Choosing Your COGS Method: FIFO vs. LIFO vs. WAC

Last verified: July 2026

Most e-commerce founders pour their energy into revenue. Conversion rates, ad spend, channel mix — the numbers that feel like growth. But the number that actually determines how much money you keep, and how much tax you owe, is your Cost of Goods Sold. And specifically, how you calculate it. The method you pick isn't just an accounting preference. It shapes your gross margin, your inventory valuation, and your tax bill — sometimes by thousands of pounds or dollars a year. If you're selling across Shopify, Amazon, and wholesale at the same time, the stakes are even higher, because getting it wrong at scale compounds fast.

We've written a detailed primer on what to include in your COGS calculation — landed costs, inbound freight, duties, and all. This article picks up from there: once you know what goes into COGS, which method should you actually use?

Why your COGS calculation method matters for profitability and taxes

Your COGS method determines which unit costs get expensed when a sale happens — and which stay on your balance sheet as inventory. That single decision ripples through your P&L, your tax return, and your working capital in ways that aren't always obvious until year-end.

Here's the clearest way to think about it. You bought 100 units in January at £8 each, then another 100 in April at £11 each (supplier prices went up — as they do). You sell 100 units in May. Did you sell the £8 units or the £11 units? Physically, you might not know. But your COGS method decides that for you — and the difference is £300 in reported cost, flowing directly into your gross profit and your taxable income.

Different inventory valuation methods can significantly affect both tax liabilities and reported profitability. This isn't a technicality to delegate blindly to your accountant — it's a strategic decision that should align with your business model, your markets, and your growth stage.

And it connects directly to how clean your underlying inventory data is. If you're managing stock across multiple channels without a reliable system, your COGS figures are only as good as your stock counts. We built Ceendesis IMS partly because we ran into this ourselves — and inaccurate inventory data makes every COGS method unreliable, regardless of which one you choose.

For a fuller picture of how COGS fits into your broader financial setup, our complete guide to e-commerce accounting covers it all.

The FIFO method (First-In, First-Out): explained with e-commerce examples

FIFO assumes the oldest inventory you bought is the first inventory you sell — and for most e-commerce brands, this isn't just an accounting assumption, it's literally what happens. You ship stock from the oldest purchase order first; the new stock sits at the back. FIFO formalises that reality.

It's also the most common method in use. Around two-thirds of American companies use FIFO because it matches the natural flow of goods and is accepted under both GAAP and IFRS — which matters enormously if you're selling into the EU or UK and need to comply with international accounting standards.

A worked FIFO example

You run a Shopify brand selling skincare. In Q1 you receive:

• February: 200 units @ £6.00 each
• March: 200 units @ £7.50 each (supplier price increase)

In April, you sell 250 units.

Under FIFO, you sell the February batch first: 200 units × £6.00 = £1,200. Then the remaining 50 from March: 50 × £7.50 = £375. Total COGS = £1,575. The 150 March units still in stock are carried at £7.50 each — so your closing inventory is £1,125.

The benefit is straightforward. Your balance sheet carries inventory at more recent (and often higher) costs, which reflects current replacement value more accurately. And because you're expensing the cheaper older stock first, your gross margin looks healthier in a rising-cost environment — though your tax bill will be higher as a result.

FIFO is the default choice for fashion brands, beauty brands, food and supplement sellers, and anyone whose products have expiry dates or seasonal relevance. It tends to match how stock is physically picked in the warehouse, so your accounting isn't fighting your fulfilment process — and at scale, that alignment is worth a lot.

If you're managing inventory across Shopify and Amazon simultaneously, FIFO also gives you a consistent, auditable cost layer — which becomes important when you're reconciling multi-channel sales against a single inventory pool.

The LIFO method (Last-In, First-Out): use cases and limitations for online sellers

LIFO assumes the most recently purchased inventory is sold first. In practice, this is rarely how stock actually moves — but that's not why businesses use it. They use it for the tax advantage it creates in an inflationary environment.

