Updated March 2026

Inventory Management Glossary

Plain-language definitions for the inventory terms that matter most to Shopify merchants. Each entry includes formulas, worked examples, and tips for your store.

All Terms

Core Concepts Every Merchant Should Know

Inventory management is built on a surprisingly small set of core ideas. At its heart, the question is always the same: how much to order and when to order it. Every formula and metric in this glossary is a different angle on answering that question.

Reorder point answers when to order. It is the inventory level at which you should trigger a new purchase order, calculated from your sell rate and supplier lead time plus a safety buffer. Get this right for each SKU and you will never scramble to fix a stockout.

Safety stock answers how much buffer to keep. It protects you against two types of variability: demand variability, where customers buy more than expected, and supply variability, where your supplier delivers late. The safety stock formula balances the cost of carrying extra inventory against the cost of stocking out.

Days of supply tells you how long your current stock will last at the current sell rate. It is the most intuitive metric for day-to-day inventory monitoring. When days of supply drops below your lead time, you are cutting it close. When it drops below your lead time plus safety stock days, you should have already reordered.

Why Formulas Beat Gut Feel

Most Shopify merchants start with gut-feel inventory management: glance at stock levels, reorder when something looks low, order roughly the same amount as last time. This works when you have 20 products. It breaks down at 100 and becomes actively harmful at 500 or more.

The problem with gut feel is that it treats every product the same. But a SKU selling 50 units per day with a 7-day lead time has completely different reorder needs than a SKU selling 2 units per day with a 30-day lead time. A flat mental model, such as reorder when anything drops below 100 units, will overstock the slow mover and understock the fast mover simultaneously.

Formulas fix this by making reorder decisions SKU-specific. Each product gets its own reorder point based on its own sell rate, its own lead time, and its own demand variability. The result is less cash tied up in slow-moving inventory and fewer stockouts on your best sellers.

You do not need to be a mathematician. The formulas in this glossary are straightforward multiplication and addition. The hard part is not the math. It is gathering the inputs: accurate daily sales data, reliable lead time estimates, and some measure of demand variability. Once you have those, the formulas do the rest.

From Theory to Practice

Understanding these terms is step one. Applying them across your catalog is step two. For stores with fewer than 50 SKUs, a spreadsheet and these formulas can get the job done. Calculate reorder points for your top sellers, set calendar reminders to check stock levels weekly, and you are ahead of most merchants.

For stores with more than 50 SKUs, the manual approach does not scale. You need a tool that calculates sell rates automatically, monitors inventory levels against thresholds daily, and sends alerts when it is time to act. That is exactly what Alertr does.

Whether you use Alertr, another tool, or a spreadsheet, the underlying concepts are the same. This glossary gives you the vocabulary and formulas to make informed decisions about your inventory, regardless of which tools you use.

More Resources