Inventory Replenishment: A Practical Plan for Shopify Bestsellers

Inventory replenishment is the decision to buy more of a variant before available stock runs out, and to buy only as much as demand, lead time, and cash can support. On Shopify, that decision is a date and a quantity, not a warehouse theory.

Both sides of a bad buy hurt. IHL Group projects that inventory distortion, the combined cost of out-of-stocks and overstocks, will cost retailers $1.7 trillion in 2024, with out-of-stocks at $1.2 trillion and overstocks at $554 billion. That figure is global retail, not a Shopify benchmark. The split still applies to a small catalog: an empty bestseller loses the sale, and a slow size locks up the cash you needed for the next reorder.

What inventory replenishment means for a Shopify store

Inventory replenishment is restocking so you can keep selling. Inventory control is knowing what you already hold. You need both, and they answer different questions. Control tells you the count. Replenishment tells you whether to send a purchase order, and for how many units.

Shopify stores usually run two restock motions. One is a purchase from a supplier. The other is a transfer from a reserve location, a 3PL, or another store into the location that fulfills orders. Shopify tracks both as Incoming once a transfer, purchase order, or app has them on the way. Incoming units are not available to sell until they are received.

A useful replenishment record has four numbers for each variant: units you can still sell (Available), units already on the way that you trust, supplier lead time in days, and a recent daily sales rate. Purchase orders in Shopify record the supplier, quantities, and costs. They do not calculate the quantity for you. Turn inventory tracking on first, or those counts will not be reliable enough to order from.

Which SKUs deserve a reorder first

Reorder the variants that make the revenue, not the ones with the lowest unit count. Shopify's ABC product analysis grades each variant on the last 28 days of revenue. A-grade variants together account for 80% of revenue, B-grade the next 15%, and C-grade the last 5%. Cost per item does not enter that grade, and the 28-day window cannot be changed.

Use that grade as the priority list, then check margin before you fund the purchase order. A high-revenue, low-margin variant can still be an A. If cash is tight, fund the A variants with healthy unit margin first. A fuller method for ranking what to stock is in ABC analysis for Shopify inventory.

Do the math at variant level. A hoodie can look healthy while Medium is 8 days from a stockout and XXL has 90 days of cover. Replenish Medium. Leave XXL off the purchase order unless a campaign specifically needs it.

• First: A-grade variants whose days of cover are inside lead time plus buffer.
• Next: A-grade variants that will cross that line before your next review.
• Then: B-grade variants only if the supplier minimum order quantity is already justified by A-grade units on the same purchase order.
• Last: C-grade variants. Reorder them when a customer order requires it, not on a standing schedule.
• Skip variants with no sales in the last 28 days until you know whether demand is seasonal, paused, or gone.

When to reorder using demand, lead time, and a buffer

Place the order when days of cover drop to lead time plus buffer, not when the shelf looks thin. Days of cover equals Available units divided by average daily sales. Lead time is the count of days from sending the purchase order to those units becoming Available, including production, transit, and receiving.

If days of cover are shorter than supplier lead time, the replenishment decision is already late. The remaining choice is how large the gap will be.

Shopify's Inventory remaining per product report estimates days left as ending quantity divided by average units sold per day, using sales from the last 28 days. If a variant had no sales in that window, the value is N/A. If ending quantity is negative, days remaining is 0. Treat 0 as already late. The report does not subtract lead time, so a variant with 18 days remaining and a 21-day supplier is not safe.

The reorder point formula, demand during lead time plus safety stock, is the same idea in units. The step-by-step version is in the Shopify reorder formula. For a weekly buying rhythm, add the days until you will look again, or a fast variant can sell through the gap between reviews.

A worked reorder, with made-up numbers

These figures are an illustration, not store results. A variant sells 4 units a day on the 28-day rate. Available is 40. No incoming purchase order is reliable enough to count. Lead time is 21 days. You review stock every 7 days. You hold a 7-day buffer for a late shipment or a short demand spike. That buffer is an operating target until you have a record of actual late deliveries.

