Optimal Stock Calculator for Critical Spare Parts
Calculate how many units of each critical spare part you should keep in stock and when to place the next order.
How is optimal stock calculated?
Formulas: Cycle stock = Monthly consumption x (Lead time / 30), Safety stock = Z x Deviation x √(Lead time / 30), Optimal stock = Cycle stock + Safety stock.
The Z value depends on the desired service level: 1.28 for 90%, 1.65 for 95% and 2.33 for 99%. The higher the required service level, the higher the safety stock needed to avoid stockouts.
Frequently asked questions — Optimal Spare Parts Stock
What is safety stock and why is it necessary?
Safety stock is the additional quantity of spare parts kept in the warehouse to cover unexpected variations in demand or supplier delays. Without safety stock, any deviation from average consumption or expected lead time can cause a stockout and a production stoppage.
How does service level influence safety stock?
A 99% service level requires more safety stock than a 90% level, because it means covering a larger percentage of variable demand scenarios. For critical spare parts that can stop an entire line, a 95-99% service level is recommended; for less critical consumables, 90% is usually sufficient.
What is the reorder point and how is it used?
The reorder point is the stock level at which a new order must be placed with the supplier so it arrives before inventory runs out. It is calculated as expected consumption during the lead time plus safety stock. When available stock reaches that level, it is time to replenish.
What if I do not know the standard deviation of demand?
If you do not have enough historical data to calculate the standard deviation, you can manually enter an estimated safety stock based on the maintenance team experience, or apply a conservative rule of thumb (e.g., 20-30% of consumption during lead time) until you accumulate enough history.
Query the consumption history of every spare part in seconds
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