How To Calculate A Reorder Point

A reorder point is a number on a stock card: when inventory falls to it, you place the next purchase order. It is the simplest piece of inventory control that actually works, and it exists because ordering too late and ordering too early are both expensive.

The logic is a race between two clocks. The first is how long a supplier takes to deliver, which is the lead time. The second is how fast you consume the product. If you order when you have exactly enough stock to cover the lead time, the delivery arrives just as the last unit is sold.

That is the first half of the formula: daily usage multiplied by lead time gives the demand you must survive while waiting. Forty units a day across a seven day lead time is two hundred and eighty units, and if stock drops below that before the order lands, you run out.

The second half is the buffer. Demand is never perfectly steady, and lead times are not perfectly reliable either. Safety stock is the extra cushion that absorbs a busy week or a late delivery without a stockout, and without it the reorder point is a knife-edge calculation that fails the first time anything goes wrong.

A simple way to size the buffer is to multiply daily usage by a chosen number of safety days. Five days of cover at forty units a day is two hundred units. That number is a business decision as much as a mathematical one: it trades holding cost against the cost of a stockout.

More sophisticated versions replace the flat buffer with a statistical one, using the standard deviation of demand and of lead time and a service level target. That is the right approach for high-value items, but for most small businesses a sensible number of safety days captures most of the benefit.

The reorder point is not the same as the order quantity. The reorder point tells you when to order; the order quantity tells you how much. They are separate decisions, and the quantity is usually set by an economic order quantity calculation that balances ordering cost against holding cost.

It is also worth distinguishing the reorder point from the minimum stock level. Many systems keep both: a minimum that triggers a warning and a reorder point that triggers an order. If the two are the same number, the warning arrives at the same moment as the action and adds nothing.

Days of cover is a useful companion figure. Divide the reorder point by daily usage and you get how many days of stock the trigger represents. In the default example four hundred and eighty units at forty a day is twelve days, which is the seven day lead time plus the five day buffer, as it must be.

Seasonal businesses need to revisit the inputs rather than the formula. A daily usage figure averaged across the whole year will trigger orders far too late in the peak season and far too early in the trough. Recalculate the reorder point per season, or at least per quarter.

Finally, the reorder point only works if the stock figure it is compared against is accurate. Cycle counting, disciplined goods-in posting and prompt recording of returns and damages matter more than any refinement of the formula, because a trigger evaluated against wrong stock data fires at the wrong time.