Lesson 1 of 6
How MRP thinks
10 min
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MRP is arithmetic with dates. For each material it walks along the time axis, adds receipts, subtracts requirements, and wherever the balance would fall below zero (or below safety stock) it proposes a receipt big enough and early enough to fix it. SAP Learning describes three steps after the BOM explosion: net requirements calculation, lot-size calculation and scheduling. If you can do them on paper, you can predict and test MRP.
- Planning fileWhich materials changed since the last run and need planning
- Net requirementsRequirements minus stock and firm receipts, per date; safety stock protected
- Lot sizeLot-sizing procedure, then minimum, maximum and rounding
- Procurement typeE: planned order · F: purchase requisition or schedule line
- SchedulingDates from lead times or the routing
- BOM explosionPlanned orders create dependent requirements one level down
Net requirements
The net requirements calculation checks whether requirements (PIRs, sales orders, dependent requirements, reservations, stock transfer requirements) can be covered by available stock and firm receipts (purchase orders, production orders, firmed proposals). Non-firm proposals from the last run are not counted as supply; MRP recreates or adapts them. SAP Learning notes that safety stock, set in MRP 2 or the MRP area, is treated as a buffer that MRP replenishes when inventory falls below it.
Lot sizing
| Procedure (example key) | Proposal quantity | Good for |
|---|---|---|
| Lot-for-lot (EX) | Exactly the shortage | Expensive or customer-specific items |
| Fixed lot size (FX) | A fixed quantity; several proposals if one is not enough | Container or batch sizes |
| Replenish to maximum stock (HB) | Up to the maximum stock level in the master | Line-side supermarkets |
| Period lot sizes (weekly, monthly) | All requirements of a period in one proposal | Fewer orders for steady demand |
Common lot-sizing procedures. Keys are SAP standard examples; your system may define its own. After the procedure, minimum and maximum lot sizes and rounding are applied.
Scheduling
For in-house items, basic date scheduling subtracts the goods receipt processing time and the in-house production time from the requirement date to get the start. Lead-time scheduling uses the routing for more exact dates and, as SAP Learning notes, also creates capacity requirements. For purchased items, the planned delivery time (from the product or the info record) and GR processing time apply, plus purchasing processing time. An opening period gives the planner time to convert a planned order before it should start. If backward scheduling lands in the past, the system can switch to forward scheduling from today, which makes the finish date late; that is when you see "start date in the past" messages.
- 14 OctOpening datePlanner should convert by now (opening period 3 workdays)
- 17 OctStartBackward scheduled start of the planned order
- 22 OctFinish3 workdays in-house production time
- 23 OctAvailableAfter 1 workday GR processing; covers the requirement
- 23 OctRequirementSales order delivery date for 120 alternators
A worked example
Rotor CMC-RTR-200 in plant 1710: stock 150, safety stock 200, fixed lot size 500, planned delivery time 10 days. Dependent requirements from alternator planned orders: 280 on 20 October. Available after the requirement: 150 − 280 = −130; to restore safety stock MRP needs 330. With a fixed lot of 500, MRP proposes one requisition for 500, available by 20 October, so the order date is about two weeks earlier. Projected stock afterwards: 370. That is the number you would write as the expected result.
What would you do?
A planner sees MRP propose 1,000 rotors where she expected 330. The fixed lot size is 500 and nothing else changed. What is the most plausible explanation?
- MRP = net requirements, lot sizing, scheduling, then BOM explosion, level by level.
- Only stock and firm receipts count as supply; non-firm proposals are recreated.
- Lot-sizing procedure first, then min/max and rounding.
- Scheduling uses in-house production time or the routing, planned delivery time, GR processing time and opening period.
Check your understanding · not graded
Lesson check
1. In which order does MRP apply lot-size rules?
2. Which receipts count as supply in the net requirements calculation?
3. What does the opening period give the planner?
4. Rotor stock 150, safety stock 200, requirement 280, fixed lot 500. What does MRP propose?