Materials requirements planning (MRP) is a time-phased planning approach used to determine what materials and components are needed, how much is needed, and when orders should be released. It is especially valuable where demand for components is derived from the production schedule for finished products. Distribution requirements planning (DRP) extends the same logic to the movement and replenishment of inventory through a distribution network.
Together, MRP and DRP connect market demand, production, procurement, inventory control and logistics. In modern organisations these activities are normally integrated through manufacturing resource planning (MRP II), enterprise resource planning (ERP), and sales and operations planning (S&OP). The quality of the resulting plan depends not only on software, but also on accurate data, realistic lead times, disciplined decision rules and effective cross-functional governance.
Learning Objectives
By the end of this chapter, the learner should be able to:
explain bills of materials and the distinction between independent and dependent demand;
prepare and interpret inventory records used in MRP;
calculate gross requirements, projected available balances, net requirements and planned order releases;
apply lead-time offsetting and common lot-sizing rules;
explain MRP II, DRP and ERP integration within S&OP;
diagnose MRP nervousness and recommend stabilising controls;
evaluate the operational, managerial and governance implications of materials planning.
1. The Logic of Dependent Demand
Independent demand is demand that originates outside the production system, such as customer orders or a forecast for finished goods. Dependent demand is calculated from the requirements of another item. For example, demand for bicycle tyres depends on the number of bicycles scheduled for production and the number of tyres required per bicycle.
The central relationship is:
Component requirement = Parent quantity × Usage quantity per parent
This relationship assumes that the bill of materials (BOM) is correct and that the parent quantity is expressed in the same planning period as the component requirement. If 500 tables are planned and each table uses four legs, gross demand for legs is 2,000, before considering inventory, scheduled receipts or scrap.
MRP is appropriate when:
demand has a definable relationship with a parent item;
the production or purchasing environment is sufficiently structured;
item, BOM, inventory and lead-time data are reliable;
requirements can be planned in discrete time periods.
MRP is less effective when demand is highly unpredictable, product structures change continuously, lead times are unreliable, or records are inaccurate. In such circumstances, MRP may generate precise-looking but operationally misleading recommendations.
2. Bills of Materials
A bill of materials is a structured record of every component, subassembly, raw material and sometimes process resource required to make a parent item. It specifies the product structure, quantity per assembly and relationships among levels.
A simplified product structure is shown below.
Indented product structure showing a finished product, subassembly and components
Figure: Indented product structure showing a finished product, subassembly and components
Figure: A multilevel bill of materials and its parent–component relationships.
2.1 BOM terminology
A single-level BOM lists the immediate components of one parent. A multilevel BOM shows components, subassemblies and raw materials across several levels. The top-level item is usually level 0; its direct components are level 1, and their components are level 2.
A phantom BOM represents a temporary grouping used for planning or engineering convenience but not normally stocked or built as a separate item. A planning BOM represents an aggregate or family structure used for forecasting and capacity planning. An engineering BOM describes design relationships, whereas a manufacturing BOM describes how the product is actually assembled and controlled on the shop floor.
BOM governance requires control of effective dates, engineering change notices, alternative components, unit of measure, scrap factors and revision status. A technically correct BOM that is not synchronised with production reality can cause shortages, excess purchases and inaccurate product costing.
3. Inventory Records and MRP Data
MRP uses a time-phased record for each item. The principal fields are:
Field
Meaning
Gross requirements
Total expected demand before available supply is considered
Scheduled receipts
Existing orders expected to arrive in specified periods
Projected available balance
Expected inventory at the end of each period
Net requirements
Additional quantity needed after considering available supply
Planned order receipts
Quantity planned to become available in a period
Planned order releases
Quantity that must be ordered or started earlier, after lead-time offsetting
Safety stock
Inventory deliberately protected from uncertainty
Inventory records also require an item code, description, unit of measure, lot-size rule, lead time, safety stock, order policy, current balance, allocated quantity and open-order information. The record must distinguish physical stock from usable stock. Quarantined, damaged, expired or customer-reserved inventory may not be available for MRP purposes.
3.1 The basic MRP calculation
For period t, a general projected available balance can be expressed as:
PABt = PABt−1 + SRt + PORt − GRt
where PABt is projected available balance at the end of period t, SRt is scheduled receipt, PORt is planned order receipt, and GRt is gross requirement. The expression assumes that receipts and requirements are measured in the same units and are allocated to the same period.
A practical net-requirement rule is:
NRt = max(0, GRt + SS − PABt−1 − SRt)
where NRt is net requirement and SS is the desired safety-stock level. This formulation treats the opening balance and scheduled receipts as available before the period’s planned receipt. Organisations may use different conventions, particularly concerning whether safety stock is included in projected balances; the convention must therefore be documented.
