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Spare Parts Reorder Calculator - Safety Stock, ROP & EOQ for MRO

Calculate Reorder Points and Safety Stock for Slow-Moving and Fast-Moving Maintenance Spare Parts

Free MRO spare parts inventory calculator for maintenance planners, storeroom managers, and reliability engineers. Enter annual demand, lead time, unit cost, and service level target, and the calculator determines the reorder point (ROP), safety stock, and economic order quantity (EOQ). Uses Poisson distribution for slow-moving items (fewer than 10 uses per year) and normal distribution for fast-movers. Shows the total inventory cost including holding cost, ordering cost, and stockout risk.

Pro Tip: The biggest inventory mistake in maintenance storerooms is treating all parts the same. A $5 V-belt and a $5,000 motor control board have completely different demand patterns, lead times, and stockout consequences. The V-belt is a fast-mover with predictable demand; stock it using normal distribution ROP with a 90% service level. The motor control board might fail once every three years with a 12-week lead time; use Poisson distribution with a 98%+ service level because the consequence of being out of stock is a $50,000 production loss. Always segment your inventory by criticality and demand velocity before applying any formula.

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Start Using This Tool See How It Works

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Spare Parts Reorder Calculator
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How It Works

  1. Enter Demand Data

    Input the average annual demand (usage) for the part. For parts consumed during PM tasks, multiply the PM frequency by the quantity per task. For failure-driven replacements, use historical consumption data from your CMMS work order history.

  2. Set Lead Time

    Enter the supplier lead time in days or weeks. Use the realistic lead time, not the best-case number. If the vendor says 2-4 weeks, use 4 weeks. If you need to include internal procurement processing time, add that to the vendor lead time.

  3. Specify Service Level

    Choose the target service level (probability of not stocking out during a replenishment cycle). Typical values: 90% for non-critical consumables, 95% for important maintenance parts, 98-99% for critical spares where stockout means production loss.

  4. Enter Cost Parameters

    Input the unit cost of the part, the annual holding cost rate (typically 20-30% of part value for industrial storerooms), and the cost per purchase order (administrative cost of placing an order, typically $50-150 in industrial settings).

  5. Review Results

    See the calculated reorder point, safety stock quantity, economic order quantity, and total annual inventory cost breakdown (holding + ordering + expected stockout cost). The calculator indicates whether Poisson or normal distribution was used based on demand velocity.

Built For

  • Maintenance planners setting min/max levels in CMMS for PM-driven consumable parts like filters, belts, and seals
  • Storeroom managers optimizing safety stock for slow-moving critical spares like specialty bearings and control boards
  • Reliability engineers calculating optimal stocking levels for insurance spares on critical equipment
  • Procurement specialists determining economic order quantities to balance holding costs and ordering costs
  • Plant managers justifying storeroom inventory investment to finance departments with quantified stockout risk
  • CMMS administrators building automated reorder triggers based on calculated ROP values
  • Maintenance supervisors evaluating vendor-managed inventory proposals against internally optimized stocking levels

Features & Capabilities

Dual Distribution Model

Automatically selects between Poisson distribution (for slow-moving items with fewer than 10 annual demands) and normal distribution (for fast-movers with 10+ annual demands). Slow-moving spare parts do not follow normal distribution assumptions, making Poisson essential for accurate safety stock.

Reorder Point Calculation

ROP = average demand during lead time + safety stock. For normal distribution: safety stock = z-score times standard deviation of demand during lead time. For Poisson: safety stock is derived from the cumulative Poisson probability function at the target service level.

Economic Order Quantity

Classic EOQ formula: sqrt(2 times annual demand times ordering cost / annual holding cost per unit). Shows the order quantity that minimizes total annual inventory cost (holding + ordering). Includes adjustments for minimum order quantities and price breaks.

Total Cost Breakdown

Displays annual holding cost, annual ordering cost, and expected annual stockout cost as separate line items. This makes it clear whether the dominant cost driver is carrying too much inventory or ordering too frequently.

Criticality Adjustment

Select criticality level (A/B/C) to automatically adjust service level targets. Critical-A parts default to 98% service level, important-B parts to 95%, and routine-C parts to 90%. Override manually for specific situations.

Lead Time Variability

Optionally enter lead time standard deviation to account for unreliable suppliers. Variable lead times significantly increase safety stock requirements and are a common source of stockout when ignored.

Frequently Asked Questions

Normal distribution works well for fast-moving items where demand is roughly continuous and symmetric around the mean. Poisson distribution is appropriate for slow-moving items where demand is discrete, intermittent, and lumpy. A part used 50 times per year can be modeled as normal. A part used 2 times per year follows a Poisson pattern because the demand in any given month is either 0 or 1, never negative, and not symmetric. Using normal distribution for a 2-per-year part produces safety stock values that are too low because normal distribution allows for fractional and negative demand, which does not happen in reality.
Industrial storerooms typically use 20-30% of part value per year as the holding cost rate. This includes the cost of capital (opportunity cost of money tied up in inventory, typically 8-12%), storage space and utilities (3-5%), insurance and taxes (2-3%), obsolescence risk (3-8%), and handling and tracking labor (2-4%). For critical spares that sit on the shelf for years, obsolescence risk is higher. For fast-moving consumables, obsolescence is minimal but handling cost per unit is higher due to frequent transactions.
Parts with no demand history are insurance spares: you stock them not because they are consumed regularly but because the consequence of not having one when needed is severe. For these parts, do not use statistical reorder models. Instead, stock one unit if the lead time to procure one exceeds your acceptable downtime window, and verify periodically that the part is still available from the manufacturer. If the equipment is truly critical, stock two units to cover the possibility of a defective replacement part.
For parts where a stockout causes immediate production loss or safety risk, target 98-99% service level. For important but non-critical parts, 95% is reasonable. For routine consumables where a brief delay is acceptable, 90% balances inventory cost against risk. The optimal service level depends on the stockout cost relative to the holding cost. If a stockout costs $50,000 in lost production and the part costs $200, even a 99.5% service level is justified because the expected stockout cost dwarfs the holding cost of an extra unit on the shelf.
Review stocking levels annually for most parts, and quarterly for high-value or critical spares. Triggers for immediate review include: significant changes in equipment utilization (more or fewer operating hours), vendor lead time changes (new supplier, discontinued part), or a pattern of recent stockouts or excess inventory. If you change PM frequencies, update the demand forecast for all parts consumed during those PMs. Many CMMS systems can automate demand tracking and flag parts where actual consumption deviates significantly from the forecast used to set ROP.
Disclaimer: This calculator provides inventory parameter estimates based on statistical demand models. Actual stocking decisions should consider equipment criticality, supplier reliability, obsolescence risk, and operational context. For safety-critical spares, consult with reliability engineers and operations management. ToolGrit is not responsible for inventory management decisions or stockout consequences.

Learn More

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MRO Spare Parts Inventory: Reorder Points, Safety Stock, and Storeroom Strategy

How to calculate reorder points and safety stock for maintenance spare parts, handle slow-moving items with Poisson distribution, and build a storeroom strategy that balances cost against stockout risk.

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