Material Requirements Planning (Mrp)
What Is Material Requirements Planning (MRP)?
Material requirements planning (MRP) is a method for calculating which materials and components are needed for production, how much is needed, and when they must be available. Used in manufacturing and supply chain management, it connects a production schedule with bills of materials, inventory records, and lead times to generate time-phased purchasing and production proposals.
MRP addresses three practical questions: what must be made or purchased, in what quantity, and by what date? Its central purpose is to coordinate material availability with planned production while accounting for existing stock and expected receipts. The term describes both the planning method and the software functionality that performs these calculations.
In ecommerce, MRP is particularly relevant to businesses that manufacture, assemble, or commission the production of products they sell. It is less directly applicable to a retailer that only purchases finished goods for resale, although such a retailer may use related inventory and replenishment planning methods.
Where MRP Fits in the Supply Chain
MRP sits between demand planning and operational execution. Forecasts and customer orders help establish a production schedule; MRP translates that schedule into requirements for the parts, ingredients, subassemblies, and other materials needed to produce the finished goods.
Its outputs inform procurement and manufacturing decisions. Buyers may use the results to prepare purchase orders, while production planners may use them to propose work orders. Receiving and warehouse transactions then update the inventory information used in subsequent planning calculations.
MRP does not itself transport goods, clear imports through customs, allocate parcels to carriers, or confirm customer delivery. Those activities affect or depend on material availability, but they belong to different operational processes. For example, an inbound shipment delay can change a component's expected receipt date, requiring the production plan to be reviewed.
Main Inputs and Outputs
| Planning element | Meaning | Role in MRP |
|---|---|---|
| Master production schedule | A time-phased plan for producing specified finished products or other scheduled items. | Establishes the quantities and dates from which material demand is calculated. |
| Bill of materials (BOM) | A structured record of the components and quantities required to make an item. | Connects finished-product requirements to lower-level material requirements. |
| Inventory records | Information about stock balances, allocations, and relevant inventory status. | Identifies material that can satisfy demand. |
| Scheduled receipts | Expected incoming quantities from existing purchase or production orders. | Reduces additional requirements when receipts are available in time. |
| Planning parameters | Lead times, lot-sizing rules, safety stock settings, and other applicable controls. | Influences proposed order quantities and release dates. |
| Planned orders and exception messages | Proposals for supply and notices of planning conflicts. | Supports decisions to order, produce, expedite, reschedule, or review supply. |
A planned order is not necessarily an authorized purchase order or production order. Approval, conversion, and release depend on the organization's procedures and software configuration. This distinction matters because a calculated requirement does not prove that a supplier has accepted an order or that production has begun.
How MRP Works
1. Establish finished-product requirements
The process begins with a production schedule stating which items are required and when. Customer orders, forecasts, or a combination of both may inform that schedule. Demand must be interpreted consistently so that the same requirement is not counted twice through both a forecast and a corresponding customer order.
2. Expand requirements through the bill of materials
MRP uses the BOM to translate planned production into component requirements. This process is often called BOM explosion. If a finished item requires a subassembly, the calculation can continue through that subassembly to its constituent parts. The resulting requirements depend on the product structure and quantities recorded in the BOM.
3. Calculate net requirements
The system compares gross requirements with usable inventory and scheduled receipts in the relevant planning periods. Net requirements represent the additional supply needed after those balances and applicable planning rules are considered. Timing is essential: a receipt arriving after a production requirement cannot automatically satisfy that requirement on time.
4. Apply order-quantity rules
Lot-sizing rules determine how requirements become proposed order quantities. A lot-for-lot approach proposes quantities corresponding to calculated needs, while other rules may group requirements or use fixed quantities. Minimum order quantities, order multiples, and safety stock settings can affect the result where they are included in the planning setup.
5. Offset requirements by lead time
MRP works backward from the required availability date using relevant purchasing or manufacturing lead times. This produces proposed order-release dates. For multilevel products, timing must also account for the sequence in which components become subassemblies and subassemblies become finished goods.
6. Review exceptions and update the plan
Changes in demand, stock balances, supplier commitments, or production progress can make an earlier plan unsuitable. Subsequent calculations may identify shortages or recommend changes to existing supply. Planners must assess whether these proposals are feasible before acting on them, particularly when orders are already in production or transit.
Illustrative Ecommerce Manufacturing Example
Suppose an ecommerce business plans to assemble 100 gift sets. Each set requires one container and two accessory packs. The following simplified example assumes no existing finished sets, no scheduled component receipts, no safety stock requirement, and no assembly losses.
| Component | Quantity per set | Gross requirement | Usable stock | Net requirement |
|---|---|---|---|---|
| Container | 1 | 100 | 30 | 70 |
| Accessory pack | 2 | 200 | 50 | 150 |
Under these assumptions, the business needs an additional 70 containers and 150 accessory packs. MRP would also determine when those components must be received and when purchasing should begin, using the assembly schedule and applicable lead times.
