What are the most important inventory KPIs for Amazon sellers?
The core set is sell-through rate and inventory turnover (how fast stock moves), days of supply (how long until you run out), GMROI (how much margin each inventory dollar returns), days sales in inventory (how long cash sits in stock), plus Amazon's own IPI score and your aged-inventory share. Track them per SKU, not just as an account average.
How do I calculate inventory KPIs?
Each is a simple ratio from your own data. Sell-through is units sold over average units on hand; turnover is COGS over average inventory value; days of supply is current stock over daily sales velocity; GMROI is gross margin over average inventory cost. You can build them all from your Seller Central sales and inventory reports.
T. Brian Jones is co-founder and CTO of Inventory Hero. He leads the engineering behind its Amazon data pipeline, demand forecasting, and the AI platform that lets sellers talk to their live inventory, sales, and supplier data in plain language.
It depends on the product, but many FBA sellers aim for a turnover in the range of roughly 4 to 12 times a year, which corresponds to holding one to three months of stock. Higher turnover ties up less cash and pays less storage, but pushed too far it risks stockouts, so the right number balances the two.
Read article
The inventory KPIs that matter for an Amazon FBA seller fall into four jobs: measuring efficiency (how fast stock moves), profitability (how much each inventory dollar earns), health (whether Amazon and your own systems flag problems), and timing (how long until you run out). The short version: track a small set of ratios, per SKU, and let each one drive a decision. Below is the full set with formulas, what good looks like, and how they fit together.
Here is the core set, each a simple ratio you can build from your own data:
KPI
Formula
What it tells you
Healthy signal
Sell-through rate
units sold / average units on hand
How fast a SKU is moving
Higher is better; watch the trend
Inventory turnover
COGS / average inventory value
How many times a year you cycle stock
Roughly 4 to 12 turns a year
Days of supply
current units / daily sales velocity
How long until you run out
Cover your lead time plus a buffer
Days sales in inventory (DSI)
(average inventory / COGS) x days
How long cash sits in stock
Roughly 30 to 90 days
GMROI
gross margin / average inventory cost
Margin returned per inventory dollar
Above 1; many target 2 to 4x
Inventory-to-sales ratio
inventory value / sales
Whether you hold too much for your sales
Lower, while staying in stock
Inventory accuracy
matched records / total records
Whether your counts can be trusted
Aim for 95 percent or higher
Aged-inventory share
aged units / total units
How much stock is heading for surcharges
Keep low and trending down
The healthy-signal column is directional operating ranges, not Amazon-published targets; set your own from your margin and lead time. The rest of this guide walks through each, grouped by the job it does.
These three answer the same question from different angles: are your inventory dollars working, or sitting?
Sell-through rate is units sold divided by average units on hand over a period. It is the most direct read on velocity, and it feeds Amazon's IPI. Higher is better, within reason. Run it in the sell-through rate calculator.
Inventory turnover is COGS divided by average inventory value, expressed as times per year. It is the finance-side version of the same idea, and many FBA sellers land in the rough 4 to 12 range depending on the product.1 Size it in the inventory turnover calculator.
Days sales in inventory (DSI) is the inverse of turnover in days: how many days, on average, a unit sits before it sells. Lower means cash converts faster.
Watch all three per SKU. A healthy account average can hide a handful of slow movers dragging your cash and your IPI.
Velocity is not the whole story; a fast-moving SKU with thin margin can be worse than a slower one with fat margin.
GMROI (gross margin return on investment) is your gross margin divided by the average cost of the inventory that produced it. It answers "for every dollar I have tied up in this product, how much gross margin does it return?" A GMROI above 1 means the SKU earns more in margin than it costs to hold in inventory; that is only the floor of viability, and many FBA sellers with healthy margins target something closer to 2 to 4x.
GMROI is the KPI that stops you from over-investing in a SKU that turns fast but barely makes money.
Amazon's IPI is the account-level Inventory Performance Index, driven by sell-through, excess inventory, stranded inventory, and in-stock rate. It governs your storage limits, so it is a KPI you cannot ignore even though Amazon calculates it for you.