If the lot-sizing rule requires an order larger than the net requirement, the planned order receipt is adjusted accordingly. Planned order releases are then placed in an earlier period according to the item’s lead time.
4. Lead-Time Offsetting
Lead time is the elapsed time between releasing an order and receiving or completing it. Lead-time offsetting converts a required receipt date into a release date.
Planned release period = Required receipt period − Lead time
For example, if a component is required in week 8 and its lead time is two weeks, the order should normally be released in week 6. This assumes a fixed, known lead time and a planning calendar in which the stated lead time is measured consistently.
Lead time may include purchasing, queueing, setup, processing, inspection, transport and receiving. If these elements are ignored, the system may recommend releases that appear early enough but cannot physically meet the requirement. Lead times should be periodically reviewed using actual performance rather than treated as permanent constants.
5. Lot Sizing
Lot sizing determines the quantity associated with a planned order. The choice affects setup cost, purchase cost, inventory carrying cost, obsolescence, supplier workload and responsiveness.
Common rules include:
Lot-for-lot (L4L): order exactly the net requirement. It minimises planned cycle inventory but may create many orders and setups.
Fixed order quantity: use a specified quantity, often reflecting supplier packs, machine constraints or economic considerations.
Fixed-period requirement: cover requirements for a defined number of future periods.
Economic order quantity (EOQ): balance ordering and holding costs under relatively stable, independent-demand assumptions.
Part-period balancing: add periods to a lot while the additional holding cost remains reasonably close to the ordering cost.
The classical EOQ formula is:
Q* = √(2DS ÷ H)
where Q* is the economic order quantity in units per order, D is annual demand in units per year, S is ordering or setup cost per order, and H is annual holding cost per unit per year. The model assumes relatively constant demand, known replenishment lead time, instantaneous replenishment, no shortages, and no quantity discounts. It is not automatically suitable for lumpy, dependent demand; MRP lot sizing should reflect the actual time-phased requirements and operational constraints.
Worked example: a basic MRP record
An item has opening available inventory of 40 units, safety stock of 10 units, a two-week lead time and lot-for-lot ordering. Gross requirements are 30, 50 and 20 units in weeks 1, 2 and 3. There are no scheduled receipts.
Week
Gross requirement
Opening PAB
Net requirement
Planned receipt
Ending PAB
Planned release
1
30
40
0
0
10
—
2
50
10
50
50
10
Week 0
3
20
10
20
20
10
Week 1
For week 1, 40 − 30 = 10, which meets safety stock. In week 2, available inventory of 10 is insufficient to cover the 50-unit requirement while retaining 10 units of safety stock; therefore, the net requirement is 50 units. With a two-week lead time, its release is required in week 0. The week 3 order is released in week 1. If the planning horizon begins at week 1, the week 2 recommendation is already late and requires an expedite, rescheduling or revised requirement.
6. MRP Explosion and Regeneration
MRP explosion begins with the master production schedule (MPS) for finished products. The system uses the BOM to calculate gross requirements for level-1 components. Those requirements are then offset and planned; the resulting planned production of subassemblies becomes demand for their components. Explosion continues through all levels.
Two operating approaches are common:
Regenerative MRP: recomputes the complete plan at scheduled intervals. It is computationally straightforward but may change many recommendations.
Net-change MRP: updates only records affected by changes in transactions, inventory, BOMs or schedules. It can respond quickly but requires strong data discipline and careful exception management.
MRP outputs are normally recommendations, not automatic truth. Planners review exception messages such as expedite, defer, cancel, release, reschedule and shortage alerts. Human judgement remains important where suppliers, capacity, quality, transport or customer priorities are uncertain.
7. MRP Nervousness
MRP nervousness is excessive and unstable change in planned orders caused by small changes in demand, inventory, lead times or schedules. A modest change to the MPS can propagate through multiple BOM levels, creating new dates, quantities and priorities for suppliers and production teams.
Consequences include:
frequent order cancellations and expedites;
supplier distrust and higher procurement costs;
unstable shop-floor priorities;
increased transport and overtime;
excess inventory alongside shortages;
administrative effort spent reacting rather than improving flow.
Nervousness is often caused by short planning horizons, inaccurate records, variable lead times, inappropriate lot sizes, excessive safety-stock changes, frozen-schedule violations and poor communication between sales, production and procurement.
Controls include a time fence, frozen and slushy planning zones, firm planned orders, longer-term visibility to suppliers, exception thresholds, accurate inventory transactions, stable lot-sizing policies and collaboration before major schedule changes. A time fence does not eliminate flexibility; it identifies where changes require managerial approval because their operational impact is high.
