Every stockout is a sale you already earned and then lost — and every unit of overstock is cash sitting on a shelf. Good inventory forecasting sits between those two failures: it tells youwhat to reorder, how much, and when. This guide shows the simple maths behind forecasting on Shopify, how to do it by hand, and how to automate it.

Why inventory forecasting matters

  • Stockouts cost you the sale, hurt your search ranking on Shopify, and send loyal customers to competitors.
  • Overstock ties up cash, fills storage, and often ends in markdowns.
  • Guessing from memory doesn't scale past a handful of SKUs — you need a repeatable method.

The four numbers you actually need

Forecasting sounds complex, but it comes down to four inputs per product:

  • Sales velocity — how many units you sell per day (average over a recent period).
  • Lead time — how many days from placing a purchase order to stock on the shelf.
  • Safety stock — a buffer for demand spikes and late deliveries.
  • Current stock on hand — what you have right now.

The reorder point formula

The core of forecasting is the reorder point — the stock level at which you should place a new order:

Reorder point = (sales velocity × lead time) + safety stock

When your stock on hand drops to the reorder point, it's time to reorder. A quick worked example:

A product sells 5/day, your supplier's lead time is 14 days, and you keep 3 days of safety stock (15 units).
Reorder point = (5 × 14) + 15 = 85 units. Order quantity for a 30-day cover = 5 × 30 = 150 units.

So when this product hits 85 units on hand, you place an order for about 150.

How to forecast inventory manually (and why it breaks)

You can do this in a spreadsheet: export sales, compute a per-day average, add lead time and a buffer, and compare to current stock. It works — until you have dozens of SKUs, seasonal demand, multiple suppliers with different lead times, and products that trend up or down week to week. Manual sheets go stale the day after you build them, and a single missed row becomes a stockout.

How to automate it on Shopify with Foreshelf

Foreshelf does this maths for every product automatically, and shows its work:

  1. Install the app. Add Foreshelf — Inventory Forecasting from the Shopify App Store. It's read-only and never changes your store.
  2. Let it sync. Foreshelf reads your sales and inventory history and computes each product's sales velocity, days of stock left and suggested reorder quantity.
  3. Read the status. Every product shows a clear label — Reorder now, Reorder soon, Stockout, Overstock, No sales or Healthy — so you see what needs attention at a glance.
  4. Set your rules. Adjust lead time and safety buffer per shop, supplier or product, and the recommendations adapt.
  5. Order by supplier. Open the reorder report grouped by supplier, tweak quantities, and export to CSV as your purchase order.

Crucially, every recommendation explains itself — "sells 5/day, 12 days left, 14-day lead → reorder 90 now" — so you stay in control instead of trusting a black-box number.

Coming from Stocky?

Shopify is retiring Stocky, leaving many merchants without their reorder tool. Foreshelf is the easy replacement: import your Stocky CSV to bring suppliers and lead times across, and get the plain-English explanations Stocky never gave you.

Best practices

  • Use a recent window for sales velocity (e.g. the last 30–60 days) so trends are current.
  • Set realistic lead times — include supplier processing, shipping and receiving, not just transit.
  • Raise safety stock for volatile or high-margin items; a stockout on your best product costs the most.
  • Review weekly, and let email alerts catch the products that hit their reorder point between reviews.

Frequently asked questions

Will a forecasting app change my inventory?

It shouldn't need to. Foreshelf is read-only — it only reads your data and recommends; you place the orders.

How much sales history do I need?

Even a few weeks helps; the more history, the more accurate the velocity and forecast.

Is my customer data safe?

Foreshelf stores no customer personal data — only aggregated daily sales counts used to compute velocity.

Forecasting isn't about predicting the future perfectly — it's about never being surprised. Get the four numbers right, watch the reorder point, and you'll stop losing sales to stockouts and stop burying cash in overstock.