Key takeaways
- ERP is the system of record for orders, inventory and finance.
- MRP calculates what to buy and make, but assumes infinite capacity.
- APS schedules against real, finite capacity and turns the plan into an executable sequence.
- Most manufacturers run APS alongside ERP rather than replacing it.
Three systems, three jobs
Manufacturers usually meet these acronyms in the order they buy them: first an ERP, then the MRP module inside it, and finally — when the schedule keeps breaking — an advanced planning and scheduling (APS) system. Each answers a different question.
- ERP — What did we sell, what do we have, what did it cost?
- MRP — What do we need to buy or make, and roughly when?
- APS — Exactly which job runs on which resource, in which order, starting when?
Where MRP falls short
MRP explodes the bill of materials and offsets requirements by fixed lead times. The hidden assumption is infinite capacity: if ten orders need the same machine on Tuesday, MRP happily plans all ten.
Planners compensate with padded lead times and spreadsheets. The result is long quoted lead times, high WIP and a schedule that is out of date by mid-morning.
What APS adds
APS takes the same demand and BOM data, then schedules each operation against the actual availability of machines, people, skills, tooling and materials. It sequences jobs to reduce changeovers and protect due dates, and it re-plans when reality changes.
| ERP | MRP | APS | |
|---|---|---|---|
| Capacity | n/a | Infinite | Finite |
| Time bucket | Days / weeks | Days / weeks | Minutes |
| Output | Transactions | Planned orders | Sequenced schedule |
| Reacts to disruptions | No | Overnight re-run | Seconds |
Modern APS platforms like SteadyFloor go one step further and push the schedule straight to the floor as work orders, so the plan and execution never drift apart.
Do you need APS?
You probably do if planners maintain a scheduling spreadsheet outside ERP, if on-time delivery is below target despite available capacity, or if a single breakdown triggers a day of re-planning. Read our APS buyer's guide for a structured evaluation.
Frequently asked questions
Does APS replace ERP?
No. ERP remains the system of record. APS reads demand, items and routings from ERP and writes back confirmations and schedules.
Is MRP part of APS?
Many APS tools include material-constrained planning, but MRP is usually still run in ERP. APS adds capacity constraints on top.
New to the topic? Start with our complete guide to advanced planning & scheduling.