MRP vs ERP: what is the difference, and which does a manufacturer need?
A worked material plan, the capacity and cost gaps it exposes, the records a standalone MRP tool must keep in step with accounting and ecommerce, and a decision table for choosing between MRP and ERP.
- 01Demand
- 02Materials
- 03Capacity
- 04Cost
- 05Decide
MRP calculates which materials to buy or make, how many and when. ERP runs the wider business on shared records, including sales, purchasing, stock, production and finance, and usually includes MRP as one module. A focused MRP tool can be enough when material planning is the main problem and accounting works well elsewhere. Choose ERP when production cost, stock across sites, customer promises and finance must agree without manual reconciliation.
A manufacturer running on spreadsheets usually feels the pain in purchasing first: components arrive late, or too many arrive at once. MRP software solves that problem directly. The harder question comes next. Should the business add a planning tool beside its accounting package, or move sales, stock, production and finance onto one ERP? The answer depends less on company size than on how many decisions have to agree with each other.
What is the difference between MRP and ERP?
MRP means material requirements planning. It answers one question: given what customers want and what we already have, which materials and components do we need to buy or make, how many, and by when? ERP means enterprise resource planning. It keeps sales, purchasing, inventory, production and accounting on one set of records, so that an order, the stock it uses and the money it produces can be traced from start to finish.
The two are not rivals at the same level. MRP is a planning method that sits inside most manufacturing ERP systems. Between them sits MRP II, manufacturing resource planning, which added capacity, scheduling and cost to material planning. SAP’s history of the terms dates MRP to the 1960s and MRP II to the 1980s, with ERP widening the scope to the whole business after that. The practical choice for a manufacturer today is usually between a focused MRP tool beside separate accounting software, and an ERP that includes MRP.
| MRP | MRP II | ERP | |
|---|---|---|---|
| Question it answers | What material is needed, how much and when? | Can the plant make it in time, and what will it cost? | Do sales, stock, production and finance agree about every order? |
| Main inputs | Demand, bills of materials, stock, open orders, lead times | MRP inputs plus routings, work centres, capacity and standard costs | Everything in MRP II plus customers, suppliers, prices, invoices, payments and the ledger |
| Main outputs | Planned purchase and work orders, shortage and reschedule messages | Capacity load, schedules, production cost | Shared transactions and reports across the business |
| Typical users | Planners and buyers | Planners, production managers, cost accountants | Sales, purchasing, warehouse, production, finance and management |
| What it leaves out | Capacity, cost, customers, cash | Most of sales, customer service and general accounting | Specialist depth such as finite scheduling or machine data may still need other systems |
How does an MRP calculation work?
MRP works backwards from demand. It takes the quantity of finished goods required in each period, subtracts what is already in stock or on order, and turns the shortfall into planned orders. It then repeats the calculation for every component in the bill of materials, offsetting each order by its lead time. The illustrative example below uses weekly periods and orders exactly the shortfall each time, a rule called lot-for-lot.
An illustrative manufacturer assembles a pump, P-100. Each pump needs one motor and two brackets. Customers have ordered 50 pumps for week 4 and 80 for week 6. Assembly takes one week. Ten finished pumps are in stock.
| Pump P-100 | Week 3 | Week 4 | Week 5 | Week 6 |
|---|---|---|---|---|
| Gross requirement (customer demand) | 0 | 50 | 0 | 80 |
| Available from stock | 10 | 10 | 0 | 0 |
| Net requirement | 0 | 40 | 0 | 80 |
| Planned assembly order released | 40 | 0 | 80 | 0 |
The 40 pumps needed in week 4 must start assembly in week 3, and the 80 needed in week 6 must start in week 5. Those planned assembly orders become the demand for components. Motors are needed in the week assembly starts: 40 in week 3 and 80 in week 5. The motor supplier needs two weeks. Thirty motors are in stock and 20 more are already due in week 2.
| Motor M-20 | Week 1 | Week 2 | Week 3 | Week 4 | Week 5 |
|---|---|---|---|---|---|
| Gross requirement (from pump orders) | 0 | 0 | 40 | 0 | 80 |
| Scheduled receipt (already ordered) | 0 | 20 | 0 | 0 | 0 |
| Projected stock at end of week | 30 | 50 | 10 | 10 | 0 |
| Net requirement | 0 | 0 | 0 | 0 | 70 |
| Planned receipt | 0 | 0 | 0 | 0 | 70 |
| Planned purchase order released | 0 | 0 | 70 | 0 | 0 |
Week 5 needs 80 motors, but only 10 remain after week 3, so the net requirement is 70. With a two-week lead time, the purchase order for 70 motors must be released in week 3. Brackets follow the same logic: two per pump means 80 needed in week 3 and 160 in week 5. With 100 in stock and a one-week lead time, MRP plans a purchase of 140 brackets released in week 4.
What does MRP leave out?
