Material requirements planning, almost always called MRP, is one of the foundational ideas in manufacturing. It is also one that is described either too vaguely to be useful or too technically to be clear. This piece explains MRP thoroughly and plainly: what it is, what it needs, how it works, what it produces, and where it can go wrong.
What MRP is
Material requirements planning is a calculation that answers one question precisely: to meet the demand for our products, what components and materials do we need to make or buy, in what quantities, and by when? Every manufacturer has to answer this question constantly. MRP is the systematic way of answering it, rather than relying on memory, intuition, and a spreadsheet.
It exists because the question is genuinely hard once products are anything but trivial. A product is made of components; those components may themselves be sub-assemblies of further components; each has its own stock level, its own lead time, its own supplier. To plan correctly, all of that has to be considered together. MRP is the method that does so.
The three inputs MRP needs
MRP rests on three inputs, and the quality of all three decides the quality of the plan.
Demand. What finished products are needed, in what quantity, by when. This is usually expressed as the master production schedule, and it comes from confirmed sales orders and forecasts.
The bill of materials. For each product, the full structure of what it is made from, every component and sub-assembly and the quantities. MRP uses the BOM to translate demand for finished goods into demand for components.
Inventory and supply data. What is currently in stock, and what is already on order and due to arrive. MRP uses this to work out what is genuinely still needed, as opposed to already covered.
Each input also carries timing information, lead times, how long it takes to make or buy each item, which MRP needs to plan not just quantities but dates.
How the MRP calculation works
MRP works through a logical sequence. It starts with demand for a finished product. It uses the bill of materials to explode that demand: if 100 units of a product are needed and each uses 4 of a component, then 400 of that component are required. It does this through every level of the BOM, so demand for finished goods becomes demand for sub-assemblies and then for raw components, this is called the BOM explosion.
For each item, MRP then calculates the net requirement: gross requirement, minus stock on hand, minus supply already on order. What remains is what genuinely still has to be made or bought. Then MRP applies the lead time to work out when each action must start so the item is ready when needed, this is called offsetting or back-scheduling. The result is a plan with both quantities and dates.
What MRP produces
The output of MRP is a set of planned actions: purchase orders to raise, with quantities and dates, for bought items; and manufacturing orders to start, with quantities and dates, for made items. It also surfaces problems: where a requirement cannot be met in time given current lead times, MRP flags it, so the planner sees the conflict while there is still room to act. In short, MRP turns the question "what do we need" into a concrete, dated to-do list for purchasing and production.
Where MRP goes wrong
MRP has a well-known weakness, and it is honest to state it plainly: MRP is only as good as its inputs. If the bill of materials is wrong, MRP plans the wrong components. If lead times are optimistic, MRP plans actions that start too late. If stock figures do not match reality, MRP plans against stock that is not there, or ignores stock that is. MRP does not detect bad data; it calculates confidently from whatever it is given. This is why an MRP implementation that struggles almost always has a data problem, not a software problem, and why accurate master data is the real precondition for MRP to be useful.
A second limitation: classic MRP plans materials but, in its basic form, assumes capacity is available. Modern manufacturing systems extend MRP with capacity planning so the plan is checked against the real limits of machines and people. A manufacturer should understand whether its planning checks capacity or only materials.
MRP today
MRP is rarely bought as a standalone product now. It is a core capability inside manufacturing ERP, where it sits alongside the sales, inventory, and purchasing that produce its inputs and consume its outputs. That integration is what keeps MRP's inputs current and so keeps its plans trustworthy.
The takeaway
Material requirements planning is the calculation that turns demand into a dated plan of what to make and buy. It needs three accurate inputs, demand, the bill of materials, and inventory and supply data, and it explodes, nets, and time-offsets them into a concrete plan. Its great strength is doing this completely and reliably; its great weakness is that bad data produces a confident wrong plan. For how we approach manufacturing planning, see our manufacturing work.