MRP Net Requirements Calculation: Step-by-Step with Examples

The MRP net requirements calculation is the core engine that drives material requirements planning. It is the mathematical process that transforms demand into actionable purchase and production orders. Understanding this calculation is essential for anyone implementing, operating, or troubleshooting an MRP system. In this guide, we walk through the complete calculation step by step with detailed examples that show exactly how MRP determines what to order and when.

For the broader context, see our complete MRP guide and MRP inputs and outputs.

The Net Requirements Formula

At its core, the MRP net requirement for any period is:

Net Requirement = Gross Requirement - Scheduled Receipts - Projected On-Hand + Safety Stock

If the result is positive, there is a shortage that requires a planned order. If zero or negative, existing inventory and incoming supply cover the need.

Let us define each term:

Step-by-Step MRP Calculation Example

Let us walk through a complete MRP calculation for Component X, used in Product A.

Given data:

• Product A requires 3 units of Component X per assembly (BOM)
• Beginning on-hand inventory: 500 units
• Safety stock: 100 units
• Lead time: 2 weeks
• Lot sizing: Lot-for-lot
• An open PO for 300 units arriving in Week 2

Master Production Schedule for Product A:

Step 1: Calculate Gross Requirements

Multiply the MPS by the BOM quantity (3 units of Component X per Product A):

Step 2: Build the MRP Planning Grid

Now we build the time-phased planning grid, calculating period by period:

Week 1:

• Projected On-Hand (start): 500
• Gross Requirement: 150
• Scheduled Receipts: 0
• Projected On-Hand (end): 500 - 150 = 350
• 350 > 100 (safety stock), so no planned order needed

Week 2:

• Projected On-Hand (start): 350
• Gross Requirement: 240
• Scheduled Receipts: 300 (open PO)
• Projected On-Hand (end): 350 - 240 + 300 = 410
• 410 > 100, no planned order needed

Week 3:

• Projected On-Hand (start): 410
• Gross Requirement: 180
• Scheduled Receipts: 0
• Projected On-Hand (end): 410 - 180 = 230
• 230 > 100, no planned order needed

Week 4:

• Projected On-Hand (start): 230
• Gross Requirement: 300
• Scheduled Receipts: 0
• Projected On-Hand (end): 230 - 300 = -70
• Below zero! Net requirement exists.
• Net Requirement: 300 - 230 + 100 (safety stock) = 170 units
• Planned Order Receipt: 170 in Week 4
• Planned Order Release: 170 in Week 2 (Week 4 - 2 weeks lead time)

Week 5:

• Projected On-Hand (start): 100 (safety stock level after planned order)
• Gross Requirement: 210
• Net Requirement: 210 - 100 + 100 = 210 units
• Planned Order Receipt: 210 in Week 5
• Planned Order Release: 210 in Week 3

Week 6:

• Projected On-Hand (start): 100
• Gross Requirement: 270
• Net Requirement: 270 - 100 + 100 = 270 units
• Planned Order Receipt: 270 in Week 6
• Planned Order Release: 270 in Week 4

Complete MRP Grid

Reading the Results

MRP tells the planner:

• Week 2: Place an order for 170 units (to arrive Week 4)
• Week 3: Place an order for 210 units (to arrive Week 5)
• Week 4: Place an order for 270 units (to arrive Week 6)

The projected on-hand never drops below the 100-unit safety stock level because MRP planned orders to maintain that buffer.

Multi-Level BOM Explosion

The example above shows a single-level calculation. In practice, Component X might itself be made from sub-components, creating a multi-level BOM. MRP processes levels from top to bottom using low-level codes:

1. Calculate net requirements for Level 0 (finished goods)
2. Generate planned production orders for Level 0
3. Those planned orders become gross requirements for Level 1 components
4. Calculate net requirements for Level 1
5. Continue down through every BOM level

Low-level coding ensures that if an item appears at multiple BOM levels (common parts used in different sub-assemblies), MRP waits until all parent demand is calculated before netting. This prevents under-ordering.

Impact of Lot Sizing on the Calculation

The example above used lot-for-lot (order exactly the net requirement). Different lot sizing methods change the planned order quantities:

With EOQ of 500, the Week 4 order covers Week 5 demand entirely and partially covers Week 6, reducing the number of orders but increasing inventory.

Impact of Safety Stock

Safety stock changes the trigger point for planned orders. Without safety stock, MRP only generates orders when projected on-hand goes negative. With safety stock, MRP triggers orders earlier to maintain the buffer.

Compare the same scenario with 0 vs 100 safety stock:

Safety stock increases order quantities and triggers orders in the same or earlier periods. For more on sizing safety stock appropriately, see our safety stock in MRP guide.

Regenerative vs Net Change MRP

There are two approaches to running the MRP calculation:

Regenerative MRP recalculates net requirements for every item in the system, regardless of whether anything changed. It provides a complete refresh but is computationally intensive.

Net Change MRP only recalculates items affected by changes since the last run (new orders, inventory adjustments, BOM changes). It is faster and can be run more frequently.

Most modern MRP systems, including RMDB, support net change processing for rapid recalculation as conditions change.

Common Calculation Errors and How to Avoid Them

Avoiding these common MRP mistakes ensures your net requirements calculations produce actionable, reliable results.

Frequently Asked Questions

Let MRP Do the Math

Stop calculating net requirements in spreadsheets. RMDB from User Solutions automates MRP calculations across your entire BOM structure, with real-time recalculation as demand and supply change.

Schedule a free demo to see automated net requirements planning for your operation.