Material Requirements Planning (MRP) and Enterprise Resource Planning (ERP) are closely connected, but they are not the same thing. MRP is planning logic used to calculate what material is needed, how much is needed and when it is needed. ERP is the broader enterprise system that integrates transactions and data across functions such as procurement, inventory, manufacturing, sales and finance.
In many companies, MRP runs inside the ERP system rather than as a separate standalone application.
MRP vs ERP at a Glance
| Area | MRP | ERP |
|---|---|---|
| Primary purpose | Calculate time-phased material requirements | Integrate enterprise transactions, data and processes |
| Main focus | Material supply and dependent demand | Finance, procurement, sales, inventory, manufacturing and other functions |
| Typical inputs | Master schedule/demand, BOM, inventory, scheduled receipts, lead times, lot-sizing rules | Master data plus transactions across business functions |
| Typical outputs | Planned orders, purchase/manufacturing recommendations, reschedule messages | Purchase orders, inventory transactions, production orders, invoices, financial postings and integrated reports |
| Forecasting? | MRP consumes planning demand; it is not itself the demand-forecasting process | ERP may contain or integrate forecasting/planning applications |
How MRP Works
Oracle describes MRP as a process that calculates net requirements by evaluating the master schedule, bills of material, scheduled receipts, on-hand inventory balances, lead times and order modifiers. The planning engine then recommends new or rescheduled supply.
The basic logic is:
Gross requirement − available inventory − scheduled receipts + required buffers = net requirement
The actual calculation is time-phased and depends on product structure, lead times, lot sizes, safety stock, existing orders and other planning parameters.
Core MRP Inputs
1. Master Schedule or Demand
MRP needs a statement of what finished goods or parent items are required and when. This may come from a master production schedule, demand schedule, sales orders, forecasts or a combination depending on the planning environment.
2. Bill of Materials (BOM)
The BOM defines the product structure: which components are required to produce each parent item and in what quantity.
If one finished product requires 2 units of Component A and the plan calls for 500 finished products, the gross component requirement is initially 1,000 units before inventory and scheduled receipts are netted.
3. Inventory and Scheduled Receipts
MRP considers usable on-hand inventory and existing purchase or production orders expected to arrive. Without accurate records, the planning recommendation will be wrong even if the planning algorithm works perfectly.
4. Lead Times
Lead times determine when a planned order must be released to make the material available by the required date. Incorrect lead times are a common cause of unrealistic MRP messages.
5. Lot-Sizing and Planning Rules
Order minimums, fixed lot sizes, multiples, safety stock and other parameters affect the quantity and timing of recommendations.
Worked MRP Example
A manufacturer needs 100 finished units in Week 6. Each finished unit requires 3 units of Component X.
Gross Component X requirement = 100 × 3 = 300 units.
The company has:
- 80 usable units on hand.
- 50 units already scheduled to arrive before the requirement date.
Ignoring safety stock and lot-sizing for simplicity:
Net requirement = 300 − 80 − 50 = 170 units
If Component X has a two-week procurement lead time, MRP will time the planned supply so the material is available for Week 6, which means the order needs to be released early enough to respect that lead time.
Dependent vs Independent Demand
MRP is particularly important for dependent demand: demand for components derived from demand for a parent item. Finished-goods demand may be forecast or customer driven; component demand is then calculated by exploding the BOM.
This is why saying that “MRP forecasts demand” is misleading. Forecasting determines or contributes to the independent-demand signal; MRP translates that demand into time-phased material requirements.
Typical MRP Outputs
- Planned purchase orders.
- Planned manufacturing orders.
- Reschedule-in or reschedule-out recommendations.
- Cancellation recommendations.
- Exception messages.
- Projected inventory positions.
A planner should review these recommendations rather than blindly converting every message into an order.
What ERP Adds
An ERP system connects the planning output to enterprise execution. For example:
MRP planned purchase → Buyer converts to PO → Supplier delivers → Warehouse posts receipt → Inventory updates → Invoice is matched → Finance posts liability and payment
The same system may also connect customer orders, production, inventory, costing and accounting, providing a shared transaction record.
MRP Does Not Automatically Solve Capacity
Traditional MRP is primarily material focused. Oracle notes that MRP can assume sufficient capacity unless separate capacity-planning logic is used to validate the material plan. A material plan can therefore be technically feasible from a component perspective while exceeding machine or labour capacity.
Why MRP Produces Bad Recommendations
The most common problems are often master-data or process problems:
- Incorrect BOM quantities.
- Wrong lead times.
- Inventory inaccuracies.
- Outdated open purchase orders.
- Unrealistic master schedule.
- Incorrect lot sizes or minimum order quantities.
- Safety stock set without understanding demand variability.
- Planner messages ignored for long periods.
MRP vs Reorder Point
A reorder-point method triggers replenishment when inventory falls to a defined level. MRP instead uses time-phased demand and product structure to calculate future material needs. Reorder-point methods can work well for simpler independent-demand items; MRP is particularly valuable where multi-level BOM relationships and scheduled production matter.
Useful Planning KPIs
- Schedule adherence.
- Material availability.
- Planner exception age.
- Inventory accuracy.
- Supplier lead-time adherence.
- Shortage / stockout frequency.
- Excess and obsolete inventory.
- Planned vs actual order dates and quantities.
Common Interview Question
Question: What is the difference between MRP and ERP?
Strong answer: MRP is the planning logic that calculates time-phased material requirements using demand or a master schedule, BOMs, inventory, scheduled receipts and lead times. ERP is the broader enterprise platform that integrates those planning results with procurement, inventory, manufacturing, sales, finance and other transactions.
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