Inventory is not one number. Physical stock can be committed, unavailable, incoming, or ready to sell. Replenishment decisions add a second clock: how fast demand consumes stock versus how long supply takes to arrive. Separate those states before changing purchase plans.
🧭 Jump to a term
Product identity and stock states: Stock keeping unit (SKU) · On-hand inventory · Available inventory · Allocated inventory · Available to promise (ATP) · Stockout · Backorder · Preorder
Replenishment decisions: Reorder point · Reorder quantity · Safety stock · Inventory turnover · Days inventory outstanding (DIO)
Inventory control and accuracy: Dead stock · Cycle count · Inventory shrinkage · Inventory accuracy
⭐ Know these first
Start with Stock keeping unit (SKU), Available to promise (ATP), Reorder point, Safety stock, Inventory turnover. Then follow the grouped learning order below.
📎 How to read this page
What it means gives the precise meaning. Operator translation gives the version you might hear in a real ecommerce meeting. In real life shows an illustrative example. Watch out and the confusion boxes show where a familiar term can mislead.
📈 Read the relationships first
These combinations are diagnostic hypotheses, not proof of causality. Compare the same period and scope, then investigate the mechanism.
On-hand stock + low available stock
Usually means: Means some units may be committed, unavailable, or otherwise ineligible. Check next: inventory state by location and variant.
Reorder point + long supplier lead time
Usually means: Can mean replenishment begins too late for demand during the lead time. Check next: demand variability and confirmed supply dates.
High inventory turnover + stockouts
Usually means: May indicate lean stock is reducing availability. Check next: service level, lost demand, and replenishment variability.
Inventory records + cycle count variance
Usually means: Can reveal receiving, picking, transfer, damage, or system process errors. Check next: Investigate before adjusting the balance.
Product identity and stock states
01 · 🟢 Core
Stock keeping unit (SKU) = Identifier for a distinct sellable item
🧠 What it means
An internal identifier used to distinguish a specific product or variant for inventory and order management.
💬 OPERATOR TRANSLATION
“The code the business uses to tell one variant from another.”
🛍️ In real life
A shirt's size and color variants each have distinct SKUs in the store's catalog.
⚠️ SKU formats are business-defined and are not necessarily universal product identifiers.
🔗 Related: On-hand inventory · Available inventory · Allocated inventory · ↑ all terms
02 · 🔵 Operations
On-hand inventory = Physical units recorded at a location
🧠 What it means
The quantity of units a business records as physically present at a specified location.
💬 OPERATOR TRANSLATION
“What the system says is physically in the warehouse or store.”
🛍️ In real life
A stock count shows 120 units on the shelf and in the back room.
⚠️ On-hand can be stale or inaccurate until receiving and counts are reconciled.
🔗 Related: Stock keeping unit (SKU) · Available inventory · Allocated inventory · ↑ all terms
03 · 🔵 Operations
Available inventory = Stock eligible for new demand
🧠 What it means
The quantity of on-hand inventory available to sell after accounting for reservations, holds, or other business rules.
💬 OPERATOR TRANSLATION
“What can be promised to a new order right now.”
🛍️ In real life
A store has 120 on-hand units and 18 reserved; available stock is 102 before other holds.
⚠️ Systems vary in how they treat safety stock, damaged units, and channel allocations.
🔗 Related: Stock keeping unit (SKU) · On-hand inventory · Allocated inventory · ↑ all terms
04 · 🔵 Operations
Allocated inventory = Stock assigned to a purpose or demand
🧠 What it means
Units set aside for specific orders, channels, transfers, or other planned demand and not generally available to new orders.
💬 OPERATOR TRANSLATION
“Stock already promised or earmarked for a specific use.”
🛍️ In real life
A warehouse allocates 12 units to paid orders before calculating free-to-sell quantity.
⚠️ Allocation rules vary across systems and may include temporary reservations.
🔗 Related: Stock keeping unit (SKU) · On-hand inventory · Available inventory · ↑ all terms
05 · 🟢 Core
Available to promise (ATP) = Quantity that can be committed to customers
🧠 What it means
The quantity a business can promise by a date, based on eligible on-hand stock and expected supply or demand under its planning rules.
