Inventory Accounting Methods for Ecommerce

Why Your Inventory Method Matters

The inventory accounting method you choose determines which cost gets assigned to each unit you sell and which cost remains on your balance sheet as unsold inventory. When your purchase costs are constant, all three methods produce identical results. The differences emerge when your costs change between purchases, which they almost always do. Supplier price changes, shipping cost fluctuations, currency exchange rate movements, and tariff adjustments all cause your per-unit cost to vary from one purchase order to the next.

The method you select affects your cost of goods sold, your gross profit, your inventory value on the balance sheet, your tax liability, and your reported profit margins. It also affects how much work your bookkeeping requires, because some methods demand more detailed tracking than others. Once you choose a method, the IRS expects you to use it consistently. Changing methods requires filing Form 3115 (Application for Change in Accounting Method), so choose wisely at the start.

FIFO: First In, First Out

How FIFO Works

FIFO assumes that the first units you purchased are the first units you sell. When a sale occurs, the cost assigned to that sale comes from your oldest inventory layer. As older layers are used up, costs from newer purchases move to the front of the queue.

Example: You make three purchases of the same product over three months.

• January: 100 units at $10.00 each
• March: 100 units at $10.50 each
• May: 100 units at $11.00 each

By June, you have sold 150 units total. Under FIFO, the first 100 units sold are assigned the $10.00 January cost, and the next 50 units are assigned the $10.50 March cost. Your COGS for those 150 units is (100 x $10.00) + (50 x $10.50) = $1,525. Your remaining 150 units of inventory are valued at (50 x $10.50) + (100 x $11.00) = $1,625.

When FIFO Is Best

FIFO is the best choice for most ecommerce businesses for several reasons. It matches the physical flow of most inventory, where you sell older stock before newer stock. It is the default method in QuickBooks Online and most accounting software, reducing setup complexity. It produces a balance sheet inventory value that closely reflects current market prices because the remaining inventory consists of your most recent purchases. And it is allowed under both US GAAP and IFRS international standards, which matters if you ever seek investment or operate internationally.

In an environment where supplier costs are rising (which has been the trend for most consumer products over the past several years), FIFO results in lower COGS and higher reported profit because the cheaper, older units are matched against revenue first. This means higher income taxes compared to LIFO, but it also means your financial statements show a more accurate current inventory value and your reported gross margins reflect the profitability of recent pricing decisions.

FIFO Drawbacks

The main disadvantage of FIFO in a rising-cost environment is higher tax liability compared to LIFO. Because FIFO assigns lower costs (older, cheaper purchases) to sold units, it reports higher profit and therefore higher taxes. For a business with significant inventory and rising costs, this tax difference can be meaningful. The trade-off is more accurate financial reporting versus tax optimization, and most small ecommerce sellers prioritize accurate reporting and simpler administration over the tax benefit of LIFO.

LIFO: Last In, First Out

How LIFO Works

LIFO assumes that the most recently purchased units are the first ones sold. When a sale occurs, the cost assigned to that sale comes from your newest inventory layer.

Using the same three purchases from the FIFO example, 150 units sold under LIFO would be costed as: the first 100 units use the $11.00 May cost, and the next 50 units use the $10.50 March cost. COGS for those 150 units is (100 x $11.00) + (50 x $10.50) = $1,625. Your remaining 150 units of inventory are valued at (100 x $10.00) + (50 x $10.50) = $1,525.

Compare the two methods for the same 150 units sold: FIFO COGS is $1,525 with $1,625 in ending inventory. LIFO COGS is $1,625 with $1,525 in ending inventory. The $100 difference in COGS flows directly to reported profit and tax liability.

When LIFO Makes Sense

LIFO reduces taxable income when costs are rising because it assigns higher costs (newer, more expensive purchases) to sold units, resulting in higher COGS and lower reported profit. For a business with $500,000 in annual COGS experiencing 5% annual cost inflation, the LIFO tax benefit could be $1,000 to $3,000 per year depending on tax rates and the timing of purchases. For very large inventory businesses with millions in COGS, the benefit is proportionally larger.

