A reorder point is not the quantity to buy

A reorder point tells a business when its stock position calls for replenishment under a particular policy. It does not, by itself, tell the buyer how many units to order. The trigger, the order quantity and the date the goods become usable are separate parts of the plan.

Fifty is the trigger; eighty is the order

Consider an invented workshop using ten identical replacement cartridges each business day. At 8 a.m. on Monday it has fifty usable cartridges, no open supply orders and no unfilled customer demand. It uses cartridges during the day; stock left after each day's use carries forward. Nothing is used over the weekend.

An order placed at 8 a.m. Monday is expected to arrive ready for use at 8 a.m. Friday. The workshop therefore expects to use forty cartridges while waiting: ten on each of Monday, Tuesday, Wednesday and Thursday. It has separately chosen a ten-cartridge buffer.

Assume stock movements are monitored as they happen. For this example, the policy calls for an order when the stock position reaches or falls below fifty: forty expected uses during the wait, plus the ten-unit buffer. The selected replenishment quantity is eighty. That quantity is supplied as part of the policy, not calculated from the reorder point or claimed to be the cheapest order.

With no order outstanding, Monday's fifty cartridges meet the trigger. The workshop orders eighty. The trigger does not mean buying fifty or restoring the shelf to fifty. The fifty identifies the point at which the supplied policy calls for action.

Paul Schönsleben's inventory explanation at ETH Zurich separates replenishment lead time from batch size and describes safety stock as a buffer against variation. The time that matters ends when the goods are available to use, not when someone sends an order confirmation.

Follow the cartridges until Friday

Under the supplied demand, fifty cartridges become forty after Monday, thirty after Tuesday, twenty after Wednesday and ten after Thursday. If the eighty ordered units arrive as expected on Friday morning, usable stock rises from ten to ninety. Friday's ten uses leave eighty.

The ten-unit buffer was not an extra order. It was part of the stock already held when the order was placed. Nor did the eighty-unit delivery leave eighty immediately: ten units remained from the earlier stock when the shipment arrived.

An open order changes the stock position

Suppose a second buyer checks the same records just after Monday's order is confirmed, before any cartridges have been used. The shelf still holds fifty. But eighty more are now ordered for Friday morning.

For this policy, count those eligible incoming units alongside stock on hand: fifty plus eighty gives a stock position of 130. There is no unfilled demand or other adjustment in this example. The position is above the fifty-unit trigger, so another eighty-unit order is not called for merely because the shelf still holds fifty.

The accompanying order-point discussion includes scheduled receipts when assessing supply during the lead time. A relevant open order belongs in the check. A canceled order, an unsuitable item or a shipment due after the need cannot be treated as the same supply.

The buffer is not a delivery guarantee

Now suppose the Friday shipment does not arrive. The ten cartridges left after Thursday cover Friday's ten uses, leaving zero. If delivery is still missing when work resumes on Monday, the next ten uses cannot be supplied from this stock. The workshop needs an actual alternative before promising that work.

The original 130-unit position did not mean 130 cartridges were on the shelf. The missing shipment still needs investigation, even if it remains marked open. A higher-than-expected daily requirement could also use the buffer sooner.

This example does not calculate an optimal buffer, a service probability or an order size. Those require the business's demand, delivery variability, costs and service requirements. Here, the narrower check is whether the trigger, incoming order and usable-stock calendar agree.

What ten more cartridges can cover

Compare a second supplied policy for the same workshop. It still expects ten uses per business day and a four-business-day wait. It chooses a twenty-cartridge buffer instead of ten, so its trigger becomes sixty: forty expected uses plus twenty. The order quantity remains eighty.

This is an alternative starting position, not an extra ten cartridges appearing after the first order. Assume the workshop actually holds sixty usable cartridges at 8 a.m. Monday, with no open orders or unfilled demand. It places one eighty-unit order then. Its position after ordering is 140, not 130.

Suppose the shipment arrives ready for use at 8 a.m. the following Tuesday, before that day's work. Nothing else changes. The sixty cartridges fall to twenty after Thursday, ten after Friday and zero after the following Monday. The shipment arrives before Tuesday's next ten uses.

The original fifty-cartridge starting stock covers Friday but leaves Monday's ten uses without supply. The alternative sixty covers that additional business day. Ten more units buy one more day at the supplied rate; they do not make the supplier more reliable.

Now change daily use to twenty cartridges from the opening Monday, while keeping the alternative starting sixty and an expected Friday arrival. Monday leaves forty, Tuesday twenty and Wednesday zero. Thursday's twenty uses cannot be supplied. The larger buffer has not covered this different demand.

The extra ten cartridges also need funding and space. They had to be obtained and held before the order point was reached. No unit price, storage cost or expiry has been supplied, so this comparison cannot price that commitment or choose the better policy. It shows the particular interruption each stock position can cover.

Before increasing a buffer, name the delay or demand change it is intended to absorb. Run that change through the actual usage calendar. Keep the order quantity separate, and identify the supplier action or service change needed if the buffer runs out.

For one item, put the reorder trigger beside the quantity to buy. Then find the open orders and the date each can actually serve demand. The bulk-order comparison examines whether extra quantity has a useful destination. Ordering sooner and buying more are different choices.