💬 OPERATOR TRANSLATION
“What can we confidently commit to customers by this delivery date?”
🛍️ In real life
A planner uses current stock and confirmed inbound supply to set a delivery promise for next week.
⚠️ ATP depends on timing, reservations, supply certainty, and the system's rules.
🔗 Related: Stock keeping unit (SKU) · On-hand inventory · Available inventory · ↑ all terms
06 · 🔵 Operations
Stockout = No available units to fulfill demand
🧠 What it means
A condition where no eligible inventory is available to meet current demand for an item or location.
💬 OPERATOR TRANSLATION
“The customer wants it, but sellable stock is gone.”
🛍️ In real life
A product page marks a color unavailable after all units are reserved or sold.
⚠️ A zero sellable balance can coexist with physical units that are damaged or held.
🔗 Related: Stock keeping unit (SKU) · On-hand inventory · Available inventory · ↑ all terms
07 · 🔵 Operations
Backorder = Order accepted for unavailable stock
🧠 What it means
An order or line item accepted when inventory is unavailable, with fulfillment expected after replenishment under the stated terms.
💬 OPERATOR TRANSLATION
“Take the order now, then ship when stock returns—if the promise is clear.”
🛍️ In real life
A customer orders a temporarily unavailable item with an estimated later ship date.
⚠️ Show expected timing and cancellation terms; don't imply immediate availability.
🔗 Related: Stock keeping unit (SKU) · On-hand inventory · Available inventory · ↑ all terms
08 · 🔵 Operations
Preorder = Order placed before regular availability
🧠 What it means
An order placed before a product is available for ordinary fulfillment, often before its release or first stock receipt.
💬 OPERATOR TRANSLATION
“Let customers reserve a future product before it is normally available.”
🛍️ In real life
A retailer accepts orders for a launch item and states its planned ship window.
⚠️ Make timing and uncertainty clear; preorder rules depend on market and policy.
🔗 Related: Stock keeping unit (SKU) · On-hand inventory · Available inventory · ↑ all terms
Replenishment decisions
09 · 🟢 Core
Reorder point = Inventory threshold that triggers replenishment
🧠 What it means
A stock level at which a business should initiate replenishment, commonly based on expected demand during lead time plus a buffer.
💬 OPERATOR TRANSLATION
“When available stock falls to this line, start the next supply action.”
🛍️ In real life
A planner triggers a purchase order when stock reaches expected lead-time demand plus safety stock.
⚠️ The threshold depends on demand, lead time, review cadence, and service target.
🔗 Related: Reorder quantity · Safety stock · Inventory turnover · ↑ all terms
10 · 🔵 Operations
Reorder quantity = Amount ordered or replenished
🧠 What it means
The quantity a business chooses to add when replenishing inventory, based on its plan, constraints, and replenishment policy.
💬 OPERATOR TRANSLATION
“How many units should the next replenishment bring in?”
🛍️ In real life
A buyer orders enough to cover the target stock level while respecting case-pack and supplier constraints.
⚠️ Reorder quantity is distinct from reorder point; one triggers, the other sizes replenishment.
🔗 Related: Reorder point · Safety stock · Inventory turnover · ↑ all terms
11 · 🟢 Core
Safety stock = Buffer inventory against uncertainty
🧠 What it means
Additional inventory held to reduce the risk of stockouts from variability in demand, lead time, or supply.
💬 OPERATOR TRANSLATION
“A buffer for demand and supply uncertainty, with a carrying cost.”
🛍️ In real life
A retailer holds extra units because supplier lead time varies during peak season.
⚠️ More buffer reduces some stockout risk but increases holding and obsolescence costs.
🔗 Related: Reorder point · Reorder quantity · Inventory turnover · ↑ all terms
12 · 🟢 Core
Inventory turnover = Cost of goods sold relative to average inventory
🧠 What it means
A measure commonly calculated as cost of goods sold divided by average inventory value over a stated period.
