From the operations archive

One stock number was telling three different stories

A growing beauty and accessories seller was discussing purchases, current stock, expected demand and the possibility of regional warehousing. Read message by message, the business seemed well supplied. When the thread was reconstructed by product, some units were already reserved, some were physically sellable, and some existed only as supplier or inbound commitments. Those are not interchangeable forms of stock.

Ordering from the largest number could delay replenishment until a fast-moving SKU ran out. Ordering from the sales forecast alone could lock cash into slower variants. The lead time was also a chain: supplier confirmation, production or domestic delivery, receiving, inspection and release. A single optimistic number would make the calculator precise-looking and operationally false.

The team separated usable stock, reserved demand and confirmed inbound supply, then compared recent paid demand with the full replenishment window and a deliberate buffer. The reorder point became a trigger for review, not an automatic purchase command. Its value came from exposing assumptions: if demand, lead time, defect rate or inbound confidence changed, the decision changed visibly too.

Reality check: This is a privacy-safe composite drawn from reviewed operations conversations. Markets and product families are broadened, and identifying details are removed. Read the dated field note: A quiet day — and writing out the whole story for one client.

The basic formula

A common starting point is: reorder point = average daily unit sales × replenishment lead time in days + safety stock. Shopify uses the same structure in its reorder-point guidance. The output is the available-stock level at which a new replenishment decision should be triggered.

Example

Suppose one variant sells an average of 8 units per day. Production and movement to the fulfilment warehouse take 18 days in total, and the seller chooses 40 units of safety stock.

Lead-time demand: 8 × 18 = 144 units.
Safety stock: 40 units.
Reorder point: 144 + 40 = 184 available units.

When that SKU reaches 184 genuinely available units, the planned reorder should be released. This is not a prediction that exactly 184 units will sell; it is a trigger designed to cover normal demand during replenishment plus a chosen buffer.

Use the complete lead time

For China fulfilment, replenishment may include supplier confirmation, material or production time, domestic movement, receiving, QC and release into sellable stock. Count the time until units become available, not merely the factory's production estimate.

Choose a safety-stock method that fits the evidence

Simple buffer: a fixed number of days of average sales.
Variability buffer: extra units based on recent demand spikes and late supplier performance.
Event buffer: a deliberate temporary increase before a campaign or seasonal period.

New products have little sales history. Use a small staged commitment, a downside plan and frequent review instead of pretending a precise formula can remove uncertainty.

Calculate by SKU, not product family

A product may have plenty of total stock while its best-selling size or colour is almost gone. Calculate sales velocity, lead time and available units for each variant that must be picked separately. Component stock can also constrain a bundle even when the main item is abundant.

Use the right stock number

Shopify distinguishes available, committed, unavailable, incoming and on-hand stock. The reorder trigger should not treat damaged, QC-held, reserved or merely incoming units as currently available. Show incoming purchase orders separately and confirm their expected release date.

Review the inputs

Weekly for fast-moving, launch or campaign SKUs.
After a supplier, material, packaging or route change.
When lead-time performance or demand variability changes materially.
Before major holidays or marketing events.

Also decide the reorder quantity separately. The trigger says when to act; order quantity must balance MOQ, cash, storage, shelf life, version risk and expected demand.

Calculate the trigger from usable inventory

A reorder point is useful only when its inputs match the physical workflow and the exact SKU. Start simple, record actual lead times and sales, then improve the buffer with evidence.

Interactive worksheet

Calculate the reorder trigger for one SKU

Use one variant at a time. The tool uses only the figures you enter, runs in this page and submits nothing while you type. It separates the reorder point from the stock position you compare against it.

Formula and inventory-state boundary: Shopify’s reorder-point guide and Shopify’s inventory-state definitions. This is not a shipping-eligibility check; batteries, liquids, powders, magnets and other restricted goods still need route confirmation.

Operating rule: Set the reorder trigger by SKU from verified sales pace, production and inbound lead time, a realistic buffer and sellable stock already available. Recalculate when demand or lead time changes; one store-wide number hides the variant that will stock out first.

A Practical Reorder-Point Calculator for China Fulfilment