Production Planning and Scheduling: MPS, MRP, Capacity, and How to Stop Running Out of Both

The planning hierarchy

Production planning operates at three levels, each with a different time horizon and planning unit.

Sales and operations planning (S&OP). The top-level process that aligns sales demand with supply capacity over a 3 to 18 month horizon. S&OP works in aggregate units (product families, production lines, total revenue) and answers: do we have enough capacity to serve the demand we expect? Where are the gaps, and how will we close them, through capacity additions, demand shaping, inventory builds, or customer commitment adjustments? S&OP outputs inform the master production schedule.

Master production schedule (MPS). The MPS converts the S&OP aggregate plan into a SKU-level production schedule over a rolling horizon of 4 to 12 weeks. It specifies what finished goods will be produced, in what quantities, and in what weeks. The MPS is the primary driver of all downstream planning: material requirements, capacity loading, procurement signals. A reliable MPS, one that is followed and not constantly changed, is the foundation of a stable supply chain. An unstable MPS that changes frequently is a major source of inventory waste, premium freight, and supplier relationship damage.

Material requirements planning (MRP). MRP takes the MPS as input and calculates the material requirements, what components, raw materials, and subassemblies are needed, in what quantities, and when, by exploding the bill of materials (BOM) against the planned production schedule and netting against existing inventory and open purchase orders. MRP outputs are planned purchase orders (what to buy) and planned production orders (what to make) with due dates derived from the MPS schedule and supplier and manufacturing lead times.

The master production schedule in practice

An MPS entry for a given SKU and week contains three numbers: gross requirement (how much demand must be satisfied), scheduled receipts (what is already in process, in production or in transit from suppliers), and projected on-hand inventory. The planning engine calculates whether projected on-hand will fall below the safety stock target, and if so, generates a planned production order to cover the shortfall.

Projected on-hand = Prior period on-hand + Scheduled receipts − Gross requirements

When projected on-hand falls below the safety stock target at any point in the planning horizon, a planned order is generated for the period offset back from the need date by the production lead time. If the production lead time for a product is 3 weeks and stock will run out in week 8, the planned order is due to start in week 5.

The time fence is a key MPS concept: the portion of the planning horizon close to the current date during which the schedule should be frozen against system-generated changes. Inside the time fence, only manual changes are made, automatic replanning inside this window would destabilize work-in-process and disrupt material that has already been committed. The time fence is typically set at or slightly inside the cumulative manufacturing lead time.

MRP: the logic and the limitations

MRP starts with the MPS and works backward through the BOM, level by level. For each component at each level, it calculates: how many are needed (gross requirement), how many are already available (on-hand inventory) or expected (scheduled receipts), and therefore how many must be procured or manufactured (net requirement). The net requirement is then offset backward in time by the lead time to generate a planned order with the correct due date.

MRP assumes infinite capacity, it plans based on what needs to happen without checking whether production capacity is available to execute. This is MRP’s most significant limitation: the MRP plan can be physically impossible to execute if capacity is insufficient. Capacity requirements planning (CRP) is the companion process that applies the MRP plan to work center capacities to identify overloads. When overloads are found, the MPS must be adjusted, by pulling orders forward, pushing them back, adding capacity, or renegotiating due dates with customers.

MRP also assumes that lead times are fixed. In practice, manufacturing lead times vary with shop floor loading, a work center running at 95 percent capacity takes longer to process a job than one at 70 percent. This feedback loop between capacity loading and lead time is the core challenge in shop-floor scheduling.

Capacity planning: rough-cut and detailed

Capacity planning has two levels corresponding to the planning hierarchy.

Rough-cut capacity planning (RCCP) operates at the MPS level and answers whether the proposed MPS is feasible before MRP is run. RCCP uses a simplified capacity model, key resources (bottleneck machines, critical skills) and their load per unit of production, to quickly check whether the MPS overloads any critical resource. If it does, the MPS is adjusted before MRP is run, avoiding the waste of planning material requirements against an infeasible schedule.

Capacity requirements planning (CRP) operates at the MRP level and provides a detailed work center-by-work center load analysis for each time period. CRP identifies where planned orders create capacity overloads and where there is slack capacity available. Planners use CRP to reschedule orders, authorize overtime or additional shifts, or subcontract work to external processors.

The bottleneck work center is the limiting resource, the one whose capacity constrains total system throughput. The Theory of Constraints (TOC) framework argues that scheduling decisions should be anchored to the bottleneck: the bottleneck should never be idle, and the schedule of all other resources should be subordinated to the bottleneck’s schedule. Non-bottleneck resources running at maximum efficiency but producing faster than the bottleneck can absorb simply creates work-in-process inventory rather than throughput.

Lot sizing in production planning

MRP generates net requirements at the component level, but actual production and purchase orders are placed in quantities that reflect lot sizing rules. The lot sizing decision balances setup cost and carrying cost, exactly as the EOQ model does for purchased items.

Lot-for-lot (L4L): produce exactly what is needed each period. Minimizes inventory, but may result in many small production runs with high setup cost if setups are expensive.

Fixed order quantity: always produce in a set quantity (similar to EOQ for manufacturing). Balances setup cost and carrying cost over a planning horizon.

Period order quantity (POQ): produce enough to cover a fixed number of periods of demand. Reduces the frequency of setups while keeping lot sizes responsive to actual demand patterns.

The EOQ post covers the trade-off in detail. For manufacturing, the equivalent of ordering cost is the setup cost, the cost of a changeover between products. When setup times are long and costly, lot sizes should be larger. When setups are short (as in SMED, single-minute exchange of die, lean manufacturing practice), smaller lots and more frequent production runs become economical.