8. MRP II and Capacity Integration
MRP II extends material planning to manufacturing resources. It integrates material requirements with rough-cut capacity planning, detailed capacity requirements planning, purchasing, shop-floor control, labour, tooling and financial evaluation.
The typical logic is iterative:
S&OP establishes an aggregate plan for product families and resources.
The MPS translates the aggregate plan into end-item quantities and dates.
MRP explodes the MPS into component and material requirements.
Capacity checks test whether work centres, labour and suppliers can support the plan.
The plan is revised until material and capacity feasibility are sufficiently aligned.
Purchasing and production execute approved orders, while feedback updates the plan.
A material plan that ignores capacity is not a feasible operating plan. Conversely, capacity planning without material availability may schedule work that cannot start. MRP II therefore provides a closed-loop relationship among demand, materials, capacity, execution and financial consequences.
9. Distribution Requirements Planning
DRP applies time-phased planning to distribution inventory. It starts with demand at distribution centres, regional warehouses or customer-facing locations and calculates replenishment requirements from a supplying warehouse or plant.
A DRP record commonly includes forecast or customer demand, scheduled receipts, projected available balance, safety stock, planned replenishment receipts and planned shipment releases. The same balance logic applies, but transportation lead time and shipment calendars become particularly important.
For a distribution location, a simplified projected balance is:
PABt = PABt−1 + SRt + PORt − Demandt
Here PORt is the planned replenishment receipt at the location. The corresponding planned shipment from the supplying node may be offset by transportation and handling lead time. DRP therefore coordinates inventory positioning across a network rather than merely calculating a factory’s component requirements.
DRP is useful for:
synchronising plant production with warehouse replenishment;
setting deployment priorities when supply is constrained;
reducing emergency transfers and transport costs;
maintaining target service levels across locations;
providing a common plan for logistics, production and sales.
Forecast consumption, promotions, seasonality, minimum shipment quantities and transportation capacity must be considered. A mathematically neat DRP schedule can still fail if a truck, route, dock or warehouse slot is unavailable.
10. ERP Integration and S&OP
ERP integrates transactions and master data across functions such as sales, purchasing, inventory, production, finance, quality and logistics. An ERP system may contain MRP, DRP, warehouse management, supplier collaboration and advanced planning modules. Its value comes from shared data and coordinated workflows, not simply from installing software.
The planning hierarchy is usually:
Planning level
Typical horizon
Main purpose
Strategic
Years
Network, capacity and sourcing decisions
S&OP
Months to approximately 18 months
Balance demand, supply, inventory and financial objectives
MPS
Weeks to months
Define end-item production quantities and dates
MRP/DRP
Days to weeks or months
Plan components, materials and distribution replenishment
Execution
Hours to days
Release, dispatch, receive and report actual work
S&OP is a cross-functional decision process rather than a software report. A typical cycle includes data review, demand review, supply review, reconciliation of scenarios and an executive decision. The agreed plan should identify assumptions, constraints, risks, inventory targets and decisions requiring escalation.
ERP integration creates important controls: one item master, controlled BOM revisions, transaction traceability, role-based approvals, audit trails and consistent units of measure. However, integration can also spread an error rapidly. A wrong unit conversion, obsolete BOM or duplicated item code may affect purchasing, production, inventory valuation and customer service simultaneously.
11. Illustrative Case: Northstar Appliances
Background
Northstar Appliances assembles 1,000 compact washing machines per month. Each machine requires one control module, one motor and two water hoses. The motor is purchased; the control module is assembled internally from one circuit board and one housing. Northstar operates a central plant and two regional distribution centres.
Problem
Sales introduces a promotion that increases the forecast for the following month by 20%. The sales department changes the MPS immediately, while procurement retains a six-week motor lead time and the distribution centres have not updated their replenishment plans. The ERP system generates multiple expedites and cancellations. At the same time, one warehouse accumulates excess hoses while the other experiences a shortage.
Analysis
The demand increase for 1,000 additional machines creates requirements for 1,000 motors, 1,000 control modules and 2,000 hoses. Each additional control module requires one circuit board and one housing. The MRP explosion identifies the additional material requirements, but the motor lead time means that part of the requirement cannot be met through normal purchasing. DRP shows that stock is positioned incorrectly: total network inventory is adequate for some items, but not at the locations where demand occurs.
The planning team also discovers that the motor lead time in ERP is six weeks, while actual average lead time is eight weeks during the promotion season. The MPS was changed inside the frozen time fence without an executive review, causing nervousness throughout the system.
