Divide the gross margin dollars a product generated over a period by the average cost of the inventory you held to produce it. If a SKU generated 40,000 dollars of gross margin on an average inventory cost of 20,000 dollars, its GMROI is 2.0, meaning it returned two dollars of margin for every dollar tied up in stock.
What is a good GMROI?
Above 1.0 means the SKU returns more gross margin than the cost of the inventory held to earn it, which is the basic bar. Many retailers target a GMROI well above 2 or 3, but the right number depends on your margins and cost of capital. Compare SKUs against each other and against your own target rather than a universal figure.
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.
What is the difference between GMROI and inventory turnover?
Turnover measures only speed (how many times you cycle stock); GMROI combines speed with profitability. GMROI equals your gross margin percentage times your sales-to-stock ratio, so a fast-moving but thin-margin SKU and a slow but high-margin one can have the same GMROI. It is the better single measure of how well inventory dollars are working.
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GMROI, gross margin return on investment, is your gross margin divided by the average cost of the inventory that produced it: the margin each inventory dollar returns. The short version: it answers a question turnover cannot, which is not just how fast a SKU moves but how much profit it makes per dollar of cash you have tied up. Below is the formula, a worked example, and why it is the better single measure of a SKU's use of your capital.
GMROI = gross margin dollars / average inventory cost
Gross margin is your sales minus the cost of goods sold; average inventory cost is the average landed cost of the stock you held. The result is a multiple: how many dollars of margin each dollar of inventory returned.
For an FBA seller, use a gross margin that is net of Amazon's referral and fulfillment fees, not just sales minus factory cost. Those fees are real cost of doing business, and leaving them out overstates GMROI on exactly the fee-heavy SKUs where the metric matters most. Pull the margin figure from your own profitability numbers after Amazon fees, and take average inventory cost as the FBA inventory value at the start and end of the period, divided by two.
Say a SKU generated 40,000 dollars of gross margin over the year on an average inventory cost of 20,000 dollars:
Input
Value
Gross margin
$40,000
Average inventory cost
$20,000
GMROI
40,000 / 20,000 = 2.0
A GMROI of 2.0 means the SKU returned two dollars of gross margin for every dollar you had tied up in its inventory. Above 1.0 is the basic bar (it earns more than it costs to hold); many sellers target well above 2.
GMROI = gross margin percentage x (sales / average inventory at cost)
One caution before you use it: the multiplier here is your sales-to-inventory ratio, which is not the same number as the COGS-based inventory turnover covered separately. Because sales exceed COGS by your margin, the sales-based ratio is the larger of the two, so do not plug a COGS-based turnover figure into this formula or you will overstate GMROI.
With that settled, this is why GMROI is powerful: it rewards both fat margins and fast movement. A product with a 20 percent margin and a sales-to-stock ratio of 10 has the same GMROI (2.0) as one with a 50 percent margin and a ratio of 4. It lets you compare a slow, high-margin SKU against a fast, thin one on equal footing, which neither turnover nor sell-through can do alone. You can size the speed side in the inventory turnover calculator.
On Amazon, cash and storage are both finite, so how well each inventory dollar works is the real constraint on growth. GMROI is the number that captures it:
It guides where to put cash. When you cannot fund every reorder, the high-GMROI SKUs deserve the cash first; they turn it into margin fastest.
It flags the traps. A SKU that turns fast but barely makes money looks healthy on turnover and terrible on GMROI. GMROI catches it.
It accounts for FBA fees indirectly. Because gross margin is after cost of goods, and you should be judging real margin after Amazon fees, a low GMROI on a fee-heavy SKU tells you the fees are eating the return.
The real floor for GMROI is your cost of capital, the return that same cash could earn elsewhere or what it costs you to borrow it. A SKU has to return more in gross margin than it costs you to keep the money tied up in its inventory, or it is a poor use of cash no matter how profitable it looks per unit.
Work it through on the numbers above. If your cost of capital is 15 percent a year and you hold 20,000 dollars of average inventory in a SKU, that stock costs you about 3,000 dollars a year just to carry. A GMROI of 2.0 returns 40,000 dollars of gross margin on that 20,000 dollars, so it clears the bar many times over. A GMROI of 1.1, by contrast, returns only a slim margin over the cost of the stock, and once you subtract the carrying cost it can be losing money on a cash-return basis even while it shows a per-unit profit. For the days-based view of the same tied-up cash, see days sales in inventory.
That is the discipline GMROI enforces: judge a SKU not by whether it turns a profit, but by whether it turns enough profit to justify the cash locked in its inventory.
Rank SKUs by GMROI when deciding where to invest limited cash. Fund the high-return products first.
Investigate the low-GMROI SKUs. Either the margin is too thin (reprice or renegotiate cost) or the turnover is too slow (clear the overstock). GMROI tells you a SKU has a problem; margin and turnover tell you which one.
Set a target from your cost of capital. Your GMROI floor should comfortably clear what that cash would earn elsewhere.
As a rough anchor, traditional retail cites a GMROI above 3 as healthy. FBA sellers carrying higher per-unit fees often treat anything above 2 as solid, but the more useful comparison is ranking your own SKUs against each other: your best cash-users and your worst show up immediately, and that ranking is what drives where the next dollar goes. Read it per SKU, never as a blended account figure, for the same reason turnover is read per SKU: the average hides the products that most need attention.
GMROI is gross margin over average inventory cost, or equivalently margin percentage times your sales-to-stock ratio, and it is the cleanest single read on how hard your inventory dollars work. Use it to rank where cash goes, to catch fast-but-unprofitable SKUs, and alongside days of supply so you optimize for profit without stocking out. For the wider metric set, see the inventory KPIs that matter.