LIFO Drawbacks

LIFO has significant practical drawbacks for ecommerce businesses. It is not allowed under IFRS (International Financial Reporting Standards), which eliminates it for internationally reporting businesses. It produces a balance sheet inventory value based on the oldest, cheapest costs, which can severely understate the current market value of your inventory. QuickBooks Online does not natively support LIFO, requiring manual journal entries or workarounds. And the IRS LIFO conformity rule requires that if you use LIFO for tax purposes, you must also use it for financial reporting, which means you cannot use LIFO for taxes and FIFO for bank loan applications or investor reports.

Most ecommerce businesses with under $5 million in annual revenue do not benefit enough from LIFO to justify its complexity. The tax savings are modest at smaller scales, and the administrative burden and software limitations outweigh the benefits. Consult a CPA before choosing LIFO, as the method change is difficult to reverse.

Weighted Average Cost

How Weighted Average Works

The weighted average method calculates a single average cost for all units in inventory and assigns that average cost to every unit sold. Each time you receive new inventory at a different cost, the average is recalculated.

Using the same three purchases: after the January purchase, your average cost is $10.00. After the March purchase, you have 200 units with a total cost of $2,050 (100 x $10.00 + 100 x $10.50), so the weighted average is $10.25. After the May purchase, you have 300 units with a total cost of $3,150, so the weighted average is $10.50. Every unit sold is assigned the current weighted average cost at the time of sale.

When Weighted Average Is Best

Weighted average works well for businesses with large quantities of interchangeable, low-cost items where tracking individual purchase layers is impractical. If you sell craft supplies, hardware, or consumables where individual units are identical and purchase costs vary slightly between orders, weighted average simplifies the math without sacrificing meaningful accuracy.

Weighted average is also appropriate for businesses with very frequent purchases at slightly varying costs. If you restock weekly from different suppliers at slightly different prices, maintaining FIFO cost layers for every purchase creates administrative overhead that weighted average eliminates. The resulting COGS and inventory values fall between FIFO and LIFO, providing a reasonable middle-ground representation of your costs.

Weighted Average Drawbacks

Weighted average obscures individual purchase cost tracking. You cannot see which specific purchase lot is being depleted because all units share the same blended cost. If you need to track profitability by supplier, by purchase order, or by season, FIFO provides better visibility. Weighted average also produces less intuitive results when costs change significantly between purchases because the blending smooths out cost spikes and drops that FIFO would make visible.

Choosing the Right Method for Your Business

Choose FIFO if: you sell consumer products with varying costs between purchases, you use QuickBooks Online or similar accounting software that defaults to FIFO, you want your balance sheet to reflect current inventory values, you operate internationally or may seek investment, or you prefer the simplest setup with the most widely supported method. FIFO is the right default for the majority of ecommerce sellers.

Choose weighted average if: you sell high-volume, low-cost interchangeable items, your purchase costs vary only slightly between orders, you restock frequently from multiple suppliers, or you want simpler calculations than FIFO without the limitations of LIFO. Weighted average is the second most common method for ecommerce businesses.

Choose LIFO if: you have a CPA advising that the tax savings justify the administrative complexity, your COGS exceeds $1 million annually and costs are consistently rising, you operate entirely within the US with no international reporting requirements, and your accounting software or system supports LIFO tracking. LIFO is uncommon for small to mid-size ecommerce businesses.

Physical Inventory Counts and Adjustments

Regardless of which method you use, your accounting records need to match your actual physical inventory. Shrinkage from damage, theft, counting errors, and administrative mistakes causes your book inventory to diverge from reality over time. The IRS expects you to perform at least one physical inventory count per year, typically at year end, and many businesses benefit from quarterly counts.

When a physical count reveals a discrepancy, you record an inventory adjustment. If your books show 500 units but the physical count finds 485, you write down 15 units. The cost of those 15 units (using your selected accounting method) moves from inventory to an expense account, typically labeled Inventory Shrinkage or Inventory Adjustment. This adjustment reduces your total inventory asset and increases your expenses, lowering your taxable profit for the period.

For Amazon FBA sellers, Amazon provides inventory adjustment reports that detail lost, damaged, and disposed units. These reports serve as documentation for inventory write-downs and should be reconciled against your accounting records monthly. Amazon does reimburse for some lost inventory, and those reimbursements should be recorded as income or as an offset to your shrinkage expense.