The connection to inventory policy

Production planning directly determines finished goods inventory levels. A plan that produces in large batches to amortize setup cost creates high cycle stock. A plan with long manufacturing lead times creates high safety stock requirements. A plan that is frequently changed disrupts supplier delivery schedules and increases the risk of component shortages, which in turn forces safety stock increases upstream.

The safety stock calculator inputs, lead time, lead time variability, demand variability, are all influenced by planning decisions. Reducing manufacturing lead time (by reducing lot sizes, reducing WIP, or improving scheduling stability) directly reduces the safety stock required at the finished goods level. See the Safety Stock Calculator to quantify the inventory impact of lead time changes.

Schedule stability is a particularly underappreciated input. A frozen MPS that rarely changes allows suppliers to deliver on predictable schedules, reduces expediting costs, and keeps safety stock requirements lower than a volatile schedule that forces suppliers to build buffer of their own.

S&OP: aligning demand and supply at the executive level

S&OP is the governance process that prevents the planning hierarchy from fracturing under competing functional pressures. Without S&OP, sales commits to customers based on optimistic forecasts, operations plans production based on conservative forecasts, and finance plans revenue based on a third set of numbers. The gaps between these plans create inventory surprises, customer service failures, and missed financial targets.

A working S&OP process produces a single agreed plan, one set of demand numbers, one production plan, one inventory projection, reviewed and approved by senior leadership on a monthly cadence. The five-step S&OP process: data gathering (actuals versus plan), demand review (sales forecast with assumptions), supply review (capacity and material feasibility), pre-S&OP reconciliation of gaps, and executive S&OP decision meeting.

Common mistakes

• Running MRP without first validating MPS feasibility. MRP run against an infeasible MPS produces purchase orders and production orders that cannot be executed on time. Capacity constraints at a bottleneck work center become component shortages when the planned work is delayed. Run rough-cut capacity planning before MRP to ensure the MPS is achievable before committing to the downstream plan.
• Treating MRP output as a firm schedule. MRP produces planned orders, not firm commitments. Planners must review, filter, and release MRP outputs rather than automatically converting every planned order to a firm purchase order or production order. A planned order inside the planning time fence that conflicts with a firm commitment must be resolved manually.
• Ignoring the bottleneck in scheduling decisions. A detailed schedule that maximizes efficiency at every work center independently will overload the bottleneck and underutilize other resources. Schedule to the bottleneck first, then subordinate the rest of the schedule to keep the bottleneck fed and producing.
• Setting safety stock without accounting for planning variability. Safety stock calculations (see the Safety Stock Calculator) assume a distribution of lead time and demand variability. If the planning process introduces additional variability, through frequent MPS changes, unreliable BOM accuracy, or poor schedule adherence, safety stock calculated from historical demand and lead time data will be insufficient. Fix the planning variability before relying on safety stock alone to cover it.

Frequently asked questions

What is the difference between MPS and MRP?

The master production schedule (MPS) is a SKU-level finished goods production plan over a rolling horizon, it specifies what will be built, in what quantities, and in which time periods. Material requirements planning (MRP) takes the MPS as input and calculates the component, raw material, and subassembly requirements needed to execute the MPS, netting against existing inventory and open orders to generate planned purchase orders and production orders. MPS plans what to produce; MRP plans what to buy and make to support that production plan.

What is a planning time fence in MRP?

A planning time fence is the portion of the MPS planning horizon closest to the current date during which the system is not permitted to make automatic schedule changes. Inside the time fence, changes to planned orders require manual intervention by a planner. This prevents MRP from automatically rescheduling work-in-process and committed material in response to demand fluctuations that occur after production has already been initiated. The time fence is typically set at or slightly inside the cumulative manufacturing lead time for the product.

What is rough-cut capacity planning (RCCP)?

Rough-cut capacity planning (RCCP) is a high-level feasibility check applied to the MPS before MRP is run. It uses a simplified model of key resources (bottleneck machines, critical skills, warehouse space) and their load per unit of planned production to quickly check whether the proposed MPS overloads any critical resource. If RCCP finds an overload, the MPS is adjusted before MRP is run, avoiding the work of planning material requirements against a schedule that cannot be physically executed.

How does production planning affect inventory levels?

Production planning drives inventory through three channels. First, lot sizing: larger production batches reduce setup frequency but increase average cycle stock. Second, lead time: longer manufacturing lead times require more safety stock at the finished goods level to cover variability during the replenishment period. Third, schedule stability: a frequently changed MPS forces suppliers to hold excess material and the factory to hold excess WIP to absorb schedule changes, both adding inventory cost. Reducing lot sizes, manufacturing lead times, and MPS volatility all reduce total inventory levels.

What is the Theory of Constraints and how does it apply to scheduling?

The Theory of Constraints (TOC), developed by Eliyahu Goldratt, holds that every system has at least one constraint (bottleneck) that limits its overall throughput, and that improving non-constraints does not improve system throughput. In scheduling, TOC implies: identify the bottleneck resource, never let it be idle (it should always have work queued), and subordinate all other scheduling decisions to keeping the bottleneck fed and flowing. Non-bottleneck resources should not be scheduled for maximum efficiency independently, doing so creates WIP inventory that accumulates in front of the bottleneck rather than increasing throughput.

What is S&OP and why does it matter for production planning?

Sales and operations planning (S&OP) is the monthly executive-level process that aligns the demand plan (from sales and marketing) with the supply plan (from operations and procurement) and produces a single agreed plan reviewed by senior leadership. It matters for production planning because without it, sales, operations, and finance operate from different numbers, different demand assumptions lead to different inventory and capacity decisions, and the gaps between plans create the expediting, stockouts, and inventory surprises that cost-cut production planning is supposed to prevent. S&OP produces the aggregate plan that drives the MPS, which drives MRP.