4 Inventory Costing Methods Explained: How to Pick the Right One for Your Business

You bought the same product three times this year, and you paid a different price each time. Now you've sold one unit. How much did it cost you?

That's not a trick question. It's the exact problem inventory costing methods exist to solve. The answer you give changes your cost of goods sold, your gross margin, your tax liability, and how your inventory shows up on the balance sheet. Pick one method and your profits look higher. Pick another and your tax bill goes down. Same product, same sale, different financial picture depending on which cost you assign to it.

For midsized ecommerce and distribution brands managing hundreds or thousands of SKUs across multiple channels, this isn't an academic exercise. The inventory costing method you choose will affect every financial decision you make, from pricing strategy to purchasing timing to how confidently you can answer the question "are we actually profitable on this product?"

There are four methods accepted under U.S. GAAP. Here's how each one works, what it does to the numbers, and which type of business it fits best.

1. FIFO: First In, First Out

FIFO assumes that the oldest inventory you purchased is the first inventory you sell. The cost assigned to each sale comes from your earliest purchase, and whatever remains in stock is valued at your most recent purchase price.

Here's a simple example. You buy 100 units of a product at $10 each in January, then another 100 units at $12 each in March. In April, you sell 100 units. Under FIFO, the cost of that sale is $10 per unit (the January batch), because those units came in first. Your remaining 100 units are valued at $12 each on the balance sheet.

FIFO tends to produce higher reported profits when costs are rising, because it matches older, cheaper costs against current revenue. That also means ending inventory value is closer to current market prices, which gives the balance sheet a more accurate picture of what the stock is actually worth.

For most ecommerce brands, FIFO is the default choice, and for good reason. It matches the way physical inventory actually moves. If you're selling consumer products, you're almost certainly shipping the oldest stock first, especially if you're managing expiration dates, lot numbers, or seasonal SKUs. FIFO reflects that reality. It's also the only method accepted under IFRS (International Financial Reporting Standards), which matters if you sell internationally or ever plan to.

The trade-off is taxes. When costs are rising, FIFO reports higher profits, which means a higher tax bill. For a business growing rapidly and reinvesting every dollar, that tax hit can sting.

2. LIFO: Last In, First Out

LIFO flips the assumption. It assigns the cost of your most recent purchase to each sale, leaving the oldest costs sitting in ending inventory.

Using the same example: you bought 100 units at $10 in January and 100 at $12 in March. You sell 100 units in April. Under LIFO, the cost of that sale is $12 per unit (the March batch), because those units came in last. Your remaining 100 units are valued at $10 each.

LIFO produces lower reported profits when costs are rising, because it matches newer, higher costs against revenue. That means a lower tax bill, which is the primary reason businesses choose it. The cash savings from reduced taxes can be significant for companies with large, fast-turning inventory.

There are real downsides, though. Ending inventory value will be understated relative to current market prices, because it carries old costs that may not reflect what the products are actually worth today. That makes the balance sheet less useful for assessing the true value of stock on hand. And LIFO is only allowed under U.S. GAAP. If you sell internationally or report under IFRS, it's not an option.

LIFO can make sense for commodity-type businesses where costs fluctuate significantly and tax management is a priority. For most ecommerce brands selling consumer products, it's less common because it doesn't match the physical flow of goods and it distorts inventory visibility on the balance sheet.

3. Weighted Average Cost

The weighted average method takes the total cost of all inventory available for sale during a period and divides it by the total number of units. Every unit, whether it was purchased in January or March, gets the same average cost assigned to it.

Back to our example: 100 units at $10 plus 100 units at $12 equals $2,200 total cost for 200 units. The weighted average cost per unit is $11. When you sell 100 units, your cost of goods sold is $1,100, and your remaining 100 units are valued at $1,100.

The weighted average smooths out price fluctuations. You don't get the highs of FIFO or the lows of LIFO. COGS and ending inventory land somewhere in the middle, which produces more stable period-to-period financial results.

This method works well for businesses selling large quantities of similar or interchangeable products where individual unit tracking isn't practical. If you're a distribution company moving pallets of commodity goods and your purchase prices shift from order to order, weighted average gives you a clean, defensible cost without the complexity of tracking which specific batch each sale came from.

The downside is precision. When product costs are volatile or you need to understand margin at the individual SKU or batch level, weighted average blurs the picture. You know what things cost on average, but you don't know what any specific unit actually cost you.

4. Specific Identification

Specific identification does exactly what it sounds like: it tracks the actual cost of each individual unit and assigns that exact cost when the unit is sold. There's no assumption about flow. You know precisely what you paid for the item that just shipped.

This method is most practical for businesses selling high-value, low-volume, or one-of-a-kind items. If you're selling custom furniture, vintage watches, or specialty equipment where each piece has a different acquisition cost and is tracked individually (often by serial number), specific identification gives you the most accurate margin data possible.

For high-volume ecommerce brands moving thousands of identical SKUs, specific identification is typically impractical. Tracking the individual cost of every unit through receiving, storage, and fulfillment creates overhead that outweighs the precision benefit. But if your catalog includes a mix of commodity products and high-value specialty items, you might use specific identification for the specialty line and FIFO or weighted average for everything else.

How to Choose the Right Method for Your Business

The best inventory costing method depends on three factors: what you sell, how your costs behave, and what financial outcome matters most to your business right now.

If your products are perishable, seasonal, or physically rotated on a first-in-first-out basis (which covers most ecommerce and retail brands), FIFO will give you the most accurate match between your cost flow assumption and your actual operations. It's the most widely used method for a reason.

If your costs are rising steadily and tax management is a top priority, LIFO can reduce your tax liability, but at the cost of a less accurate balance sheet. Discuss this with your accountant before committing, because switching methods later requires IRS approval.

If you move high volumes of interchangeable goods and want stable, predictable cost reporting, weighted average will keep things simple without the tracking complexity of FIFO or LIFO.

If you sell high-value items that are individually tracked, specific identification will give you exact margins. It's the most precise method but only practical for low-volume, high-value inventory.

One thing worth knowing: once you choose a method, you're expected to stick with it. GAAP requires consistency in inventory valuation. You can change methods, but you'll need to justify the switch and potentially restate prior periods. Getting it right the first time saves your finance team a headache down the road.

Why an ERP System Will Make This Decision Easier

In a spreadsheet or basic accounting tool, managing inventory costing is a manual, error-prone process. Your team has to track purchase costs, assign them to sales based on the method you've chosen, and calculate COGS and ending inventory values at the end of every period. As transaction volume grows, this becomes one of the most time-consuming parts of the financial close.

An ERP system like Acumatica's Inventory Management module will handle this automatically. You'll configure your costing method at the item level (meaning different product lines can use different methods if needed), and the system will apply the correct cost to every transaction as it happens. COGS will calculate in real time. Ending inventory values will update continuously. Your inventory turnover metrics will reflect actual costs instead of spreadsheet approximations.

That automation becomes especially important for ecommerce brands selling across multiple channels. When the same SKU sells on BigCommerce, Amazon, and through a wholesale account, you'll need the costing method applied consistently across every transaction regardless of where the sale originated. ERP will make that automatic. Spreadsheets make it a project.

If you're evaluating whether your current systems can handle the financial complexity your business is growing into, our guide to the best ERP for eCommerce compares the platforms built for this exact challenge. And if you want to see where you stand today, take our ERP Readiness Quiz below to find out.