Fixed Period Interval
How is a fixed period interval different from a reorder point system?
Fixed period interval is time-triggered, reviewing inventory on a set schedule; a reorder point system is inventory-triggered, ordering when stock drops to a predetermined level.
What drives the order quantity in fixed period ordering?
The gap between the target inventory level and the inventory position at review, adjusted for demand expected during the protection period (review interval plus lead time) and safety stock.
Why is safety stock usually higher in fixed period review than in continuous review?
Demand can occur unnoticed between scheduled reviews, so safety stock must absorb both demand variability and lead-time uncertainty across a longer exposure window.
A fixed period interval is an inventory control method where stock is reviewed and replenished at predetermined time intervals, such as weekly or monthly, rather than continuously. The order quantity varies based on current inventory position, expected demand during the protection period (review interval plus lead time), and safety stock.
On the shop floor, fixed period interval ordering suits raw materials, MRO supplies, and supplier-managed items where continuous tracking is impractical. Receiving teams typically count inventory just before the scheduled review date, reconcile system quantities against physical stock, and release purchase orders to restore a predetermined target level. For example, a plant reviews fasteners every Friday; with a target of 10,000 units, 3,200 on hand, and expected consumption of 4,500 before the next receipt, procurement orders 6,800 units. This cadence aligns with supplier order windows and reduces daily ordering workload, but it shifts risk into the time between reviews. The protection period must therefore include both review interval and lead time, forcing the target level to carry enough safety stock for demand spikes and delivery variability. When applied to line-side supermarkets or raw-material staging, it prevents over-ordering while keeping production flowing.
Stockout between reviews: When demand accelerates after the review date, inventory is not checked again until the next fixed interval, so the item can hit zero before replenishment triggers, stopping A-critical lines and causing schedule misses.
Shorts on receipts: If planners calculate the order only from current on-hand and ignore demand during both review period and lead time, the order arrives too late or too small, creating chronic expedites and repeated partial supplier fills.
Overbuffering and aging stock: Inflating safety stock to compensate for long review intervals or volatile demand raises carrying cost, creates obsolescence, and congests pallet space and line-side supermarkets.