Most Shopify merchants reorder by gut feel — and find out they got it wrong only when a best-seller hits zero. Here's the simple math that tells you exactly when (and how much) to reorder, with a worked example you can copy today.
A practical guide from the team behind Replenish · updated for 2026
Running out of a product is expensive twice over. You lose the sale you could have made, and you can lose the customer to a competitor who had it in stock. Order too early or too much, on the other hand, and your cash sits frozen on a shelf as dead stock.
The good news: knowing when to reorder isn't guesswork. It comes down to one number per product — the reorder point — and that number comes from a formula simple enough to sanity-check by hand. Once you understand it, you'll know whether any app (or spreadsheet) is giving you a sensible answer, instead of trusting a black box.
Your reorder point is the stock level at which you should place a new order — early enough that the shipment arrives before you run out.
Three inputs, each easy to find:
The first part — daily sales × lead time — is the inventory you'll burn through while you wait for the new shipment. Safety stock is the cushion on top so a normal demand spike doesn't sink you before the truck arrives.
Take one product — say a bag of coffee beans — and run it through the formula.
The reorder point tells you when. A second, related formula tells you how much.
These are two different numbers, and keeping them apart is the whole trick. The subtlety: you subtract the stock you'll still have at delivery — not what's on the shelf today, because some of today's stock sells out while you wait. Order on time — the day stock touches the 168-unit reorder point — and the math is clean:
Notice what fell out: ordering on time, the quantity is exactly your lead-time demand — 8/day × 14 days. You refill what the wait consumes, and the safety buffer rolls forward intact. The reorder point (168) is the trigger level, not the order size.
If you've already fallen behind, the math changes. Caught at 25 on hand, those 25 are gone in about 3 days — nothing survives the 14-day wait. Stock at delivery is 0, so you order the full 168… and still eat roughly 11 days of stockout you can no longer avoid. Being late costs you twice: a bigger order and lost sales.
This is the discipline that keeps cash healthy: you order enough to comfortably bridge the lead time and buffer — not a random round number, and not "let's grab 500 while they're on sale" (the classic way dead stock is born). The one thing you can safely subtract is genuinely incoming stock — units already on the way that will arrive with (or before) your new order — so you don't double-order a product whose restock is mid-transit.
The formula is easy. The mistakes are almost always in the two inputs that feed it.
A supplier says "7–10 days" and you plan around 7. In reality, between order processing, production, and shipping, your last five orders averaged 18 days. Plan around what actually happened, not the brochure. An under-estimated lead time is the single most common cause of a stockout — you ordered "on time" against a number that was never real.
Average a full year of sales and a product that triples every December looks calm and steady all year. Then Q4 arrives, real demand is 3× your "average," and you run out in the most important weeks. Use a recent, relevant window (often the last 30–90 days), and treat seasonal products differently from steady ones.
You can do this by hand in a spreadsheet. Here's the loop — per product.
Export your Shopify orders and total the units sold per product, then divide by the number of days to get daily velocity.
Look at your last few purchase orders: how many days from "ordered" to "received"? Use that, not the quote.
Start with ~7 days of sales. Increase it for unpredictable demand or unreliable suppliers; decrease it for steady sellers from a fast, dependable source.
(velocity × lead time) + safety stock. When stock crosses that line, order the reorder quantity. The catch: you have to re-check every product, every day, as velocity and lead times drift.
Doing this by hand works for a handful of SKUs. Past that, the daily re-checking is exactly the kind of chore software should own. That's what Replenish does: it reads your Shopify sales history, your stock levels, and your supplier lead times, then computes the reorder point and quantity for every product, every day — and ranks them by urgency so you see what's actually at risk first.
The part that matters most: every recommendation shows its math. Not a black-box "reorder 168 units," but the full reasoning, in plain language:
Because the numbers are deterministic arithmetic — not an AI guess — you can verify every recommendation or spot when something looks off. That transparency is the whole point: you should be able to trust a reorder number because you can see where it came from.
Replenish computes the reorder point and quantity for every product, every day — and shows the math behind each one. Built as a Stocky replacement, from $10/mo.
Install Replenish on ShopifyFrom $10/mo · 14-day free trial · no credit card required during trial