When Should You Reorder Stock? A Practical Guide for eCommerce Sellers

Having stock on the shelf does not necessarily mean there is time to wait. The question is whether your available units can cover expected demand until the next delivery arrives, with a buffer for uncertainty.

This guide walks through a worked example using the same inputs as my Stock Reorder Calculator. All products, prices and quantities below are illustrative.

Start with available stock, not just the warehouse total

Suppose you have 100 units of Camera A in the warehouse, but 20 are already reserved for orders. That leaves 80 units available for new demand:

Available stock = on-hand stock − reserved units

100 − 20 = 80 units

Use consistent units throughout the calculation. If sales are counted as individual cameras, enter stock and incoming quantities as individual cameras too. Carton size is a separate input used when rounding the purchase quantity.

Calculate the reorder point

A reorder point is a stock threshold for reviewing replenishment. For a simple constant-demand example with no existing incoming delivery, use:

Reorder point = average daily sales × supplier lead time + safety stock

This baseline formula is also described in Shopify’s reorder-point guide. It is a starting model; incoming orders and changing demand require a closer look at timing.

For Camera A, assume:

• Average daily sales: 5 units.
• Supplier lead time: 14 days.
• Safety stock: 20 units.
• Available stock: 80 units.
• Existing incoming deliveries: none.

Expected demand during the lead time is 5 × 14 = 70 units. Adding the safety buffer gives a reorder point of 90 units.

With 80 available units, the product has already crossed that threshold. Under these assumptions, it needs attention now, even though stock has not run out.

Days of cover is useful, but it is not the whole decision

Current days of cover = available stock ÷ average daily sales

80 ÷ 5 = 16 days

Sixteen days of stock may sound comfortable beside a 14-day lead time. But after 14 days of expected sales, only 10 units remain. That is below the chosen 20-unit safety buffer.

For comparison, if available stock were 100 units instead of 80, the same example would reach the 90-unit threshold in about two days. Those two figures refer to available stock; avoid accidentally subtracting reservations twice.

Safety stock is an input you choose for the planning scenario. The calculator does not estimate an optimal buffer from demand variability or guarantee a service level. Review the buffer when your sales pattern or supplier reliability changes.

Decide how much to order

The reorder point addresses timing. Order quantity is a separate calculation.

In the calculator, “target cover after lead time” means the additional days of demand you want to cover beyond the supplier lead time. Set it to 30 days for this example:

Target stock for the planning horizon = 5 × (14 + 30) + 20 = 240 units

With 80 available units and no incoming delivery, the unrounded requirement is:

240 − 80 = 160 units

Now account for purchasing constraints. Suppose the supplier’s minimum order is 24 units and cartons contain 12 units. The requirement exceeds the minimum, but 160 is not a whole number of cartons. Round up to 14 cartons:

14 × 12 = 168 units

At an illustrative unit purchase price of £15, the planned goods cost is £2,520. This excludes tax, shipping and other fees. It also assumes the demand estimate remains suitable throughout the planning horizon.

This is a target-cover planning approach, not an economic order quantity model. Check storage capacity, shelf life and the supplier’s actual terms before turning a suggestion into a purchase order.

Check when incoming stock arrives

An incoming quantity alone cannot show whether it prevents a shortage. Its arrival time matters.

Consider a different product with 20 available units, expected sales of four units per day and a normal replenishment lead time of 10 days. An existing delivery of 40 units is due in eight days.

The current stock covers five days. The existing delivery arrives after that stock would be exhausted. A new order placed today on the normal 10-day lead time would arrive later still. Adding the 40 incoming units to today’s available balance would conceal the gap.

That situation calls for a separate operational action: investigate an earlier delivery, a transfer or a change to the customer promise. A larger normal-speed purchase does not resolve the earlier timing problem.

The calculator supports one incoming delivery per product, entered as units and “due in days”. Zero means today, before demand. If you have multiple shipments, review their dates separately rather than combining them into one invented arrival date.

Read the status and quantity together

The calculator separates urgency from the quantity needed for your chosen target cover:

• Stockout risk: projected demand exhausts stock before a new normal-lead-time order could arrive.
• Order now: the lead-period projection reaches or falls below the chosen safety stock.
• Stock sufficient: the lead-period projection remains above the safety threshold.
• Review demand: daily demand is zero and the assumptions need review.

A “Stock sufficient” product can still show a positive target-cover quantity. Treat that as a planning scenario, not an instruction to buy immediately. With zero demand, a quantity may simply reflect a safety-stock shortfall; confirm the business reason before ordering.

Try the example in the calculator

Open the Stock Reorder Calculator and use these Camera A inputs:

• On hand: 100; reserved: 20.
• Daily sales: 5; lead days: 14; safety units: 20.
• Incoming units: 0; due in days: 0.
• Minimum order quantity: 24; carton size: 12; unit cost: 15.
• Target cover after lead time: 30 days; currency: GBP.

The expected results are 80 available units, 16 days of current cover, a 90-unit reorder point, an “Order now” status and a suggested purchase of 168 units costing £2,520 before extra charges.

You can compare several products and export the buying plan as CSV. To reuse your inputs later, export the input file before closing or refreshing the page. The tool does not save your inventory or connect to a marketplace account.

Keep the assumptions visible

Review the expected daily sales, reservations and supplier dates before relying on a result. The model uses constant demand and carries projected deficits forward; it does not separately simulate lost sales, seasonal peaks, uncertain delivery dates or multiple future shipments.

The useful outcome is a decision you can explain: which product needs attention, when stock is likely to reach a threshold and what quantity fits the chosen coverage and carton rules.

For another example of turning repetitive marketplace work into a practical tool, see my eBay pricing and SKU automation case study. You can also explore all tools.