MRP (material requirements planning) is the calculation that turns demand for finished products into a dated plan of what to make and buy, at every level. It answers three linked questions: what is needed, in what quantity, and by when.
Why it matters
Deciding to make a hundred finished products seems simple. But each is made of sub-assemblies, themselves made of components, themselves from raw materials, each with its quantity, current stock, and own lead time. Computing by hand what to order, in what quantity and by what exact date, for thousands of nested references, is humanly impossible. MRP is the machine that does this calculation, without error and at scale.
Its invention marked a historic turning point in industry: it allowed a move from stock management by guesswork, reference by reference, to an integrated plan that unrolls all demand down to raw materials. It is today the beating heart of most manufacturing ERPs, the invisible mechanics that, every night, recompute what the factory must launch and what purchasing must order.
MRP distinguishes two natures of demand, and that is its whole power. Independent demand (finished products) is forecast, because it comes from the market. Dependent demand (components) is not forecast: it is calculated exactly from the finished products, via the bill of materials.
It is a costly, frequent error to statistically forecast components as if they were independent. You then pile up useless safety stock on parts whose need is perfectly deducible. The expert rule is simple: forecast what is independent, calculate what is dependent. Confusing the two means paying twice.
The mechanism
MRP runs a logical three-step sequence, from three indispensable inputs: demand, the bill of materials (BOM), and the state of stock and open orders.
First step, explosion. MRP starts from finished-product demand and unrolls it through the BOM, level by level. On the chart, a finished product (N0) breaks down into sub-assemblies (N1), themselves into components (N2). At each descent, quantities multiply: if a product needs two wheels and you make a hundred, that is two hundred wheels; and if each wheel has thirty-six spokes, that is seven thousand two hundred spokes. Finished-product demand thus becomes a cascade of gross requirements.
Second step, netting: for each item, on-hand stock and supply already on the way are subtracted from the gross requirement. What remains is the net requirement, what genuinely must be procured. Third step, offsetting (or back-scheduling): each order's release date is pulled back by the item's lead time, cascading across all levels. The result is a concrete plan: purchase and work orders, each with its quantity and date.
Figure 1. The BOM explosion: a finished product (N0) unrolls into sub-assemblies (N1) then components (N2), quantities multiplying at each level. Illustrative schematic.
The traps
The MRP calculation is simple arithmetic. What makes it fail is its input data.
MRP explodes demand through the BOM: if it is wrong (wrong quantity, forgotten component, outdated version), the error propagates and multiplies across all lower levels. A reliable BOM is not a detail, it is the condition of the whole calculation.
Offsetting pulls dates back by the lead time. If the entered lead times are optimistic or never updated, the whole plan shifts and stockouts follow. Real lead times must be tracked and revised regularly, not set once and forgotten.
At each small demand change, MRP recomputes everything and can upend hundreds of orders, sowing panic among buyers. Guardrails (frozen horizon, firm orders, lot-sizing periods) are indispensable to stabilize the plan and make it executable.
Computing it
MRP is only reliable if its three inputs are. The calculation itself is a mechanical chain.
Demand (master production schedule and orders), each product's multi-level BOM, and the state of stock and open orders. These three must be accurate and current: that is where everything is decided.
Unroll demand through the BOM to get each item's gross requirement, accumulating when a component serves several products. A shared component sums the needs of all its parents.
Subtract on-hand stock and scheduled receipts from the gross requirement (see formula above). The net requirement is what must genuinely be ordered or made, no more, no less.
Pull each need date back by the lead time to find the release date, cascading across all levels. The output is a set of dated orders: purchases to raise, manufacturing to launch.
Neighboring concepts
MRP extends the master schedule and connects with lead time and stock notions.
From knowledge to action
An MRP is only worth its data: BOMs, lead times, stocks. Our Planning, Forecasting & S&OP file makes these foundations reliable before optimizing the calculation.