How do you learn business reorder points quickly and correctly when a shop sometimes runs out of stock even though its last purchase seemed large enough? Start with the question how much demand might occur while the business is waiting for more stock to arrive? A reorder point is a stock or inventory-position threshold that prompts a replenishment decision. This eduKateSG Business Quick Notes lesson shows you how to calculate expected demand during supplier lead time, understand an illustrative safety-stock buffer and avoid double ordering when goods are already on the way. You will learn to use quantities and dates together rather than copying a fixed stock number from another business.
- Choose one SKU: do not mix different sizes or colours of stock.
- Estimate demand: use a clear daily or weekly consumption assumption.
- Know lead time: measure the relevant time from placing or accepting the order to usable receipt.
- Choose a buffer: understand the purpose and cost of safety stock.
- Check inventory position: review on-hand stock, open orders and outstanding customer demand.
- Decide and monitor: calculate a trigger, then check whether actual shortages and excess stock improve.
Teaching step 1 — calculate lead-time demand. Imagine a shop uses an average of five A5 notebooks per day and the supplier normally requires four days between the relevant order event and usable receipt. Expected demand during that lead time is 5 × 4 = 20 notebooks. This is the starting requirement: stock must cover expected usage while replacements travel through the supply process. The number is only an estimate, because demand and delivery time may change. A weekend market, a supplier delay or a different product mix could make actual usage larger or smaller. Label the unit and period before multiplying.
Teaching step 2 — understand a simple safety-stock buffer. Suppose the manager chooses an illustrative buffer of eight notebooks to cover some uncertainty. The elementary reorder point is expected lead-time demand 20 + safety stock 8 = 28 notebooks. The eight-unit buffer is a hypothetical choice for learning, not a statistically derived recommendation. A real business should use relevant demand and lead-time variation, desired service level, stockout cost and inventory-holding cost to determine the buffer. This is why a reorder point cannot simply be copied from a different shop. One business may need a higher service level because a missing item creates serious problems; another may carry costly stock that becomes obsolete.
Teaching step 3 — do not reorder blindly when a shipment is already coming. Suppose the shop has 18 usable notebooks on hand, another 15 notebooks on an outstanding purchase order, and seven notebooks in backorders that have not yet been fulfilled. Under a simple inventory-position definition, position is on hand 18 + on order 15 − backorders 7 = 26 notebooks. That is below the illustrative reorder point of 28, so it signals a replenishment review. But it does not automatically tell the buyer to order exactly two notebooks. The chosen order-up-to level, order minimum, supplier availability, reservations, other commitments and timing still matter. The aim is to recognise what is already committed or on its way instead of counting only the shelf quantity.
The five-number reorder point card
- Demand rate: expected units consumed per defined period.
- Supplier lead time: how many compatible periods replenishment is expected to take.
- Lead-time demand: demand rate multiplied by lead time in the simple model.
- Safety stock: an additional buffer, chosen using relevant uncertainty and costs.
- Reorder point: lead-time demand plus safety stock under this elementary calculation.
Make sure units match. If demand is measured per week and lead time is reported in days, convert one before multiplying. A shop using 35 notebooks per week with a four-day lead time should not calculate 35 × 4 = 140 notebooks without changing the units. Under an illustrative seven-day demand week, 35 per week is five per day, giving 20 units over four days.
Reorder point and order quantity are different
The reorder point helps answer when should the business consider replenishment? Order quantity answers how many should it buy? Those are separate decisions. A shop may reorder when inventory position falls to 28, but choose to order 50 units because its chosen order-up-to level, supplier minimum or purchase cycle requires a larger amount. Ordering two units just because inventory position is two below the threshold may leave the business immediately below the same trigger after placing the order, or conflict with realistic order constraints.
For a beginner, it is enough to understand that the trigger is a warning or decision point, not a shopping-cart quantity. Before approving a purchase, check current stock, confirmed demand, units already on order, storage capacity and available cash.
Stock on hand and inventory position
- On-hand stock: physically present units, subject to condition and relevant status.
- On-order stock: units expected from authorised but unreceived supply orders.
- Backorders: outstanding customer demand that could not yet be fulfilled under the chosen system.
- Inventory position: on-hand plus on-order less backorders in the simple model.
