69% of online shoppers will buy from a competitor if your product is out of stock. Not "consider buying." They'll leave your store, find someone else, and check out in under three minutes. Globally, stockouts cost retailers an estimated $984 billion per year in missed sales — and Shopify inventory forecasting is how growing stores stop the bleeding.

But stockouts aren't just lost revenue today. 40% of shoppers who hit an out-of-stock page won't come back. One empty product page can cost you a customer for life. And if you overcorrect by ordering too much, you're sitting on cash you can't spend on ads, new products, or payroll. Shopify inventory forecasting sits right in the middle of this problem — and most merchants with fewer than 500 orders per month are still guessing.

Why "I'll Just Reorder When It's Low" Doesn't Work

Eyeballing inventory works when you sell 10 products and check your dashboard every morning. It stops working the moment you add a second sales channel, run a promotion, or sell anything seasonal.

The gap between "low stock" and "actually received by your warehouse" is called lead time — and it's longer than most merchants think. If your supplier takes 14 days to ship, and you notice you're out of stock today, you're losing two weeks of sales on that SKU. That's 14 days of sending traffic to a page that can't convert.

Inventory forecasting replaces gut feel with a system. You calculate how fast each product sells, how long it takes to restock, and when to place orders so stock arrives before you hit zero. It's not complicated — it just requires doing the math once and checking it monthly.

Calculate Your Reorder Point for Every SKU

The reorder point is the inventory level that triggers a new purchase order. When stock drops to this number, it's time to reorder. The formula:

Reorder Point = (Average Daily Sales × Lead Time in Days) + Safety Stock

Example: You sell 8 units per day of a phone case. Your supplier takes 10 days to deliver. Your safety stock is 20 units (more on this below). Your reorder point is (8 × 10) + 20 = 100 units. When inventory hits 100, place the order.

To find your average daily sales, pull the "Sales by Product" report in Shopify Analytics. Look at the last 90 days — long enough to smooth out weekly spikes, short enough to reflect current demand. Divide total units sold by 90.

Do this for every SKU that accounts for meaningful revenue. If you have 200 products, you probably only need to forecast the top 30-50 that drive 80% of your sales.

Set Safety Stock Based on How Much Risk You Can Absorb

Safety stock is your buffer. It protects you when demand spikes unexpectedly or your supplier ships late. The right amount depends on two things: how volatile your sales are and how reliable your supplier is.

A simple approach for stores doing under $50K/month in revenue:

High-velocity, high-margin SKUs (your A-items): Hold 10-15% of monthly sales as safety stock. These are the products you absolutely cannot afford to run out of.
Mid-tier products (B-items): Hold 5-10%. A brief stockout hurts but won't tank your month.
Slow movers (C-items): Hold minimal safety stock or none. The carrying cost of extra inventory often exceeds the cost of a short stockout on a product that sells twice a week.

If your supplier has missed delivery windows before, increase safety stock on those SKUs. If they're consistently on time, you can run leaner. Track supplier reliability for 3-6 months and adjust.

Use Shopify's Built-In Reports for Demand Planning

You don't need a $200/month app to start forecasting. Shopify gives you enough data to build a basic demand plan with reports you already have access to.

Sales by Product report: Shows units sold per product over any time range. Export 12 months of data and look for seasonal patterns. If a product sold 3x more in November than in March, your reorder point for Q4 needs to be 3x higher.

ABC analysis (manual): Sort your products by total revenue over the last 90 days. The top 20% by revenue are your A-items — forecast these carefully. The next 30% are B-items. Everything else is C. Focus your forecasting energy on A and B items.

Inventory snapshot comparison: Check your current inventory against your calculated reorder points weekly. A simple spreadsheet with columns for Product, Current Stock, Daily Sales Rate, Lead Time, Safety Stock, and Reorder Point is enough. When Current Stock drops below Reorder Point, order.

This manual method works well up to about 100 active SKUs. Beyond that, the spreadsheet becomes a second job.

When Should You Upgrade to a Shopify Inventory Forecasting App?

If you're spending more than 3 hours per week on inventory spreadsheets, or if you've had two or more stockouts on A-items in the past quarter, a dedicated app will pay for itself.

Since Shopify deprecated Stocky (removed from the App Store in February 2026, fully shutting down August 31, 2026), the replacement landscape has matured. Here's what's worth considering based on store size:

Under $50K/month revenue: Sensible Forecasting ($29/month) covers basic demand planning and reorder alerts. It's the closest replacement to what Stocky offered in terms of simplicity.
$50K-$500K/month: Inventory Planner or Prediko. Prediko's AI engine is trained on over 25 million SKUs, which gives it strong pattern recognition for seasonal products. Both generate purchase orders directly from forecasts.
Multi-channel sellers: Sumtracker ($49/month) syncs inventory across Shopify, Amazon, and other channels while providing forecasting. If you sell on more than one platform, centralized stock counts matter more than fancy AI.

For a broader look at inventory tools beyond forecasting, see our best Shopify inventory management apps roundup. The common mistake is buying a forecasting app before you understand your own numbers. Run the manual method for at least one month first. You'll learn which products are volatile, which suppliers are unreliable, and what your real lead times look like — not what your supplier promises.

Forecast Seasonal Products Separately

Standard reorder point formulas assume relatively steady demand. Seasonal products break this assumption.

If you sell sunglasses, your March-to-July sales might be 5x your November numbers. Using a 90-day average in August will massively overestimate demand for fall, leaving you overstocked. Using the same average in March will underestimate summer demand, leaving you empty when traffic peaks.

For seasonal SKUs, use year-over-year data instead of rolling averages. Compare this April to last April. If you don't have a full year of data yet, use month-over-month growth rates to estimate the curve.

Build a seasonal calendar for your top 20 products. Mark the months where each product's sales deviate more than 30% from its annual average. During those months, override your standard reorder point with a seasonal multiplier.

Turn Forecasting Data Into Purchasing Decisions

Knowing your reorder point is step one. Acting on it consistently is where most merchants fail.

Set a weekly inventory review — same day, same time. Every Friday morning, compare current stock to reorder points. Products below the line get a purchase order that day. No exceptions, no "I'll do it Monday."

Group purchase orders by supplier when possible. Ordering from the same supplier weekly instead of placing one large order monthly gives you more flexibility and reduces the amount of cash tied up in inventory at any given time. It also means shorter gaps between reorders, which lets you run with lower safety stock.

Track your forecast accuracy quarterly. Compare what you predicted you'd sell against what actually sold. If your forecasts are consistently off by more than 20%, your input data needs work — either your lead times are wrong, your sales averages are stale, or you're not accounting for promotions and marketing pushes that spike demand.

If you're using EasySell for your order forms, the analytics dashboard shows which products move fastest through your forms — useful data for identifying velocity changes before they show up in Shopify's standard reports.

Start this week. Pull your Sales by Product report for the last 90 days, calculate reorder points for your top 10 SKUs, and set a Friday morning calendar reminder to check stock levels. That one hour of setup prevents the stockout that costs you customers who never come back.