued at $6.00 each, totaling $480.

Advantages of FIFO

• Matches physical flow. Most e-commerce businesses actually sell older inventory before newer inventory, making FIFO the most intuitive and representative method.
• Balance sheet accuracy. Because remaining inventory is valued at the most recent costs, FIFO typically reflects the current market value of your inventory more accurately.
• Accepted under both GAAP and IFRS. FIFO is universally accepted across accounting standards, making it the safest choice for businesses that may undergo audits or seek external financing.

Disadvantages of FIFO

• Higher taxable income in rising cost environments. When costs are increasing (which is common due to inflation, supplier price hikes, and rising freight rates), FIFO assigns the older, lower costs to sales, resulting in higher reported margins and higher tax liability.
• Can overstate profitability. Reporting higher margins because you are matching old costs against current revenue can create a misleading picture of how profitable your business is today.

LIFO: Last In, First Out

LIFO assumes that the most recently purchased inventory is sold first. Using the same example, under LIFO the first 100 units sold would carry the March cost of $6.00, and the remaining 20 units would carry the January cost of $5.00. Total COGS would be $700 (100 x $6.00 + 20 x $5.00), and remaining inventory would be valued at $5.00 per unit, totaling $400.

Advantages of LIFO

• Lower taxable income in inflationary periods. By matching the most recent (and typically highest) costs against revenue, LIFO reduces reported gross margin and therefore reduces tax liability. This can provide a meaningful cash flow advantage.
• Better matching of current costs to current revenue. LIFO more accurately reflects what it costs to replace the inventory you just sold, providing a more realistic view of current profitability.

Disadvantages of LIFO

• Not permitted under IFRS. If your business operates internationally or may be acquired by a company that reports under IFRS, LIFO creates a compatibility issue. It is only permitted under US GAAP.
• Understates inventory value on the balance sheet. Remaining inventory is valued at the oldest costs, which may be significantly below current replacement cost. This can make your balance sheet appear weaker to lenders or investors.
• Does not match physical flow. Most e-commerce businesses do not actually sell their newest inventory first, making LIFO a poor representation of physical operations.

Weighted Average Cost

The weighted average method calculates a single average cost per unit based on all available inventory. After each purchase, the average is recalculated. Using the same example, after both purchase orders you have 200 units with a total cost of $1,100 ($500 + $600), yielding a weighted average cost of $5.50 per unit. When you sell 120 units, COGS is $660 (120 x $5.50), and remaining inventory is valued at $440 (80 x $5.50).

Advantages of Weighted Average

• Simplicity. You maintain a single cost per SKU that updates with each purchase order. There are no cost layers to track, making it the easiest method to implement and maintain.
• Smooths cost volatility. Price fluctuations between purchase orders are averaged out rather than creating distinct cost layers. This produces more stable margin reporting over time.
• Accepted under both GAAP and IFRS. Like FIFO, weighted average is universally accepted.

Disadvantages of Weighted Average

• Less precise than FIFO or LIFO. By blending all costs into a single average, you lose visibility into the actual cost of specific inventory tranches. This can mask situations where a particular purchase order had unusually high or low costs.
• Can lag behind current costs. In periods of rapidly rising costs, the weighted average is pulled down by older, cheaper inventory, understating the true current replacement cost.

Which Method Should Your E-Commerce Business Use?

The right choice depends on your business circumstances, but for most small to mid-size e-commerce businesses, FIFO or weighted average are the practical options. Here is a framework for deciding:

• Choose FIFO if you want the most accurate balance sheet inventory valuation, you sell internationally (IFRS compatibility), or your business is preparing for a potential acquisition or audit where standard compliance is important.
• Choose weighted average if simplicity is a priority, you have high SKU counts that make layer tracking impractical, or your costs do not fluctuate dramatically between purchase orders.
• Choose LIFO if you operate exclusively in the US, your costs are consistently rising, and minimizing current tax liability is a priority. Be aware of the balance sheet and IFRS limitations.

Regardless of which method you choose, the critical requirement is consistency. Switching methods between periods creates comparability problems and may require disclosure in financial statements. Choose a method, implement it correctly, and stick with it unless there is a compelling reason to change.

Implementing Inventory Costing in Practice

Implementing any inventory costing method requires accurate COGS tracking as a foundation. You cannot apply FIFO cost layers if you do not have accurate landed costs for each purchase order. You cannot calculate a meaningful weighted average if your cost data is incomplete or inconsistent.

Most modern inventory management systems support FIFO and weighted average natively. When evaluating software, verify that it can handle the specific complexities of your business: multiple suppliers for the same SKU, partial shipments from a single purchase order, and cost adjustments after initial receipt (for example, when a duty assessment arrives weeks after goods are received). If your system cannot handle these scenarios, your inventory valuation will drift from reality over time.

Inventory costing is where accounting meets operational reality. The method you choose shapes your reported profitability, your tax bill, and the financial story your business tells. Choose deliberately and implement rigorously.

Next Steps

Understanding inventory costing is essential for accurate financial reporting and true margin calculation. Combine this knowledge with solid COGS tracking practices and a thorough understanding of marketplace fees to build a complete picture of your e-commerce profitability. Do not overlook the impact of returns and refunds on your inventory costing and overall margins. For the full framework, visit our e-commerce finance pillar page.