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Understanding How Material Requirements Planning (MRP) Works: An Insightful Guide with Example


Material Requirements Planning (MRP) is a vital component in the manufacturing sector, designed to answer three fundamental questions: What is needed? How much is needed? And when is it needed? By addressing these questions, MRP helps manufacturers ensure that materials and products are available for production and delivery to customers without maintaining excessive inventory levels. Let's delve into how MRP works and illustrate its functionality with a practical example.


The MRP Process: A Step-by-Step Breakdown

1. Demand Estimation: MRP starts with the identification of demand for the final product. This can come from customer orders (actual demand) or forecasts (predicted demand), providing the basis for all subsequent planning.


2. Bill of Materials (BOM): A BOM is a comprehensive list of raw materials, components, and assemblies needed to manufacture a product. It details the quantity of each item and the relationship to the final product.


3. Inventory Status Records: This step involves reviewing current inventory levels, including on-hand stock, quantities already ordered but not yet received, and the safety stock level, which is a buffer against unforeseen variances in demand or supply.


4. Lead Times: MRP calculates the lead times for acquiring materials and completing production processes. This ensures that materials are ordered and production is scheduled to meet demand deadlines.


5. Material Scheduling: Based on demand, BOM, inventory status, and lead times, MRP generates a detailed schedule of when each component or material must be ordered or produced.


MRP Output

The output from an MRP system typically includes three key reports:


  • Purchase Orders: Recommendations for order quantities and timings for raw materials and components.
  • Work Orders: Instructions for the production schedule, detailing what product to produce, and when.
  • Exception Reports: Notifications of any discrepancies that need attention, such as delays or shortages.


MRP in Action: A Simplified Example

Imagine a company, ABC Manufacturing, produces bicycles. A large order for 100 bicycles is due in six weeks.


Component Total Requirement Current Inventory Lead Time (Weeks) Additional Order Needed Order Placement Time (Before Due Date)
Frame 100 80 4 20 4 weeks
Tires 200 150 2 50 2 weeks
Wheels 200 180 2 20 2 weeks
Chain 100 120 1 0 N/A
Seat 100 95 3 5 3 weeks


Here's how MRP helps ABC Manufacturing meet this order:


  • Demand Estimation: The demand is clearly defined as 100 bicycles.
  • Bill of Materials (BOM): Each bicycle requires 1 frame, 2 tires, 2 wheels, 1 chain, and 1 seat. Thus, the total requirements are 100 frames, 200 tires, 200 wheels, 100 chains, and 100 seats.
  • Inventory Status Records: The current inventory shows 80 frames, 150 tires, 180 wheels, 120 chains, and 95 seats.
  • Lead Times: The lead time for frames is four weeks, for tires and wheels is two weeks, for chains is one week, and for seats is three weeks.
  • Material Scheduling: MRP calculates that to meet the order, an additional 20 frames need to be ordered four weeks before the due date, 50 tires and 20 wheels two weeks before, and 5 seats three weeks before. No additional chains are needed since the inventory exceeds the requirement.

Through this example, it's evident how MRP systems provide a structured methodology for managing production and material requirements. This ensures that materials are available for production and products are available for delivery to customers as efficiently as possible, all while keeping inventory costs low.


MRP in Reality: A Bit More Complicated

t's important to highlight that the example provided above is an oversimplification intended to illustrate the basic principles of Material Requirements Planning (MRP). In practice, MRP calculations and the overall process can be far more complex due to several factors:


  • Multiple Layers of BOMs: Real-world products often have complex Bills of Materials (BOMs) that can include numerous sub-assemblies and components, each with its own lead times, suppliers, and manufacturing processes. Managing these intricate relationships adds layers of complexity to MRP calculations.
  • Variable Lead Times: Lead times aren't always fixed. They can vary based on supplier capacity, seasonal demands, transportation issues, and other external factors. MRP systems must be dynamic enough to adjust to these changes to maintain production schedules.
  • Demand Fluctuations: Demand isn't static. It can fluctuate based on market trends, seasonality, promotions, and competitor actions. MRP systems need to continuously adjust to these changes to ensure that inventory levels are optimized without risking stockouts or overproduction.
  • Capacity Constraints: Manufacturing resources (such as labor, machinery, and work centers) have finite capacities. MRP systems must consider these constraints to ensure that production plans are feasible and do not exceed the available capacity.
  • Inventory Policies: Companies have different inventory policies and strategies (e.g., Just-In-Time, safety stock levels, and order quantity optimizations) that influence how inventory is managed. MRP must align with these policies to ensure that inventory levels are optimized across the supply chain.
  • Supplier Reliability and Quality Issues: MRP relies on the assumption that materials ordered from suppliers will arrive on time and meet quality standards. In reality, delays and quality issues can occur, necessitating adjustments to the production schedule and material orders.
  • Product Lifecycle Changes: Products undergo changes throughout their lifecycle, including modifications, updates, and phase-outs. Managing these changes within the MRP system can be challenging, especially when balancing new requirements with existing inventory.





While the example of ABC Manufacturing provides a foundational understanding of how MRP works, it's crucial to acknowledge the complexities involved in implementing MRP in a real-world context. MRP systems are powerful tools that can significantly improve efficiency and reduce costs, but they require careful management, flexible planning capabilities, and the ability to adapt to the dynamic nature of supply chains and manufacturing environments.

Thomas Meersseman
by Thomas Meersseman

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