What is MRP software? BOM explosion, netting and the buyer's guide

A purchasing manager places an order for steel that, it turns out, was already sitting in the store — reserved to a job that got cancelled last week. Two aisles over, a work order stalls because a bought-out casting nobody flagged is three weeks out on a purchase order that slipped. Both mistakes have the same root cause: a plan built on gross demand that never checked what the plant already had or already had coming. Closing that gap — turning raw demand into a netted, actionable plan — is exactly what MRP software exists to do.

This guide is written for the person evaluating the software category — a plant head, planning manager, or business owner deciding what to buy. It covers what MRP actually does step by step, the ideas that separate a real planning engine from a spreadsheet with a formula in it, and how Fast Planning Software implements each one. If you want the wider discipline first — production planning and control as a practice, independent of any tool — start with our pillar guide, What is production planning software?, and come back here when you are ready to compare software.

1. What is MRP software?

MRP — material requirements planning — is the calculation that works out, item by item and level by level, exactly what to make, what to buy, and when. MRP software runs that calculation over your real data: it takes demand from a sales or production plan, explodes each finished good through its bill of materials, nets the result against what you hold and what is coming, and turns whatever is still short into two kinds of instruction: purchase requisitions and work orders.

The starting point is always the same pair of inputs: demand (a forecast or confirmed orders) and a bill of materials (the multi-level structure of what each finished good is made of). From those, MRP derives everything else. The key word is netted. Plenty of tools can multiply demand by a BOM. What makes a real MRP engine is that nothing it suggests ignores reality:

• Every requirement is exploded — demand walks down the full BOM tree, so a finished-good number becomes a gross requirement for every sub-assembly, component and raw material beneath it
• Every requirement is netted — gross demand is reduced by stock on hand and open supply, so the plan acts only on what is genuinely short
• Every shortfall becomes a document — a purchase requisition to buy or a work order to make, not a note to chase later
• Every parameter is respected — each item's lead time and reorder level decide the timing, so the plan says when as well as how much
• Every allocation is protected — stock reserved to one plan is not offered again to another, so two plans never fight over the same material

Instead of a spreadsheet that reacts to shortages after they bite, MRP produces one coordinated plan that procurement and production act on together. For the deeper practice behind it, see our MRP — BOM explosion & netting feature.

2. Why a planning spreadsheet fails

Most plants do not start with no system. They start with a demand spreadsheet, a BOM in another tab, and a purchasing list someone updates by hand. It feels adequate until item counts, BOM depth or a stock-out exposes it. The failure modes are consistent enough to list:

• Explosion by hand does not scale. A three-level BOM across a hundred finished goods is thousands of component lines. Exploding that in a spreadsheet is a weekend's work that is stale before it is finished.
• Gross demand buys the wrong things. A spreadsheet that orders against gross demand cannot see the stock you already hold, so it buys material that is sitting in the store — and misses the item that looks covered only because of a purchase order that never arrived.
• Open supply is invisible. Open purchase orders and open work orders are supply too. A plan that ignores them either double-orders or double-makes, because it treats "coming next week" as "not here".
• Reservations get double-spent. Stock earmarked for one job looks free to the next planner. Without reservation, the same casting is promised to two work orders and one of them stalls.
• Timing is guesswork. A quantity with no lead time is only half a plan. A spreadsheet rarely knows that a long-lead casting must be ordered six weeks before the short-lead fastener.
• No safety net between runs. Between one monthly plan and the next, a spreadsheet says nothing. An item quietly falls below its reorder point and nobody notices until the line stops.

None of this fails loudly. That is what makes it dangerous: a spreadsheet never sends you a report saying you are about to buy steel you already own, or that a reserved casting has been promised twice.

3. What the software does — the MRP run, step by step

Whatever the vendor, an MRP run follows broadly the same sequence. What the software does is make each step explicit and repeatable — so a plan comes out the same way whether the plant runs ten finished goods or ten thousand component lines.

The MRP run is one chain — demand exploded, netted against stock and open supply, then split so every shortfall becomes either a purchase requisition to buy or a work order to make.

4. Core capabilities checklist

Feature lists blur together quickly. These are the eight capabilities that actually determine whether a product can run material planning — use them as your evaluation checklist.

Alongside these eight, check the make-or-buy discipline: does the system understand that a net shortfall is not just a number but a decision — buy it, or make it — and raise the right document for each? A tool that lists shortfalls but leaves you to sort them into purchase orders and work orders by hand has done the arithmetic and skipped the plan.

5. MRP vs MRP II vs ERP

Buyers often discover mid-evaluation that "MRP", "MRP II" and "ERP" answer three progressively wider questions. They build on each other rather than competing, and the best arrangement runs them on one platform rather than bolting a planning tool onto a separate system.

The point is not that one replaces the others — it is that they must share the same data. Fast Planning covers MRP and the MRP II capacity-and-control layer, and rides the same platform as the rest of the Fast Suite so the step up to full ERP is a menu, not a migration:

• MRP gives you the netted plan — purchase requisitions and work orders. See MRP — BOM explosion & netting.
• MRP II adds finite machine loading, priority scheduling on a Gantt board, and plan-vs-actual and OEE — so the plan respects real capacity and is measured against real output.
• ERP wraps both in the wider business — because Fast Planning shares one item, BOM and stock master with Fast Production, Inventory and Purchase, nothing is re-keyed at a boundary.

6. Who needs MRP software?

Not every workshop needs a dedicated engine on day one. These are the situations where the demand-spreadsheet approach reliably stops being enough:

The practical trigger is usually one of three events: material bought that was already in the store; a line stopped for a component everyone assumed was covered; or a growth stage where the number of finished goods and BOM levels outruns the one planner who used to hold it all in a spreadsheet.

7. How to evaluate MRP software

Most demos look good. The differences show up in the specifics, so evaluate against your own BOMs and stock rather than the vendor's script.

8. How Fast Planning Software implements each capability

Fast Planning Software is the MRP and production-planning product of the Fast Suite, built in Pune by Improsys under the Fast Technology brand, deployable cloud or on-premise for manufacturers across India and worldwide. It runs each capability above with real, named screens — the same ones you would see in a demo:

9. Frequently asked questions