MRP Software for Manufacturing
What the planning calculation really does, where it breaks, and an honest account of what WorkCell runs today.
Material requirements planning is a calculation, not a shop floor system. It explodes demand through the bill of material, nets gross requirements against on-hand and on-order quantities, offsets each level by its lead time, and emits planned orders that say what to buy or make and when to start it.
Demand collected and exploded through the bill of materials.
MRP is the oldest idea in manufacturing software and the most frequently oversold. The arithmetic is simple: take the demand, walk the bill of material down level by level, subtract what you already have and what is already coming, shift the result backward by the lead time, and print the list. What makes MRP hard is not the math but the inputs. Every bill of material has to be right, every on-hand balance has to be right, and every lead time has to be a number somebody still believes. Where those three are shaky, the planned orders are confidently wrong and the planner quietly goes back to the spreadsheet. This page says what MRP is, what it is not, and which parts WorkCell has shipped.
Manufacturers with repeating demand, deep multi-level bills of material, and long purchased lead times — where netting across jobs saves real money. Pure one-off job shops get less from MRP than the category marketing suggests.
Three open jobs each need the same bar stock, and each one is purchased separately because nothing looks across them. The material arrives in three receipts at three price breaks, and the rack holds more inventory than the plan ever needed while a fourth job waits on the same item.
The item record says four weeks because someone typed four weeks in 2021. The supplier has been quoting eleven. Every planned start date derived from that number is wrong by seven weeks, and the error compounds at every level of the bill of material before it reaches the promise date.
The engineer updated the drawing, the shop knows about the change, and the bill of material does not. A planning run against the old structure buys the superseded part in quantity, and the only person who catches it is the receiving clerk holding something nobody wants.
Nobody trusts the planned orders, so the floor runs on a daily shortage meeting and a whiteboard. The output is expedite freight, split purchase orders, and setups broken in half. It works, barely, and it does not survive the person who maintains the whiteboard taking a week off.
Counts happen at inventory time and drift from the day after. Netting against a balance that is wrong by two pieces turns into a planned order for the wrong quantity, and the planner learns to add a fudge factor, which guarantees the balance stays wrong.
Multi-level bills of material, versioned
The structure MRP would explode is already here: multi-level bills of material and routings with revisions and effective dating, so a job shows the structure it was released to. Getting this right is the precondition for any planning run ever being trustworthy.
Demand visible on the job
Each job carries the material it requires against the bill of material revision it was released to, so you can see what a job needs and what has been issued. What does not happen yet is a run that nets that demand across jobs for you.
Supplier catalog with prices and lead times
Purchased items carry supplier records with pricing and lead times, which is where a planning calculation gets its offsets from. Today a buyer reads them; the data is in place for the netting run rather than in a spreadsheet on somebody's desktop.
Requisitions, purchase orders, and receipts
Purchasing runs end to end: a requisition becomes a purchase order, the receipt lands against it, the material enters stock with its lot, and the cost posts against the Job. Multi-currency purchases convert on the way in.
Inventory in pieces with unit conversion
On-hand is tracked as discrete pieces with conversion between purchase, stock, and issue units, so a bar bought in feet and issued in pieces is one balance. An accurate on-hand number is the other half of what netting requires.
Finite capacity scheduling, which MRP is not
Scheduling against real machine and labor capacity is shipping, including proposal runs that let a planner test a sequence before committing it. That is the constraint MRP ignores by design, and for most high-mix shops it is the binding one.
Multi-level bills of material with revisions and effective dating.
PurchasingSupplier catalog, requisitions, purchase orders, and receipts against the job.
InventoryOn-hand in pieces, unit conversion, and lot and serial tracking.
ProductionJobs carrying their material requirements, and finite capacity scheduling.
MRP is a calculation; ERP is the system of record it reads from and writes into. ERP holds the bills of material, the inventory balances, the purchase orders, and the ledger. MRP is one scheduled job inside that system, not a parallel product.
MRP vs APSMRP assumes infinite capacity and plans material by lead time offset. APS assumes finite capacity and sequences work against the machines and people you actually have. MRP tells you to start Tuesday; APS tells you the only machine that can run it is busy until Friday.
MRP vs MESMRP is planning before the fact; MES is execution and data capture during it. MRP produces a planned order. MES reports what the operator ran, how many were good, and how many were scrapped — numbers the next MRP run depends on being true.
MRP vs SCMMRP plans inside the four walls from a bill of material and lead times you typed in. Supply chain management looks outward at supplier capacity, transport, and multi-site allocation. MRP treats a supplier as a lead-time number; SCM treats it as a constraint with its own schedule.
Deep assemblies where netting across levels and long-lead purchases matter most.
Electrical EquipmentRepeating builds with common components shared across several open orders.
ElectronicsHundreds of purchased components per assembly and volatile component lead times.
AutomotiveReleased schedules and repeating demand, where time-phased planning earns its keep.
The system of record MRP reads: bills of material, inventory, purchasing, jobs, and the ledger.
APSFinite capacity scheduling, which is shipping and is the constraint MRP ignores.
MESFloor data capture that keeps the quantities a planning run depends on honest.
SCMThe outward view of suppliers and transport that MRP reduces to a lead-time number.
Does WorkCell run MRP today?
Yes. The run explodes demand from sales orders and jobs through the effective-dated bill of material, nets it against on-hand, in-transit and open purchase orders, applies each item's lead time and order policy, and produces time-phased planned orders. A planner reviews the result and releases what they agree with, which becomes a job or a purchase order against the same records the run read. Nothing is exported and nothing has to be reconciled afterwards.
What is the difference between MRP and MRP II?
MRP plans material only: explode the bill of material, net, offset by lead time, emit planned orders. MRP II extended the same loop to capacity planning and to finance, so the plan was checked against available hours and valued in money. ERP then widened MRP II across the rest of the business.
What is the difference between MRP and ERP?
MRP is a calculation that runs inside a system. ERP is the system: the bills of material, inventory balances, purchase orders, jobs, and ledger that the calculation reads and writes. Buying standalone MRP software usually means buying a planning module that needs someone else's data to work.
Do I need MRP if I build to order?
Often less than you think. If every job is unique and material is bought per job against the customer order, the netting that MRP performs has little to net. The constraint is usually capacity and purchased lead time, which is why finite scheduling helps a high-mix shop more than a planning run.
Why do MRP implementations fail?
Almost always data, not software. The calculation is only as good as bill of material accuracy, on-hand accuracy, and lead times somebody still believes. Where any of the three is unreliable, planned orders are confidently wrong, planners stop trusting them, and the shortage meeting quietly becomes the real planning system again.
Can a planner override what the MRP run proposes?
Yes, and that is the intended shape. The run produces planned orders rather than committed ones, so a planner reads the netting, adjusts a quantity or a date where they know something the data does not, and releases what they agree with. Jobs carry their requirements against a versioned bill of material and the supplier catalog holds the prices and lead times the run used, so the judgement call and the arithmetic behind it are visible on the same screen.
One system instead of a standalone MRP.
Tell us what you are planning by hand today and we will tell you plainly whether WorkCell helps yet or whether you need an MRP run we have not shipped.