The plan above is complete as a material plan, and it still leaves the business with open questions. Suppose the assembly cell can build 60 pumps a week. MRP has scheduled 80 pumps in week 5 and does not notice the overload, because classic MRP assumes unlimited capacity. Capacity planning, added by MRP II, would show the problem: start 20 pumps in week 4, add a shift, or warn sales that part of the week 6 order will be late.
| Question | Where the answer lives | What goes wrong without it |
|---|---|---|
| Can the assembly cell build 80 pumps in week 5? | Capacity planning or scheduling | The plan looks valid but the order ships late. |
| Is the week 6 customer allowed more credit? | Accounting or credit control | Material is bought for an order that should have been held. |
| What did each pump actually cost? | Production costing and the ledger | Margin is calculated from standard costs nobody updates. |
| Which customer received pumps with a faulty motor lot? | Lot traceability across receipt, production and shipment | A recall becomes a manual search through paperwork. |
| Does the website show the right availability? | Stock owned by one system and published to sales channels | Customers order pumps that are already committed. |
An ERP answers these in the same records the material plan uses. A business with a separate MRP tool can still answer them, but each answer depends on data moving correctly between systems.
Is MRP part of ERP?
In most manufacturing ERP systems, yes. MRP runs as a module on the ERP’s own items, bills of materials, stock and orders. In ERPNext, for example, a master production schedule collects demand from sales orders, material requests, forecasts and delivery schedules, and the MRP report then calculates the quantities and dates for raw materials and components, according to the ERPNext MRP documentation. Odoo’s manufacturing documentation similarly places manufacturing orders, bills of materials and a master production schedule inside the same suite as inventory, purchasing and accounting.
The planning depth still varies. Some ERP products include capacity-aware scheduling, while others plan material with infinite capacity and rely on a separate scheduling tool. When you compare products, ask the vendor to run the pump example with your own bills of materials and lead times, then overload the bottleneck and watch what the system does. The manufacturing ERP software comparison sets out that proof script, and ERPNext is one open-source option that includes MRP.
What breaks when MRP and accounting are separate systems?
Many small manufacturers run a focused MRP tool beside an accounting package and an online store. That can work well. It also means the business, or its integration, must keep several records in step every day. Each arrow in the map below is a handoff that can fail without anyone noticing until month-end.
- 01Customer demandEcommerce or B2B portal to MRP
Sales order lines, quantities, requested dates and customer reference
- 02Stock promised to customersMRP to sales channels
Available quantity by item and location, with a timestamp
- 03Purchasing commitmentsMRP to accounting
Purchase orders, goods received and supplier bill matching
- 04Stock and production valueMRP to accounting
Inventory value, work in progress, production cost and variances as journal entries
- 05Invoice and paymentMRP or commerce to accounting
Shipped quantities, invoices, credit notes and payment status
The usual failure points are inventory value and cost. The MRP tool knows what was consumed and produced. The accounting system holds the ledger. If the journal entries that move value between raw materials, work in progress and finished goods are summarised, delayed or keyed by hand, the stock value in the ledger drifts away from the stock in the building. Customer orders arriving twice, availability published from two places, and invoices raised for the ordered rather than the shipped quantity are the other common faults. The ecommerce ERP integration guide covers the ownership rules that prevent them, and the order-to-cash guide follows one order through each handoff.
A separate MRP tool is a reasonable design when those handoffs are few, automated and reconciled. It becomes expensive when staff spend days each month explaining the difference between two systems.
Do you need MRP or ERP?
Decide by the problem you need to solve and how many decisions must agree, not by headcount or revenue alone.
| Situation | Likely fit | Why | Check before deciding |
|---|---|---|---|
| Purchasing shortages are the main problem, and accounting already works well | Focused MRP tool | It solves material planning quickly with less change elsewhere. | How purchase orders, receipts and inventory value reach the accounting system. |
| Production cost and margin per job must be reliable | ERP, or MRP with strong cost posting | Cost needs material, labour, scrap and overhead in one place, then in the ledger. | Whether actual cost and work in progress post automatically and reconcile. |
| Several sites, warehouses or legal entities | ERP | Stock transfers, intercompany sales and consolidated reporting need shared records. | Multi-company accounting and transfer pricing in the shortlisted product. |
| Sales through a website, B2B portal and sales team | ERP, or MRP with tight order and stock integration | Promised dates and availability must come from one owner. | Who owns available stock, and how quickly channels are updated. |
| A bottleneck machine or team limits output | MRP II or ERP with capacity planning; possibly a specialist scheduling tool | Material planning alone ignores capacity. | Run the overloaded-week test from the pump example. |
| Lot or serial traceability is required for recalls or regulation | ERP, or MRP with full lot tracking and quality | Tracing needs receipt, production, quality and shipment records linked. | Trace one supplier lot to every customer shipment in the demonstration. |
- Choose a focused MRP tool when material planning is the main pain, the business has one site, and accounting is already accurate and accepted by finance.
- Choose ERP when cost, stock across locations, customer promises and finance must agree without a monthly reconciliation exercise.
- Plan for integration either way. Even an ERP usually connects to an online store, a carrier, a bank or a payroll service.
How do you choose and prove the system?
- 01Name the decision that fails today
Late material, overloaded capacity, unknown cost, wrong availability or slow month-end. Each points to a different system boundary.
- 02Clean the planning inputs
Check bills of materials, lead times, stock accuracy and open orders for the main product family before any demonstration.
- 03Run your own worked example
Ask each vendor to plan one product with your data, then add a capacity overload, a short supplier and a cost question.
- 04Test the handoffs
If accounting or ecommerce stays separate, prove the order, stock, purchasing and cost handoffs and their reconciliation before go-live.
Once a system is chosen, the same example becomes part of the acceptance test described in the ERP testing guide for manufacturers. If you want help mapping the decision, the ERP and manufacturing service explains how Harmatiq approaches it.
Bring one real order, the systems it crosses and the exceptions your team handles manually. We can turn that journey into an implementation scope.