When supplier costs are rising, your most recent purchases are your most expensive. Expensing those first means higher COGS, which means lower reported profit, which means a lower tax bill in that period. It's an accepted US GAAP method, not a loophole. LIFO lets companies reduce taxable income by deducting higher recent inventory costs against current revenues — especially useful when costs are climbing fast.

LIFO is prohibited under IFRS. The International Accounting Standards Board banned it because it doesn't faithfully represent inventory flows. If you're a UK brand, an EU-registered business, or selling into markets that require IFRS compliance, LIFO simply isn't an option — full stop. Most of our readers hit this wall immediately, which is why we're flagging it early rather than burying it at the end.

And even for US-based sellers who could use it, LIFO comes with a significant administrative burden. You have to track LIFO "layers" by purchase date and price, complexity that grows every year. You also can't switch back to FIFO without IRS approval and potential recapture of the tax deferral you benefited from. That's not a decision to make casually.

For most e-commerce businesses selling across multiple channels and geographies, LIFO creates more problems than it solves. The short-term tax deferral rarely justifies the ongoing compliance cost — especially when your inventory balance sheet ends up carrying artificially low valuations that don't reflect what your stock is actually worth.

For more on how your accounting method choices interact with multi-channel bookkeeping, our guide to consolidated multi-channel bookkeeping is worth a read.

The Weighted Average Cost (WAC) method: a balanced approach for stable costs

WAC calculates a single average cost per unit across all your inventory — purchases old and new — and uses that figure to value every unit sold. When prices fluctuate between purchase orders, WAC smooths those fluctuations out rather than tracking them batch by batch.

The formula is simple: divide total inventory cost by total units available.

A worked WAC example

You sell a mid-weight cotton tee. Your stock position at the start of May:

• 100 units purchased at £5.00 = £500
• 150 units purchased at £6.20 = £930

Total cost: £1,430. Total units: 250. Weighted average cost per unit: £1,430 ÷ 250 = £5.72.

You sell 180 units in May. COGS = 180 × £5.72 = £1,029.60. Remaining inventory: 70 units × £5.72 = £400.40.

Notice that every unit — regardless of which purchase order it came from — carries the same cost. That's the point. For brands selling homogenous products (the same SKU across multiple batches where quality and specification don't change), this simplifies bookkeeping considerably. You're not tracking which batch each unit came from; you're applying one consistent figure across the board.

WAC works particularly well for:

• Brands with large SKU counts and frequent restocking at varying prices
• Businesses where individual unit tracking isn't practical (bulk commodities, accessories)
• Operations where FIFO batch tracking would require system complexity that isn't justified by the margin precision gained

The downside? In a sharply rising-cost environment, WAC lags behind current prices. Your COGS figure will be lower than LIFO and potentially lower than FIFO, because you're averaging in all those older, cheaper units. That means higher reported profit — and a higher tax bill — even if your actual margins are being squeezed by new purchase prices. Run this past your accountant at year-end, particularly if supplier costs have moved significantly during the year.

For brands using multi-channel inventory management to track stock across Amazon, Shopify, and wholesale, WAC works well when your inventory management platform can automatically recalculate the average every time a new purchase comes in. That's the perpetual WAC approach — more on that in the comparison table below.

How to choose and implement the right COGS method for your brand

Four things drive the decision: your accounting standard (GAAP or IFRS), your product type, your operational capacity to track batches, and your tax planning priorities. If you're subject to IFRS, LIFO is already off the table, which simplifies things. From there, it mostly comes down to whether your products are homogenous enough for WAC to make sense, or distinct enough by batch that FIFO gives you cleaner numbers.

Practical implementation steps

First, talk to your accountant before switching methods. Changing your COGS method mid-year — or between fiscal years without proper disclosure — can create issues with HMRC, the IRS, or your statutory accounts. In the US, switching to or from LIFO requires IRS Form 970. In the UK, HMRC expects consistency unless there's a valid reason to change.

