If you sell a physical product — retail, e-commerce, a restaurant tracking food cost, a small manufacturer — inventory and Cost of Goods Sold are two of the most misunderstood numbers on your books. Not because the concepts are complicated. They’re not. It’s that the software makes it easy to record inventory the same way you’d record a software subscription or an office supply run, and that one habit quietly distorts everything downstream.
Buying inventory is not an expense
Here’s the mistake, in its most common form: you place a $12,000 order with your supplier, it hits your business credit card, and you categorize it as “Cost of Goods Sold” or “Supplies” the day it’s purchased. Your P&L takes a $12,000 hit that month. It feels correct — you spent the money, so it should show up as an expense.
It shouldn’t. Not yet.
Inventory sitting in your stockroom, on a shelf, or in a warehouse is an asset. You converted cash into product, but you still have the product — nothing has been sold. Nothing has happened yet that the P&L should reflect. That inventory belongs on the balance sheet, not the income statement, and it stays there until a customer actually buys it.
When you record the purchase as an expense immediately, two things go wrong. First, the month you bought the inventory looks artificially unprofitable — a big purchase can turn a genuinely good month into one that looks like a loss, which is exactly the kind of number that spooks an owner into a bad decision or confuses a lender reading the file. Second, when the inventory actually sells — maybe over the next six months, maybe over the next two years — there’s no cost recorded against that revenue at all, because you already expensed it. That period looks unrealistically profitable, for the same reason the purchase month looked unrealistically bad. Same dollars, two wrong pictures.
The correct sequence: the purchase increases an inventory asset account. Only when an item sells does its cost move from the balance sheet to the P&L, landing in Cost of Goods Sold, matched against the revenue that sale generated. That match — cost against the revenue it produced, in the same period — is the entire point.
What COGS actually measures
This is where a lot of confusion starts, because the name sounds like it should mean “what I spent on inventory this period.” It doesn’t.
Cost of Goods Sold is the cost of what you sold during the period — not what you bought. Those two numbers are only the same in one specific case: when the inventory you purchased exactly equals the inventory you sold, with nothing left over and nothing carried in from before. In practice, that almost never happens.
Buy more than you sell — building up stock ahead of a busy season, for instance — and your purchases exceed your COGS for that period, because some of what you bought is still sitting in inventory, unsold. Sell more than you bought — drawing down a stockpile, or a supplier delay that didn’t stop sales — and COGS exceeds purchases, because you’re selling product that was already on the shelf from an earlier period.
The relationship, in plain terms: beginning inventory, plus what you purchased during the period, minus what’s left in ending inventory, equals what you sold. That’s Cost of Goods Sold. It’s a period-matching calculation, not a running total of supplier invoices, and treating it as the latter is exactly the mistake covered above — the P&L moves with your purchasing pattern instead of your sales.
Pick a costing method, then stop relitigating it
Once you know an item sold, you still need to know what it cost — and if you buy the same product at different prices over time (your supplier raises prices, you order a different-sized batch, you switch vendors), “what it cost” isn’t automatically obvious. That’s what a costing method decides.
The two approaches worth knowing as a small business owner:
FIFO (first in, first out) assumes the oldest inventory you’re holding is the first to sell. It’s the more intuitive mental model for most physical goods — it mirrors how you’d actually want to rotate stock — and in a period of rising costs it tends to keep your reported COGS closer to your older, lower purchase prices, which pushes reported profit slightly higher.
Weighted average blends the cost of everything you’re holding into a single average cost per unit, and uses that average for every sale regardless of which specific batch it came from. It smooths out price swings between orders and is often simpler to maintain, especially for a business where individual units genuinely are interchangeable and tracking “which batch” doesn’t mean much operationally.
There’s a third method, LIFO, that’s more relevant to larger businesses with specific tax strategies — not something most small business owners need to think about, and it’s not covered here.
What matters far more than which of these two you choose is that you choose one and apply it consistently. Switching methods between periods, or letting different products in the same business get costed differently without a reason, makes your gross margin impossible to trust from one month to the next — a decline could be a real cost problem or just a costing artifact, and you won’t be able to tell which without redoing the math by hand. Pick one, document it, and let your CPA know which one you’re using — it affects both your P&L and your tax return.
Three mistakes that quietly break the numbers
Never doing a physical count. Most accounting and inventory software tracks a running number — it adds units in when you record a purchase and subtracts them out when you record a sale, and shows you what should be on the shelf. What it can’t see is shrinkage, theft, damage, a miscount at receiving, or a return that never got logged correctly. Left unchecked, that running number drifts further from reality every month, and because it drifts quietly, nothing on the P&L flags it. A periodic physical count — even once or twice a year for a small operation — is what catches the gap and lets you true it up before it’s a year old and impossible to explain.
Not updating cost when your supplier’s price changes. If your cost per unit goes up and your books are still using last year’s cost to value inventory and calculate COGS, your Cost of Goods Sold is quietly wrong — usually understated, which makes your margin look better than it actually is. This is easy to miss because nothing breaks; the transaction still posts, the software doesn’t flag it, and the error just sits there compounding every time that item sells.
Mixing inventory purchases with regular business expenses. Inventory bought for resale and, say, packaging supplies, shop equipment, or office costs are fundamentally different things and need to be recorded differently — one is an asset until sold, the other is a straightforward expense. When they land in the same account, your inventory asset balance stops meaning anything, your COGS calculation has noise baked into it, and untangling which invoice was which becomes real reconstruction work later, not a quick fix.
Why this is worth getting right
None of this is exotic accounting. It’s foundational, and it’s also one of the areas we see go wrong most often in a first cleanup — inventory purchases dumped straight into an expense account, no consistent costing method, a running inventory number nobody’s checked against reality in over a year. It doesn’t take an unusual business to end up there. It just takes inventory accounting never getting set up correctly in the first place, and nothing forcing a second look until the numbers stop making sense to whoever’s reading them — an owner sizing up margin, a lender, or a CPA at tax time.
Inventory is one line on the balance sheet — see our guide to reading your balance sheet for how it fits with everything else the statement shows you. And if your business sells across multiple channels, getting COGS right is only half the picture: how you reconcile channel payouts determines whether the revenue side of that margin calculation is accurate too — cost and revenue both have to be right before gross margin means anything.
If you’re not sure whether your inventory accounting is set up the way it should be — or you suspect it’s been drifting for a while — that’s exactly the kind of thing our bookkeeping cleanup service is built to sort out.
Not sure if your inventory numbers are telling you the truth? Book a free consultation and we’ll look at how your costing is actually set up.