- Reserved stock: units committed to specific orders, which require consistent handling in the stock system.
Be careful not to deduct the same commitment twice. Some software defines available-to-promise separately from inventory position. A reserved product still on the shelf may not be available for new sales, but the treatment of reservations and backorders depends on the system’s rules. State exactly which obligations you have subtracted. For real operations, follow the organisation’s documented inventory policy.
How stockouts can hide the true demand
Suppose a shop sells ten notebooks on Monday but runs out at noon. The recorded sales count is ten; that does not prove only ten people wanted notebooks that day. Some customers may have left without buying. A demand estimate based only on observed sales during stockouts can understate actual demand. Useful additional records may include unfulfilled orders, lost-sale reports or appropriately collected customer enquiries. But do not invent lost demand simply because the shelves were empty.
When analysing historical sales, mark the dates when items were unavailable. Averages are easier to interpret if you know whether the business could actually meet orders. A single unusually busy day also should not be treated as the standard demand rate without context.
Lead time must end with usable stock
A supplier may say “dispatch in two days” while delivery and receiving inspection require another three. If the shop needs stock ready to sell, the relevant replenishment lead time may be the time until accepted usable stock is available, not just until a truck leaves the supplier. Confirm the definition. A shipment that arrives damaged or mismatched may not restore the expected inventory.
Delivery schedules can also be affected by weekends, public holidays, cut-off times or supplier capacity. A four-day lead time in one month may not be reliable for a festival week. Review real supplier performance and distinguish confirmed commitments from historical averages.
What a buffer costs
Extra safety stock can reduce the chance of a shortage but uses money, storage and handling capacity. Suppose a buffer rises from eight to 18 notebooks and each notebook has a purchasing cost of S$4. The extra ten units tie up an additional S$40 in purchasing cost, before any storage or financing effects. That is not a complete annual holding-cost calculation. It shows why a larger buffer is not free and why safety stock should be chosen using the business’s service requirements and risks.
For products that expire, change in style or require controlled storage, carrying excess stock can be particularly costly. High-value or slow-moving products may need a different review method from inexpensive, regularly used stationery. The formula is a starting point, not a substitute for responsible inventory policy.
Your twelve-minute reorder point lesson
- Minutes 1–2: select one product code and define the demand unit.
- Minutes 3–4: calculate average demand per compatible day or week.
- Minutes 5–6: multiply by the relevant lead time.
- Minutes 7–8: add an illustrative buffer and label the assumption.
- Minutes 9–10: calculate inventory position using stock, open supply and backorders.
- Minutes 11–12: explain one reason the business might increase or decrease its buffer.
Quick practice: a supplier needs three days
A shop expects to use six pens each day, and replenishment takes three days under the chosen definition. It selects a temporary buffer of nine pens for this exercise. What is expected lead-time demand and the simplified reorder point? If inventory position is 25 pens, is the threshold reached?
Answer: Lead-time demand = 6 × 3 = 18 pens. Reorder point = 18 + 9 = 27 pens. With inventory position at 25, the business has crossed below the 27-pen review trigger. That does not by itself determine the order quantity or prove nine pens is an appropriate real-world safety-stock level.
Common reorder mistakes
- Multiplying weekly demand by days: make the units compatible.
- Ignoring units already on order: check inventory position.
- Subtracting the same reserved orders twice: follow a consistent stock definition.
- Assuming reorder point equals order size: separate the two decisions.
- Using sales during shortages as complete demand: examine availability.
- Choosing a buffer without considering costs: compare service needs and holding expense.
Check your understanding
1. Four units per day and five-day lead time: expected demand? 20 units. 2. Add six illustrative safety units: reorder point? 26 units. 3. Does crossing the threshold tell you exactly how many units to buy? No. Order quantity needs its own decision rule.
Continue the Business Quick Notes series
Start with Business Fundamentals, then read Business Inventory Management, Business Inventory Reconciliation, Business Forecasting, Business Supply Chain Management, Business Procurement, Business Cash Flow and Business Order Fulfilment.
For broader operational planning, consult the U.S. Small Business Administration business guide. All demand and buffer quantities here are hypothetical learning examples.
Remember: Reorder points start with lead-time demand and uncertainty. Check inventory position, not just the shelf, and separate the reorder trigger from the purchase quantity.