Approach and solution
Northstar takes five actions:
It creates a constrained S&OP scenario separating the promotional forecast from committed customer orders.
It checks supplier capacity and negotiates a split delivery for motors, while qualifying an approved alternate supplier.
It uses lot-for-lot planning for circuit boards and housings but preserves supplier-pack quantities for hoses.
It runs DRP for both distribution centres using actual inventory, shipment calendars and regional demand.
It establishes a two-week frozen zone, a four-week slushy zone and formal approval for changes inside the frozen zone.
The MPS is then revised to reflect confirmed motor supply rather than the unconstrained sales forecast. The ERP master data is corrected, and supplier lead-time performance is reviewed monthly.
Outcome
Northstar cannot satisfy the entire promotion at the originally requested date, but it avoids uncontrolled expedites and communicates a reliable allocation plan to customers. Excess hose inventory is redeployed between warehouses. The revised plan reduces schedule changes, improves supplier confidence and makes the capacity and service trade-offs visible to management.
Learning points
The case illustrates that MRP cannot create supply that does not exist. It can expose timing and quantity consequences, but management must resolve capacity, sourcing and priority decisions. DRP prevents a local warehouse view from dominating the network plan, while S&OP provides the governance needed to reconcile commercial ambition with operational feasibility.
12. Performance Measures and Governance
Materials planning should be assessed with balanced measures rather than a single inventory figure. Useful measures include:
inventory record accuracy;
BOM accuracy and engineering-change compliance;
schedule adherence;
supplier on-time delivery;
planned-order stability;
material availability at production start;
customer service level and fill rate;
inventory turns and days of supply;
forecast error and forecast bias;
expedite frequency and obsolete inventory.
A high inventory turnover ratio is not automatically good if it results in stockouts. Similarly, a high service level may be uneconomic if achieved through excessive emergency freight and safety stock. Measures should be interpreted together and aligned with customer promises, risk tolerance and financial objectives.
Ethical and governance issues include truthful reporting of inventory, transparent communication of shortages, responsible supplier treatment, traceability of regulated materials and avoidance of deliberately optimistic schedules. Where AI or advanced analytics are used to recommend orders, organisations should retain human accountability, document data sources, monitor bias and provide an override process. Automated recommendations should be explainable enough for planners to understand material consequences.
13. Challenges, Limitations and Best Practices
MRP and DRP are planning systems, not substitutes for operational discipline. Common limitations are poor master data, uncertain demand, variable lead times, capacity bottlenecks, inappropriate safety stocks, excessive dependence on forecasts and weak transaction discipline.
Best practices include:
establish ownership for item, BOM, routing and supplier master data;
use cycle counting and investigate root causes of record errors;
separate independent-demand forecasting from dependent-demand calculation;
validate lead times against actual performance;
use time fences and exception thresholds to control nervousness;
integrate capacity and material feasibility before releasing schedules;
conduct regular S&OP reviews using scenarios rather than one unchallenged forecast;
align lot sizes with setup, supplier, transport and obsolescence realities;
use DRP to optimise network inventory, not merely warehouse replenishment;
audit ERP interfaces, units of measure, revision dates and user permissions;
review performance using service, cost, stability and inventory measures together.
Chapter Summary
MRP converts a master production schedule into time-phased requirements for components and materials by using bills of materials, inventory records, lead times and lot-sizing rules. Dependent demand is calculated from parent requirements, whereas independent demand originates from customers or the market. The projected available balance, net requirement, planned receipt and planned release are the central elements of an MRP record.
Lead-time offsetting determines when an order must be released to meet a required receipt date. Lot-for-lot, fixed quantity, fixed-period and EOQ approaches have different assumptions and operational consequences. MRP nervousness results when small changes create excessive schedule instability; time fences, accurate data and disciplined exception management reduce it.
MRP II adds capacity and resource integration. DRP applies time-phased logic to distribution networks. ERP provides transaction integration and traceability, while S&OP balances demand, supply, inventory, capacity and financial objectives at a cross-functional level. Effective planning depends on reliable data, realistic constraints, governance and managerial judgement.
Key Terms
Bill of materials: Structured record of the components and quantities required for a parent item.
Dependent demand: Demand calculated from the requirements of another item.
Independent demand: Demand arising from customers, markets or external forecasts.
Master production schedule: Time-phased plan for producing finished goods or end items.
Gross requirement: Total demand for an item before available supply is deducted.
Projected available balance: Expected inventory remaining at the end of a planning period.
Net requirement: Additional quantity needed after considering available inventory, receipts and safety stock.
Scheduled receipt: An existing order expected to arrive in a specified period.
