What Is MRP (Material Requirements Planning) and How It Works

Every Monday morning, the production manager and the PPIC team usually wrestle with a classic dilemma: what materials should be ordered, how much, and when must they be available? A small error in the calculation can have fatal consequences, the production line halting because components arrive late, or the opposite, the warehouse crammed full of stock that piles up costs. This is where Material Requirements Planning (MRP) comes in as a systematic solution. This article thoroughly dissects what MRP is, how it works through a simple input-process-output approach, and how it differs from MRP II and ERP, complete with a practical worked example.

What Is MRP (Material Requirements Planning)?

MRP (Material Requirements Planning) is a planning method that translates the production plan for a finished product into a detailed plan of which materials and components are needed, in what quantities, and when each must be ordered or produced. In essence, it answers one operational question: “what material, how much, when?”

This method is not new. MRP was formalized by Joseph Orlicky, an engineer at IBM, in 1964, and documented in his work Material Requirements Planning in 1975. In that same year, about 700 companies had already implemented it; that figure jumped to roughly 8,000 by 1981. According to the University of Cambridge Institute for Manufacturing (IfM), MRP and its derivatives are now among the most widely used production planning and scheduling tools in the world.

Often, many discussions get it wrong by treating MRP as a software brand. In fact, MRP is fundamentally a planning logic. Its working principle is the same whether you implement it via a simple spreadsheet, a dedicated application, or as a module within a large ERP system. The only difference lies in the scale and speed of data processing, not in the underlying concept.

How Does MRP Work? The Input-Process-Output Logic

MRP works with three inputs, namely the Master Production Schedule (MPS), the Bill of Materials (BOM), and the Inventory Status File, then processes them through four steps: netting, lotting, offsetting, and exploding. The result is a Planned Order Schedule: a schedule of when and how much material must be ordered or produced for each component level.

The Three Inputs MRP Needs

Without these three pieces of data, MRP cannot calculate anything. The three complement each other:

• Master Production Schedule (MPS): the master production schedule, that is, which finished products, how many, and when they must be completed. This is the starting point that drives the entire calculation.

• Bill of Materials (BOM): a hierarchical list of all components, sub-assemblies, and raw materials along with the quantities needed per unit of product. A BOM can be multi-layered: a finished product consists of sub-assemblies, and sub-assemblies consist of yet smaller components.

• Inventory Status File: the actual inventory record, covering stock on-hand, orders already placed and in transit (on-order), and safety stock (safety stock).

The quality of these three inputs determines everything. Because MRP reads stock directly from the inventory records, data accuracy in the warehouse is an absolute requirement. This is one point where the discipline of record-keeping and inventory data accuracy in the warehouse is directly proportional to planning quality.

The Four Steps of the MRP Process

The core MRP computation is commonly explained in four standard steps according to the operations management literature:

• Netting: calculating net requirements, that is, gross requirements minus stock on hand and orders already scheduled to arrive. This is the heart of MRP arithmetic.

• Lotting (lot sizing): determining the order lot size. Net requirements are rarely ordered as is; they are grouped following an ordering policy, for example a certain minimum order or a lot-for-lot pattern.

• Offsetting: pushing the schedule back according to the lead time. If a component needs five days from ordering until it is ready to use, the order must be released five days before the date it is needed.

• Exploding: breaking requirements down to the next component level via the BOM. The requirement for one sub-assembly automatically becomes the requirement for its constituent components, and the netting cycle then repeats at that level.

An honest note for industrial-engineering readers: the order and number of these steps vary between sources. Cambridge IfM cites three steps (without separate lotting), while SAP Learning lays out five steps. In top-down implementations, the exploding step often precedes netting. That is not an error, but a different way of reading the BOM; the arithmetic logic remains the same.

The Output MRP Produces

From the process above come two main outputs. First, the Planned Order Schedule, namely a plan of when and how much material must be ordered or produced at each BOM level. Second, the Order Release Report, namely the formal order to start purchasing from suppliers or to run the production process on the factory floor. This is the concrete document received by the procurement and PPIC teams each planning cycle.

