MRP vs APS: What's the Difference, and Do You Need Both?

Key takeaways

• MRP (Material Requirements Planning) answers what to make or buy and when, by exploding the bill of materials against demand, but it assumes infinite capacity.
• APS (Advanced Planning and Scheduling) takes that material plan and schedules it against real, finite capacity and constraints.
• MRP gives you the materials; APS gives you a schedule the floor can actually run.
• Most plants need both: MRP for planning horizons in weeks, APS for executable schedules in hours and days.

MRP and APS are often confused because both touch planning, but they answer different questions. MRP decides what materials you need; APS decides when machines and people will actually do the work. Here is how they differ and why mature plants run them together.

What MRP does

MRP starts from demand (forecast plus firm orders), explodes the bill of materials, nets against on-hand inventory and open orders, and produces time-phased requirements: planned production orders and purchase orders with due dates.

Its great strength is material coordination across many levels of a product structure. Its core limitation is that classic MRP assumes infinite capacity. It will happily tell you to build 500 units next Tuesday even if Tuesday only has room for 200, because it never checks the machine.

What APS does

APS picks up where MRP stops. It takes the material plan and schedules each operation against finite capacity, considering setup times, sequence, shift calendars, and bottleneck constraints. The output is an executable schedule, not just a list of due dates.

Because APS models the real constraints, it can sequence to minimize changeovers, protect the bottleneck, and give realistic promise dates. This is the same finite logic covered in finite vs infinite-capacity scheduling and a full guide to APS.

A worked example

Say MRP nets demand and plans 1,000 valves to ship Friday. MRP confirms the castings and seals are available and sets the order. APS then schedules it: it sees the CNC cell is already 80% loaded, that the valve needs a 45-minute changeover, and that a higher-priority order sits ahead of it. APS sequences the work, flags that 200 units will slip to Monday, and lets you decide before you promise the customer.

MRP said it was possible on paper. APS showed what the floor can really deliver. That gap is exactly why on-time delivery suffers when plants run MRP alone.

MRP vs APS at a glance

• Question answered: MRP, what and when to order; APS, when work actually runs.
• Capacity: MRP assumes infinite; APS respects finite capacity and constraints.
• Output: MRP, planned orders and due dates; APS, a sequenced, executable schedule.
• Horizon: MRP, weeks to months; APS, hours to days.

Where OEE fits

APS is only as accurate as the capacity it assumes. If real OEE on the bottleneck is 60% but the schedule assumes 85%, even a good finite schedule over-promises. Feeding measured availability and performance into the capacity model keeps both MRP and APS honest. Book a Fabrico demo to see how live OEE data sharpens capacity planning.

Common mistakes

• Treating MRP output as a schedule. Planned orders are a starting point, not something the floor can run as-is.
• Feeding APS optimistic capacity. Garbage capacity in, unrealistic promises out.
• Skipping one or the other. APS without MRP loses material coordination; MRP without APS loses executability.

Frequently asked questions

Does APS replace MRP?

No. APS complements MRP. MRP handles material netting and time-phasing; APS schedules that plan against real capacity. Many systems run them in sequence.

Can ERP do APS?

Most ERP systems include MRP but only basic, infinite-capacity scheduling. True finite-capacity APS is usually a specialized module or system layered on top.