Planned order receipt: An MRP recommendation for supply to become available in a specified period.
Planned order release: The order or production start date calculated by offsetting a planned receipt by lead time.
Lead time: Time between releasing an order or job and receiving or completing it.
Lot sizing: Method used to determine the quantity of a planned order.
Lot-for-lot: Lot-sizing rule that orders exactly the calculated net requirement.
MRP nervousness: Unstable changes in planned orders caused by changes in requirements or planning data.
MRP II: Extension of MRP that integrates materials with capacity, labour, finance and manufacturing resources.
Distribution requirements planning: Time-phased planning of replenishment through a distribution network.
Enterprise resource planning: Integrated system supporting transactions and planning across organisational functions.
Sales and operations planning: Cross-functional process for balancing demand, supply, resources and financial objectives.
Time fence: Defined planning boundary within which schedule changes are restricted or controlled.
Review Questions
Short Answer Questions
Distinguish between independent demand and dependent demand, giving one example of each.
What information is contained in a bill of materials?
Explain the difference between a planned order receipt and a planned order release.
State two causes and two consequences of MRP nervousness.
How does DRP extend the logic of MRP?
Long Answer Questions
Explain the complete logic of MRP from the master production schedule through BOM explosion, netting, lot sizing and lead-time offsetting.
Compare lot-for-lot, fixed-order-quantity and EOQ approaches, including their assumptions and operational implications.
Discuss how MRP II, ERP and S&OP interact in an integrated planning hierarchy.
Explain the role of inventory accuracy, BOM governance and lead-time maintenance in successful MRP implementation.
Evaluate the benefits and limitations of applying DRP across a multi-location distribution network.
Analytical / Application Questions
An item has opening inventory of 25 units, safety stock of 5 units and gross requirements of 15, 30 and 10 units in weeks 1–3. There are no scheduled receipts, lot-for-lot ordering is used, and lead time is one week. Prepare the MRP record and identify any release that must occur before week 1.
A company changes its master production schedule weekly, causing frequent supplier cancellations and shop-floor expedites. Diagnose the likely sources of nervousness and design a control policy using time fences, exception thresholds and supplier communication.
Two warehouses have uneven inventory while total network stock is adequate. Explain how DRP and S&OP could be used to determine replenishment, redistribution and production priorities without increasing total inventory unnecessarily.
Multiple Choice Questions with Answers
Question 1
Which statement best describes dependent demand?
A. Demand generated directly by customer purchases
B. Demand calculated from the requirements of a parent item
C. Demand that is always statistically independent
D. Demand that cannot be planned using a BOM
Correct Answer: B. Demand calculated from the requirements of a parent item
Question 2
A product requires three units of component X. If the MPS calls for 400 products, what is the gross requirement for X before inventory netting?
A. 133 units
B. 397 units
C. 400 units
D. 1,200 units
Correct Answer: D. 1,200 units
Question 3
Which MRP field represents an existing order expected to arrive in a specified period?
A. Scheduled receipt
B. Gross requirement
C. Safety stock
D. Planned order release
Correct Answer: A. Scheduled receipt
Question 4
A component is required in week 10 and has a lead time of three weeks. In which week should it normally be released?
A. Week 6
B. Week 7
C. Week 8
D. Week 13
Correct Answer: B. Week 7
Question 5
Under lot-for-lot ordering, the planned order receipt is normally equal to the:
A. Annual demand
B. Safety stock only
C. Net requirement
D. Opening inventory
Correct Answer: C. Net requirement
Question 6
Which assumption is associated with the classical EOQ model?
A. Demand is relatively constant and known
B. All demand is generated by a multilevel BOM
C. Shortages are always required
D. Replenishment is never received in full
Correct Answer: A. Demand is relatively constant and known
Question 7
MRP nervousness primarily refers to:
A. Excessive changes in planned orders after small planning changes
B. A permanent increase in safety stock
C. The physical vibration of production equipment
D. A shortage caused only by theft
Correct Answer: A. Excessive changes in planned orders after small planning changes
Question 8
What is the primary additional focus of MRP II compared with basic MRP?
A. Advertising expenditure
B. Capacity and broader manufacturing resources
C. Customer demographic analysis
D. Product packaging design only
Correct Answer: B. Capacity and broader manufacturing resources
Question 9
Distribution requirements planning is mainly used to:
A. Calculate employee payroll
B. Plan replenishment through a distribution network
C. Replace all demand forecasts with EOQ
D. Design engineering drawings
Correct Answer: B. Plan replenishment through a distribution network
Question 10
Which process balances demand, supply, inventory, capacity and financial objectives at an aggregate level?