What is a stockout? Difference from Backorder

Stockout

The simplest storefront symptom is "Out of stock." Operationally, the state can be more complicated because physical inventory, sellable inventory, reserved inventory, and location-specific availability are not always the same number. The most useful principle is:

stockout is the condition; backorder, preorder, waitlist, substitution, or hiding the product are responses.

Location can create a partial stockout

Suppose a merchant has:

• Singapore warehouse: 0 units
• Malaysia warehouse: 30 units

If the Malaysia location is not configured to fulfill the Singapore customer's order, the customer can see an out-of-stock state despite the company owning 30 units elsewhere.

Shopify's current inventory documentation describes this kind of location effect in its own fulfillment model: inventory at a location that does not fulfill online orders may not make the product available to an online shopper.

The general lesson is that stockout should be evaluated against the fulfillment network available to the order, not against a company-wide physical count alone.

What happens after a stockout is a business decision

Block purchase

The safest option when supply is uncertain. The product stays visible as unavailable or may be hidden.

Backorder

Accept an order now because replenishment is expected. This creates a future allocation and delivery promise.

Preorder

Appropriate when the item is not normally available yet, such as a future launch.

Waitlist or back-in-stock notification

Capture customer interest without accepting an order or payment promise.

Substitute

Offer a compatible alternative when product identity is flexible enough and the customer can choose.

Reroute fulfillment

Use another location if inventory, cost, delivery promise, and operational rules make that possible. A stockout page should not prescribe one response. The right choice depends on confidence in future supply and the promise the merchant is willing to make.

Stockout messaging should match the real next step

"Sold out" suggests something different from "Temporarily out of stock" or "Ships when restocked." Useful messages can communicate:

• temporary versus indefinite unavailability
• expected restock date when sufficiently reliable
• ability to join a notification list
• backorder availability
• substitute products
• location-specific pickup availability

The message should not invent precision. "Back next Tuesday" can be worse than a range if the supplier date is uncertain.

Lost sale versus deferred demand

A stockout does not automatically mean the sale is permanently lost. Some customers:

• leave and buy from a competitor
• wait for restock
• choose a substitute
• join a notification list
• place a backorder

That distinction matters when measuring impact. A back-in-stock conversion one week later is deferred demand, while a customer who immediately switches to another brand may represent a lost sale.

Stockouts can be caused by demand or by data

Not every stockout means the merchant failed to buy enough inventory. Possible causes include:

• demand spike
• supplier delay
• incorrect inventory count
• reservations not released after cancellation
• inventory at the wrong location
• sync failure between ERP and storefront
• safety-stock threshold
• overselling from concurrent orders

The response should therefore begin by identifying whether the shortage is physical, allocational, or informational.

Useful stockout measures

Operators can track:

• percentage of active SKUs out of stock
• sessions or product views exposed to stockouts
• estimated demand while unavailable
• back-in-stock notification signups
• conversion after restock
• substitution rate
• backorder quantity
• time from stockout to replenishment

A SKU count alone can be misleading. A stockout on the highest-traffic product is more consequential than one on a dormant long-tail item. A stockout is the point where current supply and eligible demand no longer meet. Good operations identify the cause, communicate the state accurately, and choose the next promise deliberately.

Worked example

A product has 25 units physically present.

• 15 are allocated to paid, unfulfilled orders
• 5 are reserved as safety stock
• 5 appear available

• A sync issue incorrectly reports 0 available to the storefront

From the customer's perspective, there is a stockout. From the warehouse perspective, there is not a physical shortage. The correct fix is data reconciliation, not emergency replenishment.

Now reverse the example: the storefront reports 5 available, but all 25 units are committed. That creates overselling instead of a visible stockout.

What passes and what does not

• A warehouse might contain 10 units that are:
• reserved for existing orders
• damaged or under inspection
• allocated to another sales channel
• at a location that cannot fulfill the current customer's order
• held as safety stock
• The item can therefore be unsellable to a particular shopper even when the business owns physical units
• Conversely, some stores intentionally allow selling below zero because a product can be backordered or produced later
• That is why inventory systems often separate concepts such as on-hand quantity, available quantity, reservations, and salable quantity