Shopify Inventory for Beginners: 5 Numbers to Track

If you're new to Shopify inventory management — or you've been running your store for years but still wing your ordering decisions — this guide is for you. Inventory management sounds complex, and it can be. But for a small to medium Shopify store, you really only need to understand five numbers to make good decisions.

Everything else is either a variation of these five or a nice-to-have that matters more at scale. Master these, and you'll avoid the two most expensive inventory mistakes: running out of your best sellers and sitting on piles of stuff that won't sell.

1. Available Stock

What it is: The number of units you can actually sell right now.

This is different from "on hand" or "in stock." Your Shopify admin might show 100 units, but if 60 are committed to unfulfilled orders and 5 are damaged, you only have 35 available. Available stock is the number that determines whether a customer can buy your product — and it's the number your forecasting should be based on.

How to use it: When you're thinking about whether to reorder, look at available stock — not total stock. If available is low, it doesn't matter how high your on-hand count looks. For a deeper dive, see our guide to Shopify inventory states.

2. Sales Velocity

What it is: How many units of a product you sell per day (on average).

This is the most important number in inventory management. If you know how fast something is selling, you can predict when you'll run out. Sales velocity answers the question: "at this rate, how many days until I have zero stock?"

How to calculate it: Take the number of units sold over a period and divide by the number of days. If you sold 90 units in the last 30 days, your velocity is 3 units/day.

How to use it: Days of stock remaining = Available stock ÷ Sales velocity. If you have 45 available units and sell 3/day, you have 15 days of stock left. Now you can make an informed reorder decision. Our guide on choosing the right sales period explains which timeframe to use for calculating velocity.

3. Lead Time

What it is: The total number of days between placing a purchase order and having the stock on your shelves, ready to sell.

Lead time isn't just shipping time. It's the full journey: supplier processing your order + manufacturing (if applicable) + shipping + customs (for international) + your receiving and shelving. Most merchants undercount by only including shipping days.

How to use it: Lead time tells you how far in advance you need to order. If your lead time is 21 days and you have 25 days of stock left, you need to order in the next 4 days. If you wait 10 days, you'll be out of stock for a week before the new shipment arrives. See our lead time guide for the full breakdown.

4. Safety Stock

What it is: A buffer of extra inventory you keep to protect against the unexpected — demand spikes, supplier delays, or shipping disruptions.

Nothing in inventory management is perfectly predictable. Your supplier might take 3 days longer than usual. A viral social media post might double your sales for a week. Safety stock is your insurance policy so that these normal fluctuations don't result in a stockout.

How to use it: Express safety stock as days of extra coverage. If your sales velocity is 3 units/day and you want 7 days of safety stock, that's 21 extra units. Add this to your lead time demand when calculating reorder points. Our safety stock guide covers how to find the right level for your store.

5. Reorder Point

What it is: The inventory level at which you should place your next purchase order.

This is where the other four numbers come together. Your reorder point ensures you order early enough that new stock arrives before you run out — including your safety buffer.

The formula:

Using our example numbers: (3 units/day × 21 days lead time) + (3 units/day × 7 days safety stock) = 63 + 21 = 84 units. When your available stock drops to 84, it's time to order.

How to use it: Calculate this for each of your products. When available stock crosses below the reorder point, place your order. If you have more than a handful of products, doing this manually becomes tedious fast — which is exactly why forecasting tools exist.

That's It. Five Numbers.

If you track available stock, sales velocity, lead time, safety stock, and reorder points for your products, you'll make better inventory decisions than 90% of Shopify merchants. You won't need a degree in supply chain management. You won't need a complex ERP system. You just need these five numbers, kept up to date.

The challenge, of course, is keeping them up to date across your entire catalog. Sales velocity changes. Lead times fluctuate. Available stock depletes every day. Doing this manually for 200 products means 200 calculations that need refreshing regularly.

Or Let a Tool Do the Math

Sensible Forecasting tracks all five numbers for every product in your Shopify store, automatically. It reads your sales data, calculates velocity across your chosen time period, applies your lead time and safety stock settings, and tells you which products need reordering and when.

You configure it once — set your sales period, lead times, safety stock, and days of stock target — and it does the rest. Products are sorted by urgency, so your most pressing reorders are always at the top. And with email reports, you get a summary delivered to your inbox without having to log in.

It's $29/month with a 30-day free trial (no credit card). Five minutes to set up, and your five numbers are tracked automatically from that point on.