APS vs ERP vs MRP: What Each Does and When Manufacturers Need APS
APS, ERP, and MRP play different roles in manufacturing planning. Material requirements planning (MRP) works out which materials and orders are needed, and when. Enterprise resource planning (ERP) records the transactions that run the business: orders, inventory, purchasing, and finance. Advanced planning and scheduling (APS) decides which order runs on which resource, in what sequence, within the plant’s real capacity.
Most manufacturers run MRP inside their ERP system. They add APS when sequencing, changeovers, and shared resources start deciding whether orders ship on time.
Eyelit Technologies publishes this guide.
What is the difference between APS, ERP, and MRP?
The difference between APS, ERP, and MRP is the decision each one makes: MRP plans materials against lead times, ERP records and controls business transactions, and APS builds a detailed schedule against finite capacity. A manufacturing execution system (MES) is the execution layer: it receives the schedule, runs the work on the shop floor, and reports actual status back to planning.
| System | What it does | How it treats capacity | Typical horizon | Main output |
| MRP | Explodes demand through the bill of materials and nets it against inventory and open orders. | Basic MRP assumes capacity is available and uses fixed lead times. | Weeks to months, in time buckets | Planned purchase and production orders with due dates |
| ERP | Holds orders, bills of materials, inventory, purchasing, and finance as the system of record, and usually includes MRP. | Varies by product and configuration. Many run a rough-cut check at work center level over a period, and some add finite-capacity planning modules. | Months to a year | Transactions, inventory positions, and the material plan |
| APS | Sequences orders on specific machines, lines, and resources, taking setups, tools, skills, and material arrival into account. | Plans to finite capacity, resource by resource. | Hours to several weeks | A feasible, sequenced schedule and capacity-informed promise dates |
| MES | Releases work to the shop floor and records what happens as it happens. | Records actual production status, completions, downtime, and WIP for planning feedback. | Real time | Production records, WIP status, and actuals fed back to planning |
The capacity column is where the three differ most. MRP tells you an order is due in week 34 and which materials it needs. APS takes that order and the 40 others due on the same line, schedules them around the one machine they all share and the changeovers between them, checks that each order’s materials arrive in time, and tells you which one runs first on Tuesday.
Some ERP systems include finite-capacity planning or scheduling modules. The deciding question is whether those modules model the plant’s real constraints in the detail the schedule needs.
When does a manufacturer need APS as well as ERP?
A manufacturer needs APS as well as ERP when the material plan is sound but the schedule still fails, because the limiting factor is capacity and sequence, and the plant’s ERP planning models capacity in less detail than the problem needs.
Common signs:
- Dates slip with material on the shelf. Orders run late even though MRP shows every component available.
- Planners rebuild the schedule in spreadsheets. The ERP plan is a starting point that people rework by hand every day.
- Changeovers drive the schedule. Setup times depend on what ran before, so sequence changes output.
- Shared resources become bottlenecks. Tools, dies, ovens, or skilled operators limit several lines at once.
- Change is constant. Rush orders, late supplies, and breakdowns force replanning several times a week.
- Customers want capacity-based promise dates. Sales needs a delivery date based on real capacity before committing.
When these signs apply, APS can offer a strong business case through better schedule adherence and less manual replanning. The return depends on the plant’s constraints, the cost of putting APS in place, and how much performance there is to improve. A plant with stable demand, few product changes, and spare capacity can often run well on ERP planning alone.
How do APS, ERP, and MRP work together?
APS, ERP, and MRP work together as a planning layer on top of a transaction layer: ERP and MRP hold the orders and the material plan, APS turns them into a schedule the plant can run, and MES executes that schedule and reports back.
In practice, the data moves in a loop:
- ERP sends the inputs. Items, bills of materials, routings, resources, sales orders, forecasts, inventory, and purchase orders pass from ERP to APS.
- APS builds the schedule. It sequences the work against every constraint and returns planned start and finish times, revised order dates, and promise dates.
- ERP records the result. Updated dates flow back to ERP, so purchasing, sales, and finance work from the same plan.
- MES runs the work. The schedule goes to the shop floor, and MES records completions, downtime, and WIP.
- APS replans from actuals. Real shop floor status feeds the next scheduling run.
Manufacturers connect APS to ERP through vendor APIs, middleware or an integration platform, scheduled file exchange, or, where both vendors support it, direct database integration. The choice depends on how often the schedule needs to refresh and what the ERP system supports. Our guide to APS and ERP integration covers the options in detail.
What should manufacturers look for in an APS?
Manufacturers should look for an APS that models their real constraints, replans quickly while keeping a working schedule intact, and connects cleanly to ERP and the shop floor.
| What to check | Why it matters |
| Finite capacity across every limiting resource | The schedule is only feasible if machines, tools, labor, and materials are all modeled. |
| Sequence-dependent setups | Changeover time often decides how much a line can produce in a shift. |
| Replanning behavior | A replan that reshuffles everything creates new disruption on the shop floor. |
| Available-to-Promise and Capable-to-Promise | Sales can commit dates based on stock and on real capacity. |
| ERP and MES connections | Planning works from current orders and current shop floor status. |
| Configuration effort | Business rules change, and planners need to update the model through configuration. |
For a vendor-by-vendor view, see our comparison of the 10 best APS software platforms for manufacturers.
How does Eyelit APS fit alongside ERP and MRP?
Eyelit APS adds the finite-capacity scheduling layer that sits between the ERP material plan and execution on the shop floor.
Eyelit APS is advanced planning and scheduling software that runs on the same data model as Eyelit MES, so real-time WIP, downtime, and shop-floor changes feed updated schedules back to execution.
Eyelit APS provides a wide range of constraint types to model any company’s business rules, including feature-combination limits, dynamic spacing, run-lengths on sub-sequences, sequence-dependent setup times, and inventory simulation during sequencing. Eyelit APS uses attribute-based changeover matrices with additive or maximum setup policies and per-resource setup times. Eyelit APS models auxiliary resources — tools, dies, operators with skill matrices, and batch processing resources such as ovens and autoclaves — as hard or soft constraints.
Eyelit APS replans with memory: when demand or supply changes, it minimizes disruption to the existing schedule, with the degree of allowed change configurable at any level or attribute.
Eyelit APS delivers Available-to-Promise and Capable-to-Promise in both batch and real-time modes, with the real-time module connecting to the order management system.