💬 OPERATOR TRANSLATION
“How quickly inventory value moves through sales under this accounting basis.”
🛍️ In real life
A retailer compares annual cost of goods sold with its average inventory investment.
⚠️ State the period and valuation basis; high turnover is not automatically better if stockouts rise.
🔗 Related: Reorder point · Reorder quantity · Safety stock · ↑ all terms
13 · 🔵 Operations
Days inventory outstanding (DIO) = Average days inventory is held
🧠 What it means
A measure estimating the average number of days inventory remains before being sold, often calculated from average inventory and cost of goods sold.
💬 OPERATOR TRANSLATION
“How many days of cost-based inventory are tied up on average?”
🛍️ In real life
A finance team estimates DIO using average inventory divided by cost of goods sold, multiplied by days in the period.
⚠️ Formula inputs and period must match; seasonal averages can mislead.
🔗 Related: Reorder point · Reorder quantity · Safety stock · ↑ all terms
Inventory control and accuracy
14 · 🔵 Operations
Dead stock = Inventory with little or no expected demand
🧠 What it means
Stock that is unlikely to sell at its current price or through its current channel within a relevant planning horizon.
💬 OPERATOR TRANSLATION
“Inventory taking space and cash without a realistic path to sale.”
🛍️ In real life
A discontinued accessory has not sold for months and is removed from replenishment plans.
⚠️ Slow-moving inventory is not always dead; account for seasonality and planned launches.
🔗 Related: Cycle count · Inventory shrinkage · Inventory accuracy · ↑ all terms
15 · 🔵 Operations
Cycle count = Repeated count of selected inventory items
🧠 What it means
An inventory-control process that counts a subset of items on a recurring schedule instead of counting all stock at once.
💬 OPERATOR TRANSLATION
“Count selected products regularly and reconcile differences as you go.”
🛍️ In real life
A warehouse counts high-value SKUs weekly and other items on a rotating schedule.
⚠️ Use a documented count method and investigate discrepancies before adjustments.
🔗 Related: Dead stock · Inventory shrinkage · Inventory accuracy · ↑ all terms
16 · 🔵 Operations
Inventory shrinkage = Recorded stock missing from physical inventory
🧠 What it means
The difference between recorded inventory and the quantity physically available, potentially due to loss, damage, errors, or other causes.
💬 OPERATOR TRANSLATION
“The system expects units that the count cannot find.”
🛍️ In real life
A store count is 94 units while the inventory system records 100, creating a six-unit variance to investigate.
⚠️ Shrinkage describes a variance, not its cause; investigate before assigning blame.
🔗 Related: Dead stock · Cycle count · Inventory accuracy · ↑ all terms
17 · 🔵 Operations
Inventory accuracy = Agreement between records and physical stock
🧠 What it means
The degree to which recorded inventory quantities match verified physical counts for the same items and locations.
💬 OPERATOR TRANSLATION
“How closely the system's number matches what is actually there.”
🛍️ In real life
A cycle count compares system records with a physical count by SKU and location.
⚠️ Define the tolerance and counting scope; aggregated accuracy can hide item-level errors.
🔗 Related: Dead stock · Cycle count · Inventory shrinkage · ↑ all terms
🔀 On-hand vs available inventory
On-hand is recorded physical quantity. Available is stock eligible for sale after reservations and other holds.
🔀 Allocated stock vs available-to-promise
Allocated stock is already assigned. ATP estimates what can be promised by a date using eligible stock and expected supply.
🔀 Reorder point vs reorder quantity
The point triggers replenishment; the quantity determines how much to replenish.
🔀 Inventory turnover vs days inventory outstanding
Turnover compares cost of goods sold with average inventory; DIO estimates how many days inventory is held using a related cost basis.
🤔 Still confused?
Follow this thread: Stock keeping unit (SKU) → Reorder point → Dead stock. That sequence moves from the basic object or relationship to the decisions and checks it supports.
Sources and scope
Primary documentation checked on 27 September 2026. Platform features and eligibility can change; examples and cartoon situations are illustrative.
