MRP vs MRP II: what is the difference?

MRP answers one question: given what we must ship, what must we buy and make, and by when. It takes a master schedule of finished items, explodes it through multi-level BOMs, nets each component against stock on hand and open orders, and offsets by lead time. The output is planned purchase and work orders with release dates. Joseph Orlicky's 1975 book made the method standard, and the APICS MRP crusade of the 1970s spread it widely. MRP assumes capacity is infinite. It will happily plan 400 hours of work for a week in which you have 160. That gap is what MRP II set out to close.

MRP II, a term associated with Oliver Wight in the 1980s, kept the MRP calculation and wrapped it in a closed loop. Rough-cut capacity planning checks the master schedule against work-centre hours. Capacity requirements planning checks the detailed plan. Shop-floor control reports what actually happened so the next run starts from reality. Standard costs, routings and work centres become master data, so the plan can be priced and compared with actuals. The second word changed from requirements to resource on purpose: the system plans people and machines, not only parts. MRP II is the direct ancestor of the manufacturing modules in current ERPs.

The table sets out the main differences. Both are planning methods rather than products; the names survive mainly in textbooks and in the module lists of ERP vendors. When a vendor says MRP today, ask which column they mean, because the answer decides whether capacity and labour are planned or only materials.

Nobody sells MRP II under that name any more; the functions live inside manufacturing ERPs. The distinction still matters when you evaluate software, because products differ in exactly the way MRP and MRP II did. Some plan materials well but know nothing about hours at a work centre. Others carry routings, work-centre rates and labour logging, so a plan can be checked against capacity and a job can be costed against the plan. For a 10 to 50 person shop the MRP II ideas that pay off first are routings with work centres, labour logged against jobs and a job cost that compares actual with expected. Full capacity planning can wait until the shop runs close to its limit.