A periodic inventory system tells you what your cost of goods sold was after the quarter is over. A perpetual system tells you what it is right now. For a single channel seller with one warehouse and predictable turns, periodic is defensible and cheap. For a seller shipping from three fulfillment centers, a third party warehouse and a garage, periodic is a quarterly guess dressed up as a number, and every purchasing decision made between counts is made blind. That is the whole comparison. Everything below is about when the cheaper method stops being the rational one.

What the two methods do

Periodic inventory runs on one formula applied at period end. Beginning inventory plus purchases minus ending inventory equals cost of goods sold. You count what is left, and COGS is whatever the arithmetic says is missing. No entry is made when a unit sells. The general ledger carries a purchases account all period, and a single adjusting entry at close moves the balance where it belongs.

Perpetual inventory books the movement as it happens. Every sale reduces the inventory asset and recognizes COGS in the same transaction. The inventory balance in the ledger is supposed to equal the inventory on the shelf at any moment. Counts still happen, but they verify the system rather than produce the number.

The practical distinction is not accuracy at year end. Both methods land in roughly the same place once you have counted. The distinction is whether you have a usable number in week six.

The marketplace complication

Three things make marketplace selling harder on periodic than traditional retail.

First, your inventory is in places you cannot walk to. Units in an Amazon fulfillment center move between facilities without generating anything you would recognize as a transaction. Counting them means trusting a report from a party that is not your accountant.

Second, in transit inventory is real and often large. Goods on a container are your asset the moment title passes, which depends on the incoterms in your purchase order, not on when the pallet arrives. A periodic count taken while forty thousand dollars of stock sits on water will produce a COGS figure that is wrong in a specific and expensive direction.

Third, marketplace settlements net fees against revenue, which means the revenue side of your margin calculation is already obscured. Pairing an obscured revenue figure with a quarterly COGS estimate produces a gross margin that nobody should make decisions on.

A worked example

A seller starts the quarter with $180,000 of inventory at cost. During the quarter they buy $240,000. At quarter end they count $155,000 across all locations.

Periodic COGS is $180,000 plus $240,000 minus $155,000, which is $265,000. That figure is correct in aggregate and useless in detail. It does not tell you which SKUs consumed it. It does not separate shrinkage, damage and theft from actual sales, because all three land in the same residual. If $12,000 of that was a pallet damaged in receiving, periodic reports it as cost of goods sold, and your gross margin absorbs a loss that belongs somewhere else.

Perpetual would have booked $253,000 of COGS against specific sales and flagged a $12,000 inventory adjustment as its own line. Same total, completely different management information. One of those versions lets you find the problem in the receiving bay.

What the tax rules require

Less than most sellers assume, and the threshold is higher than most published guidance says.

Under section 471(c) of the Internal Revenue Code, a small business taxpayer meeting the gross receipts test may use a method that treats inventory as non incidental materials and supplies, or may conform to the method reflected in its applicable financial statement or books and records. The regulations were finalized in TD 9942. The inflation adjusted threshold is published annually: Revenue Procedure 2025-32 sets it at $32,000,000 of average annual gross receipts for tax years beginning in 2026.

One caution, because it trips people up. IRS Publication 538, the plain language guide most sellers reach for, carries a January 2022 revision date and still prints a $26 million threshold in more than one place. The number in the current revenue procedure governs. Check the revenue procedure, not the publication, and confirm with your own tax professional rather than treating any article as authority.

What this means in practice is that the choice between perpetual and periodic is usually a management accounting decision rather than a compliance one. The IRS will accept either from most sellers in this revenue range. Your bank, your buyer in a diligence process, and your own purchasing team are the parties who care.

When the switch pays for itself

Four signals, any one of which is usually enough.

You have stocked out of a top ten SKU in the last two quarters. Reorder points require knowing on hand quantity continuously, and periodic cannot supply it.

You sell the same SKU across more than one channel. Allocation between channels is invisible under periodic until the count.

Your gross margin swings more than a few points quarter to quarter without a pricing change. That volatility is usually a counting artifact.

You are preparing to raise money or sell. Diligence teams ask for monthly gross margin by product. Periodic cannot produce it retroactively.

The tooling question

Neither QuickBooks Online nor QuickBooks Desktop maintains marketplace inventory on its own, which is why sellers end up with a layer in between. The options are not interchangeable and each is better at something.

Webgility runs bi-directional inventory sync and maps to QuickBooks bins and locations, which is the deepest inventory feature set among the accounting focused tools and the right answer if warehouse operations are the constraint. Entriwise goes furthest on Amazon specific inventory events such as FBA inbound transfers and lost or damaged adjustments. Sellerboard maintains FIFO cost tracking and stockout adjusted forecasting, and it is better than most accounting tools at telling you when to reorder, though it is a seller analytics product rather than a bookkeeping one. ConnectBooks sits on the accounting side, syncing marketplace activity into QuickBooks Online, QuickBooks Desktop Enterprise and Xero with automated COGS and SKU level profit reporting. A2X deliberately does none of this and posts clean accrual journals instead, which is the correct choice for an accountant who wants the ledger right and the inventory system somewhere else.

Pick based on which problem is costing you money. A seller losing sales to stockouts and a seller failing an audit need different products, and the second one is cheaper to fix.

For the underlying accounting standards, the AICPA publishes guidance on inventory valuation, and the IRS small business and self employed pages cover the method change procedures. Changing an inventory method generally requires filing for a change in accounting method, which is not a decision to make in a spreadsheet the week before a filing deadline.