A Simple Example of How MRP Works

The fastest way to understand MRP is through one chained numerical example. The core calculation is a single line: net requirement = gross requirement − stock in the warehouse − orders already scheduled to arrive. Suppose a furniture factory plans to produce 1,000 units of “Table A” this week. Each Table A requires one “Tabletop”. There are 300 Tabletops in warehouse stock, and another 100 already ordered and scheduled to arrive this week.

*Note: ILLUSTRATION. The following figures are not client data; they only show MRP's arithmetic logic.*

The process follows a logical flow. First, Netting: we calculate the net requirement by subtracting stock on hand from the gross requirement. Second, Lotting: we adjust the order quantity based on the purchasing policy, for example rounding up to a certain multiple. Third, Offsetting: we schedule the order backward based on the lead time so that material arrives exactly when needed. Finally, Exploding: we break the finished-product requirement into its list of constituent components per the BOM, then repeat the calculation above for each of those components. Put simply, one production plan can trigger a very long chain of component requirements. This is why calculating everything by hand becomes very difficult and risky as the number of products or the complexity of the BOM structure increases.

MRP, MRP II, and ERP: What's the Difference?

MRP plans material requirements. MRP II (Manufacturing Resource Planning) extends it with machine capacity, labor, and finance. ERP (Enterprise Resource Planning) integrates all business functions, from finance, HR, and sales to production, in one system. In short: MRP is part of MRP II, and MRP II is the forerunner of the manufacturing module in ERP.

The three are one evolutionary line, not three competing technologies. MRP emerged in 1964. MRP II was developed by Oliver Wight in 1983 by adding capacity planning and Sales & Operations Planning; so large was it that by 1989 MRP II software accounted for roughly a third of the software industry sold to US industry, worth approximately USD 1.2 billion. The term ERP itself was coined by the research firm Gartner in 1990 to describe systems whose scope had already moved beyond manufacturing.

The question “what's the difference between MRP and ERP” is actually mis-framed. MRP is not a rival to ERP; it is one of the functions within ERP. A company running a modern ERP is automatically running MRP, only integrated with finance, procurement, and sales in one database.

Limitations of Classic MRP That Are Rarely Discussed

Classic MRP assumes a fixed lead time and depends heavily on the accuracy of BOM and inventory data; wrong data produces a wrong plan. MRP also plans materials, not machine capacity, and is prone to excessive replanning. It is precisely these limitations that drove the evolution toward MRP II, ERP, and real-time MRP such as MRP Live in SAP S/4HANA.

The four most concrete limitations deserve an honest dissection, because most articles only praise MRP without mentioning this side:

• Fixed lead time assumption. MRP uses a standard lead time from the material master data. Yet in the field, actual times fluctuate with queues, capacity load, and lot size. This static assumption becomes fragile in a dynamic production environment.

• “Garbage in, garbage out”. MRP is only as good as the data feeding it. A wrong BOM or an off inventory record directly produces a wrong plan. That is why data accuracy is a prerequisite for reliability: not about chasing 100% perfection, but about keeping data clean enough to be trusted.

• System nervousness. A small change to the master production schedule can trigger large chain-reaction changes across the entire plan. Operations management research shows that replanning too frequently actually raises total cost and reduces schedule stability.

• Material only, not capacity. Classic MRP computes material requirements but does not check whether machines and labor can execute them. It was this limitation that once gave rise to MRP II.

This is the bridge to modern MRP. SAP S/4HANA runs MRP within the Production Planning (PP) module in three modes. Classic MRP is still supported but is no longer the focus of innovation. MRP Live pushes the planning logic into the SAP HANA in-memory database, making it far faster for large volumes of material and complex BOMs, and is defined by SAP as the future architecture in its official documentation. Predictive MRP (pMRP) is a simulation tool for evaluating demand plans and detecting potential medium-term capacity bottlenecks earlier. In production-floor implementations, all three modes live within the same platform as finance and procurement, on top of a modern cloud-based ERP foundation.

Benefits of MRP and When a Company Needs It

The main benefit of MRP is inventory balance: implemented correctly, MRP prevents the stockout that halts production, curbs the excess stock that ties up capital in the warehouse, and aligns the purchasing schedule with the production schedule. The result is more controlled inventory costs and delivery commitments to customers that are easier to keep. A company needs it most when the number of SKUs and BOM levels grows until manual calculation is no longer reliable.

