What An Amazon Inventory Tracker Actually Needs To Track

The Six Numbers A Tracker Actually Needs

An Amazon inventory tracker needs six numbers in one place, updated on the same day: sellable units at FBA, units inbound or in transit, units reserved against open orders, average daily sales velocity, days of cover, and storage-age buckets (0-90, 91-180, 181-270, 271-365, 365+ days). Miss any one of these and you're either guessing at reorder timing or paying storage fees on stock you didn't know was aging.

Amazon gives you pieces of this for free. The Manage FBA Inventory report shows sellable and reserved units. The Inventory Performance Index (IPI) dashboard shows a rolled-up excess and sell-through score. The Restock Inventory tool suggests reorder quantities from its own velocity model. None of these three talk to each other, and none show you the one thing that actually prevents a stockout: your reorder point against your specific lead time.

A real tracker — spreadsheet or software — pulls these into one row per SKU and does the arithmetic Amazon's dashboards don't: how many days until you run out, and whether your next PO lands before that day.

The Math: Reorder Point, Worked Through

The formula is simple: reorder point = (average daily sales velocity × total lead time in days) + safety stock. Total lead time means manufacturing time plus freight time plus FBA check-in time — not just the number your supplier quotes for production.

Worked example: a SKU sells 40 units a day on average. Production takes 20 days, ocean freight and customs take 20 days, and FBA check-in is currently running 5 days. That's 45 days of lead time. 40 × 45 = 1,800 units needed just to cover the lead time. Add 15 days of safety stock (40 × 15 = 600 units) for demand variability and carrier delay. Reorder point: 2,400 units. If sellable inventory drops to 2,400 units and the next PO hasn't been placed, you're already late.

The number that breaks this math most often is FBA check-in time, because it moves. A SKU that checked in in 3 days in March can take 10 in November. A tracker built on a fixed lead-time assumption from the last time someone set it up will quietly under-order for months without anyone noticing until the stockout.

Where A Spreadsheet Stops Working — And What The Next Tool Actually Is

A spreadsheet works fine up to a few dozen SKUs and one or two POs in flight at a time. Past that, the manual update becomes the failure point — someone forgets to refresh the check-in number, or the reserved-units column doesn't get pulled the week everyone's busy with a launch.

Be precise about what the next tier of tool actually is, because the categories get blurred in most "best inventory software" lists. An ERP or multichannel inventory system — Linnworks, Cin7, Brightpearl and similar — runs your whole business: orders, accounting sync, warehouse management across every channel you sell on, not just Amazon. If you sell on Amazon, Shopify, and wholesale, you probably need one of these eventually. It is not an Amazon-specific fee or reimbursement tool, and nothing that only tracks Amazon inventory should try to replace it.

A research suite is built for sourcing decisions — finding products, checking competitor pricing — not for running inventory you already have. A service like Getida is genuinely good at one narrow thing: auditing FBA transactions to find reimbursement-eligible discrepancies — lost units, damaged units, overcharged fees — and filing the claims. That's real, valuable work. It is not a reorder-point tracker and doesn't try to be.

A 3PL physically stores and ships your product. It can tell you what's on its shelves, but it isn't calculating your Amazon-specific reorder point or reconciling FBA's numbers against your own — that's a different job entirely.

The Mistakes That Actually Cause Stockouts

The mistake we see most, across the group of brands we work with (Full Circle has managed more than $500M in revenue across 100+ brands), is a reorder point set once at launch and never revisited. Ad spend goes up, velocity doubles, and the reorder point that was correct in month one quietly stops covering lead time by month four. The stockout doesn't look like an inventory problem when it hits — it looks like a rank drop mid-campaign, so people go looking for an ads fix. If the SKU is genuinely out of stock, that's a timing problem in the warehouse, not a targeting problem. If the leak really is in the ad account rather than the inventory, that's a question for Dr. PPC, not this page.

Second mistake: trusting Amazon's "days of supply" figure as if it already accounts for inbound shipments in transit. It often doesn't reflect stock sitting at a port or checked in but not yet posted. Cross-check it against your own inbound tracking before delaying or placing a PO based on it alone.

Third mistake, one we've made ourselves early on with a seasonal SKU: using a trailing 30-day average velocity to set the reorder point right before a known demand spike. The average is backward-looking; the reorder point needs to be forward-looking. For anything seasonal or promo-driven, the safety-stock number needs to move ahead of the spike, not react to it.

When The Tracker And Amazon Disagree

Sometimes the tracker and Amazon disagree, and the instinct is to trust Amazon because it's the platform of record. Don't, automatically. Amazon's own inventory ledger and the Manage FBA Inventory report can lag by a day or more, and shipment reconciliation discrepancies — units that shipped but were never received, or received in a different quantity than the manifest said — are common enough that Amazon runs a claims process for exactly this.

If your tracker shows fewer sellable units than Amazon's dashboard, check the shipment reconciliation report before assuming your tracker is wrong. If it shows more, check for units stuck in "unfulfillable" or pending disposal that your tracker isn't picking up. Either way, there's a claim window — usually measured in months, not weeks — and once it closes, a genuine discrepancy becomes a write-off instead of a reimbursement. The fix that doesn't work: re-running the same report and hoping the number changes. The fix that does: filing the discrepancy while it's still inside the window.

Where Dr. Stock Fits

Dr. Stock is Amazon inventory and supply chain run as a managed product by Fable 5, from Full Circle. It's built for exactly the reconciliation and reorder-timing work described above — stockout timing, aged-inventory and storage-fee decisions, FBA fee errors, and reimbursement recovery — plus Orbit's inventory, finance, and fee tracking included at no extra cost. Purchasing decisions always go to a human, whatever autonomy level a client chooses. There's no published price; it's a demo and a 30-day free look, priced on the call. If all you need is the reorder-point math running correctly on a spreadsheet, you don't need us for that — this page is the tracker.