Second, make sure your inventory system can support the method you choose. FIFO requires tracking costs by purchase order and date. WAC requires recalculating your average cost every time a new purchase comes in. If you're running this on spreadsheets, you'll hit the limit fast — especially when you're managing returns, damaged goods, and multi-warehouse stock at the same time. Returns are a particularly sharp edge case: do you add returned units back at their original cost or the current WAC? Your system needs a clear rule and needs to apply it consistently.

The operational reality is that most growing brands need proper software to make any of these methods accurate at scale. Honestly, we've watched teams waste quarters trying to maintain FIFO layers in Excel — it falls apart the moment you have a partial shipment, a supplier substitution, or a bulk return. Operations managers running 500+ SKUs across Amazon and Shopify can't sustain that manually. That's why integrating your inventory system with your accounting stack matters: cost data should flow automatically rather than being re-entered manually at month-end.

What about subscription boxes and dropshipping?

Subscription box brands — where a curated mix of items ships monthly — typically do best with FIFO or WAC depending on whether the box contents are fixed SKUs or variable. If you're curating different products each month, WAC per product category keeps things manageable. For more on the broader operational stack, our essential tech stack for subscription box brands covers this in more detail.

Dropshipping businesses have a simpler problem: your COGS is the supplier invoice cost per order, because you're not holding inventory. FIFO and WAC don't really apply in the traditional sense — your cost per unit is whatever the supplier charged for that specific order, applied at the point of sale. The complexity there is more about capturing all the landed costs accurately (shipping, payment processing fees, duties) than choosing a valuation method.

Tax planning in 2026

Your COGS figure directly reduces your gross profit and, by extension, your taxable income — so any error here flows straight through to your tax bill. Review this carefully at year-end with your accountant, particularly if your supplier costs have been volatile, since the COGS method you're using will determine how much of that cost lands in the current tax year versus future periods. For a solid grounding in how COGS feeds into your broader financial picture, our e-commerce chart of accounts guide is a good companion read.

And while COGS is a cost-side lever, don't forget the revenue-side complexity of multi-channel selling. If your Amazon and Shopify payouts don't reconcile cleanly with your P&L, your COGS figures won't tell the full story. We covered exactly this in our piece on why Amazon and Shopify payouts don't match your books.

Frequently asked questions

What is the best COGS method for ecommerce?

FIFO is the best COGS method for most e-commerce businesses because it matches the natural physical flow of goods, is accepted under both GAAP and IFRS, and produces an inventory valuation that reflects current replacement costs. It's especially well-suited for brands selling perishable, trend-sensitive, or expiry-dated products. Weighted Average Cost is a strong alternative for brands with large, homogenous SKU counts where batch-level tracking isn't practical.

How does FIFO vs LIFO affect ecommerce taxes?

In a rising-cost environment, FIFO results in lower COGS (older, cheaper stock is expensed first), which means higher reported profit and a higher tax bill. LIFO does the opposite — expensing newer, more expensive stock first produces higher COGS and lower taxable income, deferring tax. But LIFO is prohibited under IFRS, making it unavailable to UK and EU sellers or any brand required to report under international accounting standards.

How do you calculate weighted average cost for inventory?

Divide the total cost of all inventory available for sale by the total number of units available. If you have 100 units at £5.00 and 150 units at £6.20, your total cost is £1,430 across 250 units — giving a WAC of £5.72 per unit. Under the perpetual WAC method, you recalculate this average every time a new purchase is received, so the figure stays current throughout the year.

The bottom line

FIFO, LIFO, and WAC are tools. Each one fits a different set of circumstances — accounting standard, product type, operational capacity, tax position. Pick deliberately, apply consistently, and make sure your inventory data is clean enough that whichever method you use actually produces reliable numbers. If you're curious how Ceendesis IMS can help you maintain that data layer across channels, see how it works — or if you're managing both inventory and compliance obligations across EU markets, take a look at what we do for textile compliance and EPR packaging compliance too.