But MRP is not a need for every business from day one. A small workshop or manufacturer with one or two products, a shallow BOM, and few suppliers can still run tidily with a disciplined spreadsheet. Forcing a complex planning system onto an operation that simple only adds administrative burden without commensurate benefit.

Conversely, the signs that manual MRP is no longer adequate usually appear together: SKUs already number in the hundreds, BOMs are layered across many levels, suppliers are dispersed, and demand fluctuates sharply. When a single schedule change forces the team to recalculate dozens of components by hand into the small hours, the spreadsheet has become a source of risk rather than a tool. That is the point at which MRP needs to live within an integrated system, and many companies pair it with a digital manufacturing system initiative so that material planning connects directly with real conditions on the production floor.

FAQ (Frequently Asked Questions)

What is MRP (Material Requirements Planning)?

MRP (Material Requirements Planning) is a material requirements planning system that calculates the type, quantity, and timing of procuring raw materials and components to meet the production schedule. The system turns the master production plan into a purchasing and component-build plan automatically, so that materials are available on time without excess stock piling up in the warehouse.

What does MRP stand for?

MRP stands for Material Requirements Planning, often called “Perencanaan Kebutuhan Material” in Indonesian. The term was first formalized by Joseph Orlicky (IBM) in 1964 and is now one of the most widely used production planning methods in the world, including as a core function within modern ERP systems such as SAP S/4HANA.

What are the three inputs MRP needs?

MRP needs three main inputs: (1) the Master Production Schedule (MPS), the schedule of when and how many finished products must be completed; (2) the Bill of Materials (BOM), a hierarchical list of all components and raw materials with their quantities per unit; (3) the Inventory Status File, the record of actual stock, orders in transit, and safety stock. All three must be accurate; even one error corrupts the entire plan.

What is the difference between MRP and ERP?

MRP is a material requirements planning method, one of the functions within production. ERP (Enterprise Resource Planning) is an integrated system that connects all business functions: finance, HR, sales, procurement, and production, in one platform. In short, MRP is part of ERP. The term ERP was coined by Gartner in 1990 to describe the evolution of MRP II whose scope had expanded across the whole enterprise.

What is MRP II?

MRP II (Manufacturing Resource Planning) is an extension of MRP introduced by Oliver Wight in 1983. Whereas classic MRP only plans material requirements, MRP II adds planning of machine capacity, labor, and finance, producing a more comprehensive production plan. MRP II became the forerunner of the manufacturing module in modern ERP systems.

What is the main weakness of classic MRP?

Classic MRP has four main weaknesses: (1) it assumes a fixed lead time, which in reality fluctuates; (2) it depends heavily on BOM and inventory data accuracy (“garbage in, garbage out”); (3) it is prone to system nervousness, where a small change triggers chain-reaction changes across the whole plan; (4) it plans only materials, not machine capacity. It is these limitations that drove the evolution toward MRP II and modern ERP.

When does a company need to run MRP inside a modern ERP like SAP S/4HANA?

When SKUs already number in the hundreds, BOMs have many levels, suppliers are dispersed, or demand is highly volatile, manual MRP in a spreadsheet is no longer reliable. SAP S/4HANA provides three MRP modes in the Production Planning (PP) module: classic MRP, MRP Live (HANA in-memory-based, the architecture SAP recommends), and Predictive MRP (pMRP) for medium-term capacity simulation, all within one integrated platform.

Conclusion

MRP is the heart of material planning: from three inputs (MPS, BOM, and inventory records), through four process steps, into a concrete order schedule. Understanding its input-process-output logic helps you read when a spreadsheet is still enough and when planning needs to move to an integrated system. At a complex scale, MRP does not stand alone; it lives within a modern ERP. As an SAP Platinum Partner in Indonesia through the United VARs alliance, with cross-industry manufacturing implementation experience, Soltius helps companies adopt and support production planning in SAP S/4HANA Manufacturing, from readiness assessment through post-go-live support.

To discuss your company's production planning and SAP S/4HANA Manufacturing readiness, visit soltius.co.id.