If accessory packs must be ordered in multiples of 100, the proposed purchasing quantity could be 200 rather than 150. If an existing purchase order will deliver usable containers before assembly starts, that receipt would reduce the additional requirement. These variations show why MRP is more than subtracting total stock from total demand.
Use in Cross-Border Supply Chains
For a manufacturer sourcing components internationally, MRP provides a way to connect imported-material requirements with production dates. However, it depends on the lead-time assumptions and expected receipt dates entered into the planning process. It does not independently guarantee that an international shipment will arrive as scheduled.
Depending on how the organization defines its lead times, planning may need to account for supplier preparation, international transport, customs processing, receiving, and inspection before stock becomes usable. These are operational planning considerations, not universal customs time limits or guaranteed transit periods.
A shipment arriving at a port is not necessarily available for production. It may still require clearance, inland transport, receiving, or quality approval. The relevant MRP date is the date on which the material can satisfy the production requirement, according to the organization's inventory and planning rules.
MRP and Related Planning Concepts
| Concept | Main focus | Relationship to MRP |
|---|---|---|
| Inventory management | Recording, controlling, and replenishing stock. | Provides essential stock information; MRP specifically calculates time-phased production-material requirements. |
| Reorder point planning | Triggering replenishment when an inventory position reaches a defined threshold. | Uses a different replenishment logic from deriving component needs through a production schedule and BOM. |
| Manufacturing resource planning (MRP II) | Coordinating manufacturing resources more broadly, including capacity and financial considerations. | Extends the scope beyond material requirements planning. |
| Enterprise resource planning (ERP) | Integrating information and processes across business functions. | May contain MRP functionality alongside procurement, finance, inventory, and other modules. |
| Capacity planning | Assessing whether labor, equipment, and other resources can support planned work. | Tests feasibility that a material calculation alone does not establish. |
A fundamental distinction is between independent demand and dependent demand. Demand for a finished product may come from customer orders or forecasts. Demand for its components is derived from the quantity of that product to be made. MRP is particularly associated with planning this dependent demand.
Limitations and Planning Checks
MRP results are only as useful as the records and assumptions behind them. An incorrect BOM can create the wrong component requirement. An inaccurate stock balance can hide a shortage. An unrealistic lead time can produce a release date that appears workable but cannot be achieved.
Traditional MRP calculations do not, by themselves, ensure that every proposed production order fits available machine or labor capacity. Material availability and production feasibility therefore require coordinated review. More advanced planning capabilities may address additional constraints, but those capabilities should not be assumed whenever a system is described as MRP.
Before acting on a planning result, a buyer or planner can check:
- Whether the correct BOM version applies to the planned product.
- Whether recorded inventory is physically present, usable, and not committed elsewhere.
- Whether scheduled receipts reflect current supplier or production commitments.
- Whether lead times extend to actual material availability rather than only shipment departure.
- Whether minimum quantities and order multiples are correctly represented.
- Whether proposed production dates are consistent with available capacity.
- Whether a suggested order duplicates an existing supply commitment.
MRP should therefore be understood as a structured planning method, not a promise of zero stockouts, minimum inventory, or on-time delivery. It makes requirements and timing more explicit; operational performance still depends on data quality, execution, and changing conditions.
QA
Is MRP only for large manufacturers?
No. The method is relevant wherever production requires coordinated quantities of components or materials. Its usefulness depends more on product structure and planning needs than on company size alone.
Can MRP be used by an ecommerce business?
Yes, particularly when the business manufactures or assembles products. A business that only resells finished goods may rely more directly on finished-goods replenishment methods.
What is the difference between MRP and MRP II?
MRP calculates material requirements and their timing. MRP II, or manufacturing resource planning, has a broader scope that includes other manufacturing resources and financial coordination.
Does an MRP planned order mean materials have been purchased?
No. It is a planning proposal. A purchase normally requires the relevant authorization and purchasing process, and supplier confirmation remains a separate operational matter.
Sources
- ASCM Supply Chain Dictionary — Professional terminology for material requirements planning, production scheduling, bills of materials, and related planning concepts.
- SAP: What is MRP? — Technical overview of MRP inputs, calculation processes, outputs, and relationships with broader manufacturing planning.
- Wikipedia: Material requirements planning — Supplementary reference for the method's structure, dependent demand, and data and capacity limitations.
