MRP stands for Material Requirements Planning. It is the calculation that turns what you have promised to ship into what you need to buy and build, and when each of those actions has to happen.
MRP is not software. It is a planning logic. Software executes it quickly and consistently, but the logic — and the data quality it depends on — is what determines whether the output is trustworthy.
The four inputs MRP depends on
Every MRP calculation, in every system ever built, needs the same four things. If any one of them is wrong, the output is wrong in a way that looks authoritative.
- Demand — firm sales orders, forecast, and any internal demand such as spares or stock builds.
- Bill of materials — the structured list of components and quantities required to make each item.
- Inventory status — on hand, allocated, on order, and in inspection or quarantine.
- Lead times and order policies — purchasing lead time, manufacturing lead time, lot sizes, minimums and safety stock.
What MRP actually calculates
MRP explodes demand through the bill of material level by level. At each level it nets the gross requirement against what you already have and already have on order, then offsets the remainder backwards by lead time to produce a planned order date.
The output is a set of recommendations: planned purchase orders, planned work orders, and reschedule messages telling you to pull in, push out or cancel existing orders. MRP does not make decisions. It proposes them, and a planner accepts, adjusts or ignores them.
Why MRP output gets ignored
In most small manufacturers that say MRP is not working, the arithmetic is fine. The trust is missing. Planners stop acting on MRP messages when inventory records are inaccurate, when lead times are aspirational rather than real, or when BOMs do not match what the floor actually builds.
Once planners route around the system, the data degrades further and the system becomes a very expensive record of history. Fixing this is usually a data, process and training problem rather than a software problem.
MRP, MRP II and ERP
MRP handles materials. MRP II added capacity, routings, labor and cost so that the plan could be checked against what the plant can physically do. ERP wrapped planning together with finance, sales, HR and the rest of the business.
For a small or midsized manufacturer, this history matters for one reason: you may only need the materials and capacity layers. Buying a full enterprise suite to fix a materials planning problem is a common and expensive mistake.
Common questions
- Does MRP stand for anything other than Material Requirements Planning?
- In manufacturing, MRP means Material Requirements Planning. MRP II means Manufacturing Resource Planning, which extends the same logic to capacity, labor and cost.
- Can a small manufacturer run MRP in a spreadsheet?
- Yes, up to a point. Spreadsheets work while part counts, BOM levels and order volume stay low. They break down when multiple people need the same current answer, when BOMs have several levels, or when inventory changes faster than the sheet is updated.
- Is MRP still relevant with modern demand-driven methods?
- Yes. Demand-driven and pull methods change how buffers and order signals are set, but the underlying need to relate demand to BOMs, inventory and lead time does not go away.