What Is MRP? Material Requirements Planning Explained for Modern Factories

Material requirements planning is the oldest idea in manufacturing software and still the most misunderstood. At its core it does something almost mechanical: take what you have promised to produce, explode it through the bill of materials, subtract what you already have in stock and what is already on order, and what remains is what you need to buy or make. Everything else in MRP is detail on top of that calculation.

The inputs are three, and the quality of each determines whether the output is useful or actively harmful. The master production schedule says what finished goods are needed and when. The bill of materials says what each finished good is made of, level by level. The inventory record says what is physically on hand and what is already inbound. Get any of the three wrong and MRP will confidently produce a wrong answer at scale — which is why factories with poor BOM discipline often conclude that MRP does not work, when what does not work is their data.

Lead time is where the calculation becomes time-phased rather than just arithmetic. Knowing you need 400 units of a component is not actionable; knowing you need them in week 12, and that the supplier takes five weeks, tells you to order in week 7. MRP walks backwards through every level of the BOM applying each item's lead time, producing a schedule of purchase and production orders rather than a shopping list. This is the part spreadsheets reproduce badly, because a multi-level BOM with different lead times per level is not a calculation anyone should be doing by hand.

The classic failure is the infinite capacity assumption. Standard MRP tells you what you need and when, but it does not ask whether your factory can actually make it in that window. It will happily schedule three orders onto one machine in the same week and report no problem, because capacity was never part of its model. This is not a flaw so much as a scope boundary — and it is precisely the boundary where advanced planning and scheduling picks up.

That distinction is worth being exact about, because the three acronyms get used interchangeably in vendor material and they are not the same thing. MRP determines material requirements — what and when. APS determines the executable schedule — which machine, which sequence, accounting for finite capacity, changeovers and constraints. SCP extends the plan outward across suppliers, purchasing and distribution. A factory can run MRP alone. A factory running MRP alone will usually discover its capacity problems on the shop floor rather than in the plan.

Nervousness is the practical problem that surprises new users. Because MRP recalculates the entire dependent demand chain, one small change high up — a customer pulling an order forward by three days — can cascade into dozens of changed purchase suggestions several levels down. Without damping mechanisms like planning time fences, firm planned orders and reasonable lot sizing, the system generates more churn than the purchasing team can absorb, and they start ignoring it. Ignored MRP output is worse than no MRP, because it creates the illusion of control.

Lot sizing policy is where practical judgement enters. Ordering exactly what is needed each period minimises inventory and maximises transaction cost and supplier irritation. Ordering in large economic batches does the reverse. Real configurations use a mix — lot-for-lot for expensive, low-volume, long-lead items; fixed or economic order quantities for cheap fasteners nobody wants to reorder weekly. There is no universally correct setting, which is why this is a configuration decision rather than a default.

Modern MRP differs from the 1970s original mainly in what it is connected to. When the inventory figure comes from a WMS that enforces locations rather than from a periodic count, the starting data is trustworthy. When actual consumption and scrap come back live from the MES, the plan reflects what happened rather than what was supposed to happen. When supplier confirmations flow in from an SCP layer, the inbound picture is real. The calculation has not changed; the reliability of its inputs has, and that is what turns MRP from a theoretical exercise into an operational tool.

For a factory currently planning materials in Excel, the honest starting point is not software selection. It is BOM accuracy and inventory accuracy, because MRP will amplify whatever discipline you already have. Factories that fix those two things first tend to get value in the first planning cycle. Factories that skip them tend to spend a year blaming the system.