Answer First
Managers use MRP because it systematically translates a master production schedule into time-phased orders for every component, respecting lead times, inventory on hand, and lot sizes. A simple spreadsheet quickly breaks when there are multiple levels, shared components, and changing demand—MRP does the explosion and timing logic automatically.
Real MBA Example: Simple Product with Components
A small manufacturer produces a finished product P. Each unit of P requires:
- 2 units of component A
- 1 unit of component B
Data:
- Planned demand for P: 50 units in Week 4
- Lead time: P = 1 week, A = 1 week, B = 2 weeks
- On-hand inventory at start of Week 1: P = 0, A = 10, B = 20
- Lot size: lot-for-lot (order exactly what you need)
Step-by-Step MRP Logic
1. Start with the Master Production Schedule (MPS) for P
We need 50 units of P in Week 4. With a 1-week lead time, we must start production of P in Week 3.
Planned order release for P: 50 units in Week 3.
2. Explode requirements for component A
Each P needs 2 A, so:
\[ \text{Gross requirements for A in Week 3} = 2 \times 50 = 100 \]
On-hand A at start: 10 units.
\[ \text{Net requirements for A} = 100 – 10 = 90 \]
Lead time for A is 1 week, so we must release an order for A in Week 2:
Planned order release for A: 90 units in Week 2.
3. Explode requirements for component B
Each P needs 1 B, so:
\[ \text{Gross requirements for B in Week 3} = 1 \times 50 = 50 \]
On-hand B at start: 20 units.
\[ \text{Net requirements for B} = 50 – 20 = 30 \]
Lead time for B is 2 weeks, so we must release an order for B in Week 1:
Planned order release for B: 30 units in Week 1.
4. Build the time-phased MRP tables
Finished Product P (LT = 1 week)
| Week | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Gross requirements | 0 | 0 | 0 | 50 |
| Scheduled receipts | 0 | 0 | 0 | 0 |
| Projected on-hand | 0 | 0 | 0 | 0 |
| Net requirements | 0 | 0 | 0 | 50 |
| Planned order receipts | 0 | 0 | 50 | 0 |
| Planned order releases | 0 | 0 | 0 | 50 (if LT counted differently) |
(Using the usual convention, we plan a receipt of 50 in Week 4 and release it in Week 3.)
Component A (LT = 1 week)
| Week | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Gross requirements | 0 | 0 | 100 | 0 |
| Scheduled receipts | 0 | 0 | 0 | 0 |
| Projected on-hand | 10 | 10 | 0 | 0 |
| Net requirements | 0 | 0 | 90 | 0 |
| Planned order receipts | 0 | 0 | 90 | 0 |
| Planned order releases | 0 | 90 | 0 | 0 |
Component B (LT = 2 weeks)
| Week | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Gross requirements | 0 | 0 | 50 | 0 |
| Scheduled receipts | 0 | 0 | 0 | 0 |
| Projected on-hand | 20 | 20 | 0 | 0 |
| Net requirements | 0 | 0 | 30 | 0 |
| Planned order receipts | 0 | 30 | 0 | 0 |
| Planned order releases | 30 | 0 | 0 | 0 |
Intuition
MRP “explodes” the demand for finished goods into time-phased requirements for every component, then backs up orders according to lead times and existing inventory. Instead of guessing when to order parts, managers get a precise schedule of what to order, how much, and in which week.
Common Exam Mistakes
- Forgetting to subtract on-hand inventory before computing net requirements.
- Ignoring lead times when timing planned order releases.
- Confusing gross requirements (from parents) with net requirements (after inventory).
- Mixing up planned order receipts and planned order releases.
Why This Matters
MRP is the backbone of ERP systems in manufacturing, electronics, automotive, and consumer products. It prevents stockouts, reduces excess inventory, and aligns purchasing with production. For MBAs, understanding MRP means understanding how real companies translate strategy and forecasts into day-to-day orders and schedules.
Final Summary
Managers use MRP because it converts a master production schedule into detailed, time-phased orders for every component, respecting lead times, inventory, and lot sizes. A simple spreadsheet cannot reliably handle multi-level bills of materials and shifting demand, but MRP can—making it essential for modern operations and supply chain management.
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