The Japanese manufacturing philosophy of kaizen means continuous improvement, and it’s a good mantra for anyone managing inventory. But you can only improve what you can see, and an ecommerce business has a lot to watch: suppliers, stock, orders, and customers.

Key performance indicators (KPIs) make that manageable. This guide covers the inventory KPIs worth tracking, how to calculate each one, how to analyze performance by product, market, and channel, and how to choose which ones to start with.

What are inventory KPIs?

Inventory KPIs are the measures you use to judge how well you manage your stock. Just as a doctor checks your blood pressure and temperature at a physical, inventory KPIs act as the vital signs of your business, and help you spot bottlenecks and problem areas before they get expensive.

Why inventory KPIs and metrics matter

• Better inventory management. Tracking turnover or days of inventory, for example, shows slow-moving stock that may need discounting or clearing to free up cash.
• Trends and patterns. Watching stockout rates shows which products run out often, so you can adjust stock levels, promotions, and even warehouse layout.
• Lower costs. Holding stock costs money in storage, insurance, and labor. KPIs show where you can cut those costs.
• Higher profit. Beyond stopping cost leaks, KPIs show where you can sell more and earn more from each unit of stock.

Inventory KPIs and how to calculate them

There are dozens of inventory metrics. The hard part isn’t calculating them, it’s deciding what to track and what to ignore. These are the ones that matter most, grouped by what they tell you.

Efficiency and speed

KPI	What it tells you	How to calculate it
Inventory turnover	How many times you sell and replace your stock in a period. Low turnover means slow stock and tied-up cash	Cost of goods sold (COGS) ÷ average inventory. Full guide
Days of inventory outstanding (DIO)	The average number of days stock sits before it sells. Lower is generally better	(Average inventory ÷ COGS) × 365, or 365 ÷ inventory turnover
Sell-through rate	The share of stock you sold in a period, useful for seasonal and fashion products	Units sold ÷ units available to sell × 100. Full guide
Average inventory	The value of stock you typically hold. It’s the denominator in the metrics above	(Beginning inventory value + ending inventory value) ÷ 2

Profitability and cost

KPI	What it tells you	How to calculate it
Gross margin return on inventory investment (GMROI)	How much gross profit each dollar of stock earns. It shows which products and categories pay for the space they take	Gross margin ÷ average inventory at cost
Carrying cost of inventory	The cost of holding stock: storage, insurance, taxes, labor, shrinkage, and the opportunity cost of the cash tied up in it	Total carrying costs ÷ average inventory value × 100. Full guide
Cost of goods sold (COGS)	The direct cost of the products you sold. It’s the basis for gross margin and turnover	Beginning inventory + purchases − ending inventory

Availability and replenishment

KPI	What it tells you	How to calculate it
Stockout rate	How often products aren’t available when customers want them, which means lost sales	Days out of stock ÷ days in the period × 100, per SKU. Reducing stockouts
Lead time and lead time variability	How long suppliers take to deliver, and how much that varies. Unpredictable lead times force you to hold more safety stock	Date received − date ordered. Variability is how much that gap changes between orders
Forecast error	How far your demand forecasts are from what actually sold	Actual demand − forecast demand, usually as a percentage of actual demand. Forecast accuracy
Reorder point	The stock level at which you should reorder to avoid running out	(Average daily usage × lead time) + safety stock. Reorder point guide and safety stock

Accuracy and loss

KPI	What it tells you	How to calculate it
Inventory accuracy	How closely your records match what’s on the shelves. Errors lead to oversells and stock-outs	Items where the count matches your records ÷ items counted × 100. Accuracy guide
Cycle count accuracy	How accurate your regular partial counts are, which shows how trustworthy your records are	Same as inventory accuracy, measured on each count. Cycle counting
Shrinkage	Stock lost to theft, damage, or mismanagement	Recorded inventory value − actual inventory value. Reducing shrinkage
Dead stock	Stock that hasn’t sold for a long time and ties up capital and space	Units or value with no sales over a period you define. Dead stock guide
Order fulfillment accuracy	How often orders go out right, with the correct items and quantities	Correct orders ÷ total orders × 100

Broader business metrics, such as cash flow and customer satisfaction, matter too, but they’re not specific to inventory. See our guide to ecommerce analytics for those, and our guide to inventory reports for the reports that put these KPIs in front of you.

A worked example

A business has COGS of $500,000 over a year, an average inventory value of $100,000, and gross margin of $200,000.

• Inventory turnover = $500,000 ÷ $100,000 = 5. The business sold and replaced its stock five times.
• DIO = 365 ÷ 5 = 73 days, so stock sits for about 73 days before it sells.
• GMROI = $200,000 ÷ $100,000 = 2.0, so each dollar invested in stock returned $2 of gross margin.

Analyze performance by product, market, and channel

The KPIs above tell you how your inventory is doing overall. To act on them, break them down.

• By product. Rank SKUs by sales and margin. Your best sellers show where demand is, and can point to similar products worth selling. Slow movers need a plan; see our guide to inventory reduction strategies. Check whether weak sellers perform badly all year or only in certain seasons, because that affects when you should reorder.
• By market. Compare how products perform in each region. If something sells well in France but not in the UK, you can aim promotions where demand is strong or move stock between warehouses. Strong demand with little competition may also support a higher price.
• By channel. Compare which channels perform best and worst, overall and for specific products or categories. That tells you where to invest more and where to cut back.

Inventory risk costs

Inventory risk costs are the costs of holding stock that might lose value. They’re part of your carrying cost, and cover:

• Obsolescence: stock that becomes outdated or unsellable, for example because of expiry or changes in demand
• Spoilage and damage: perishable or fragile goods that are lost or damaged
• Opportunity cost: the return you miss by tying up cash in stock
• Theft and shrinkage: stock lost to theft, damage, or errors

Start by tracking these together as one number. As your reporting matures, break them out.

Which KPIs should you track?

Tracking everything is a good way to act on nothing. What to prioritize depends on your industry, your size, and the resources and software you have. If you’re unsure, start with these five, which together show how fast your stock sells, how profitable it is, whether you have it when customers want it, and whether your records are right:

• Inventory turnover
• Sell-through rate
• GMROI
• Stockout rate
• Inventory accuracy

Once those are routine, add others, such as order fulfillment accuracy, lead time, and forecast error, as you need them. A metric that doesn’t apply to your business, such as inventory age for goods that don’t expire, can wait.

Best practices for manageable KPI tracking

• Use software, not memory. Tracking KPIs by hand, including in a spreadsheet, is slow and prone to error. Inventory software can calculate them automatically and show trends. See Linnworks reporting and insights, which offers customizable dashboards and reports for stock, orders, profitability, and fulfillment, filterable to SKU or channel level.
• Document your processes. Write down how you count stock, fulfill orders, and set purchasing policy, in a knowledge base your team can search. It reduces errors and means the business doesn’t depend on one person’s memory.
• Get the core KPIs right first, then add others gradually, so you don’t get overwhelmed.

FAQs about inventory KPIs

Ready to see these KPIs for your own stock? Request a demo of Linnworks.