At 4 units a day, 40 available units last 10 days. The supplier needs 21. An order placed today still leaves a gap of about 11 selling days unless you expedite, pause ads on that variant, or both. Expediting is a supplier negotiation, not a default. Pausing spend on a variant you cannot ship is the faster lever.

Subtract incoming only when the receipt date is inside the protection window and the supplier has been hitting it. If 80 units are already Incoming and due in 10 days from a supplier who delivers on time, the suggested order falls from 100 to 20. If that shipment has slipped before, leave it out of the math and reorder as if it might miss.

Check the last 7 days against the 28-day rate before you lock the purchase order. A sale, a creator post, or a stockout on a substitute can double the recent pace without changing the 28-day average much. Use the higher rate only if that demand will continue. How to separate a spike from a new baseline is covered in ecommerce demand forecasting.

How much to order without trapping cash

Order quantity equals daily demand times (lead time + days until the next review + buffer days), minus Available, minus trusted incoming. In the illustration, that is 4 times 35, minus 40, which is 100 units. Round to the case pack only after that number exists. Do not start from the supplier's preferred carton size.

Then apply a cash cap. If the unit cost is $12, 100 units is $1,200 before freight. If the reorder budget this week is $800, you can buy 66 units. That thins the buffer. It does not change the fact that the variant is inside the reorder window. Cut a C-grade reorder on the same supplier before you cut the A-grade quantity.

A minimum order quantity that forces you far past the target is a carrying-cost decision, not a demand decision. Take the minimum on an A variant when the extra units will sell inside a normal season. Decline it on a C variant and accept a shorter gap, or combine sizes so the minimum is filled by variants you would have bought anyway. Economic order quantity is the textbook balance of order cost and holding cost. Inventory carrying cost is what that extra carton actually costs you per month. A wider view of where cash sits is in inventory optimization.

A weekly replenishment review for a small catalog

A weekly review is enough when no A-grade variant can sell through its buffer between Mondays. Block 45 minutes. Work only the variants that can stock out before the next review, not the full catalog.

1. Open ABC product analysis and list every A variant, plus any B variant you are actively advertising.
2. Open Inventory remaining per product. Flag any of those variants with days remaining at or below lead time plus buffer.
3. On the Inventory page, read Available, not only On hand. Committed units are already in unfulfilled or draft orders and cannot be sold again.
4. Read Incoming from purchase orders and transfers. Count a shipment only if you would bet the reorder date on it.
5. Compare the last 7 days of units sold with the 28-day daily rate. Keep the 28-day rate unless a live promotion will continue.
6. Calculate the suggested order, round to the case pack, and stop when the week's cash cap is spent. A variants win ties.
7. Create the purchase order, mark it Ordered after the supplier confirms, and create the inventory transfer so the units show as Incoming.

That last step is what stops a double order next Monday. Incoming that never gets recorded looks like a hole, and the next review buys the same units again. After you receive the shipment, confirm Available increased by the quantity you accepted, not by the quantity on the PDF.

When to leave the spreadsheet for early stock alerts

Stay on the weekly review while you can see every A variant before it crosses the reorder point. Move to earlier alerts when any of these are true: an A variant can sell out inside the 7 days between reviews, lead time is longer than the review cycle and you keep discovering that late, ad spend is scaling a variant faster than Monday's sheet, or the catalog is too large to finish the review in one sitting.

Shopify Flow can send a low-stock notification. That is a threshold alert, not a lead-time calculation. A fixed alert at 10 units is late for a variant that sells 4 a day with a 21-day supplier, and early for a variant that sells 1 a week. Set any Flow threshold from days of cover, not from a single unit count you copy across the catalog. How those alerts differ from a reorder plan is covered in Shopify inventory alerts.

If bestsellers are crossing the reorder point between reviews, continuous stock monitoring is the next operating step. Stock intelligence watches inventory and flags reorder risk from demand and lead time, so the weekly review starts from a ranked list instead of a full export. The buy decision stays yours: confirm Available, trusted incoming, and the cash cap before the purchase order goes out.

This week, pull the A-grade variants and write four numbers next to each one: Available, daily sales, lead time, and days of cover. Order the ones whose cover is already inside lead time plus buffer. Leave the rest until the next review.