Inventory accuracy is the share of your inventory records that match reality. In FBA that means reconciling your own records against the FBA inventory report (Amazon does the physical counting), so the relevant signals are short-received, stranded, and phantom units. If your counts are wrong, every other KPI built on them is wrong too, so accuracy is the foundation.
Aged-inventory share is the fraction of your units that have sat long enough to approach the aged-inventory surcharge. A rising share is an early warning of dead stock.
Days of supply is current sellable units divided by daily sales velocity: the number of days your stock lasts at the current pace. Compared against your lead time, it is the single most important restock-timing number, covered in days of supply vs days of inventory.
Days of supply is forward-looking, which is what makes it the timing KPI; the efficiency KPIs above are backward-looking measures of how you did.
The metrics are easier to read together than apart. Take a single SKU over a year:
Input
Value
Annual COGS (landed)
$180,000
Average inventory value
$30,000
Gross margin generated
$60,000
Current sellable units
240
Daily sales velocity
8 units
From those five numbers you get every value-side KPI:
Turnover = 180,000 / 30,000 = 6 turns a year.
DSI = 365 / 6 = about 61 days a unit sits.
GMROI = 60,000 / 30,000 = 2.0, so each inventory dollar returns two dollars of margin.
Days of supply = 240 / 8 = 30 days of cover left.
Read as a set, this SKU is healthy: it turns six times, returns 2x on cash, and has a month of cover. Its 90-day sell-through, aged-inventory share, and accuracy you would read straight from the dashboard rather than compute here, but the same discipline applies: one SKU, every angle, then a decision.
One row in the table above deserves its own note. The inventory-to-sales ratio is your inventory value divided by your sales over the same period, and it is the blunt "are we carrying too much?" check. A rising ratio means inventory is growing faster than sales, which is cash quietly piling up in the warehouse. It is the inverse intuition of turnover: where turnover asks how many times you cycled stock, inventory-to-sales asks how heavy the pile is relative to what you are selling. Watch it climb and you are heading for overstock and the aged-inventory surcharge.
Every KPI here is only as good as the data underneath it, and a few things quietly corrupt them:
Factory-price COGS. If you cost inventory at the factory invoice instead of landed cost, your turnover and GMROI both look better than they are.
Out-of-stock days in the denominator. If you average velocity over a period that includes stockout days, you understate how fast the SKU really sells, and your days of supply reads long.
Blended multi-channel numbers. Mixing FBA and off-Amazon sales into one figure distorts every ratio; keep the channels separate.
Fix the data first. A confident decision on a wrong number is worse than no decision.
Every KPI here is built from data you already have in Seller Central:
Units sold and sales velocity come from your Business Reports (units ordered by ASIN, by date), with out-of-stock days excluded so velocity is not understated.
On-hand and aged units come from the FBA inventory and inventory-age reports.
COGS and inventory value come from your own landed cost records, not the factory price.
IPI is shown directly in your account dashboard.
The work is pulling and updating all of it per SKU on a regular cadence, which is exactly what a tool automates.
Tracking account averages only. The average hides the slow movers that cost you the most. Go per SKU.
Chasing turnover at the expense of stockouts. Very high turnover can mean you are running too lean and losing sales. Balance it against days of supply.
Ignoring GMROI. A fast SKU with no margin is not a good SKU. Velocity and profitability both matter.
Building KPIs on bad data. Wrong counts or factory-price COGS quietly corrupt every metric. Fix accuracy and use landed cost.
The inventory KPIs that matter for FBA are a small set of ratios, sell-through and turnover and DSI for efficiency, GMROI for profitability, IPI and accuracy and aged share for health, and days of supply for timing. Track them per SKU on real data, and make each one drive a decision. When cash is tight and you cannot fund every reorder, rank by GMROI and fund the highest-return SKUs first; that single rule keeps your limited capital working where it earns the most. For the system these feed, see Amazon FBA restock planning.
Inventory turnover benchmarks vary widely by industry and product; the 4 to 12 range is a common operating band cited in standard inventory-management references, not an Amazon figure. Set your own target from your margin and lead time. ↩