MRP Optimization for Manufacturers: Getting the “When” and “How Much” Right

When it’s working, MRP is one of the most valuable tools a manufacturer has. It weighs demand against what’s on hand, what’s on order, and what’s scheduled, then tells buyers and planners what they need and when they need it. The catch is that MRP can’t tell the difference between good data and bad data. It runs the same math either way.

When lead times are off, BOMs are outdated, or inventory counts don’t match what’s actually on the shelf, the suggestions start to drift. Purchase orders land too early or too late, and shortages show up on the floor with no warning. Before long, planners stop trusting the system and start keeping their own spreadsheets on the side. At that point, the ERP you invested in has become an expensive record-keeping tool.

The good news is that most MRP problems trace back to a handful of data areas, and every one of them can be fixed. Here’s where to look.

MRP Answers Two Questions: When and How Much

Every suggestion MRP makes comes down to two questions: when is the material needed, and how much is needed? Timing data answers the first question. That data includes purchase lead times, routing times, and the dates on your manufacturing orders. Quantity data answers the second question through your bills of materials (BOMs) and your inventory records.

If either side is wrong, the output is wrong. A perfect BOM won’t help if the lead time is two weeks short. An accurate lead time won’t help if the system thinks you have 500 units on the shelf when you actually have 50. Getting accurate MRP outputs means looking at both.

Getting the “When” Right

Purchase Lead Times That Reflect Reality

Most lead times in an ERP system reflect what the vendor quoted, which usually means production and transit time on their end. MRP, however, needs the full span from the moment someone decides to order to the moment the material is ready to use. That includes creating and placing the order, waiting for the vendor to confirm it, shipping, receiving and inspection, and putting the material away in the right location.

Each of those steps might only add a day or two. Together, they can add a week or more to what the system assumes, and MRP will suggest orders too late as a result.

Lead times also vary by vendor. Two suppliers of the same part rarely perform the same way, so maintaining lead times at the vendor level gives MRP a much more realistic picture. Because vendor performance changes over time, these numbers shouldn’t be set once and forgotten either. For most manufacturers, a quarterly review that compares actual receipt dates to the lead times in the system is a good rhythm.

Routing Times That Include the Gaps

Routings tell MRP how long it takes to make something, which drives when component materials need to arrive. A common mistake is entering only the run time for each operation. In practice, work rarely moves straight from one operation to the next. Parts wait in a queue, get moved between work centers, or sit while a machine is set up for the next job.

Accurate routings include buffer time between operations so they reflect how work actually flows through the shop. They should also account for the time it takes to receive the finished product and put it away in inventory. If a customer order depends on finished goods being available on a specific date, that last step matters as much as any operation on the floor.

The Production Schedule: The Input Most Often Missed

This is the one that gets overlooked most. MRP calculates material requirement dates from the dates on your manufacturing orders. If those dates don’t reflect when the orders will actually be worked, everything downstream is off.

It’s easy for this to happen. An order gets created with a default date, the floor falls behind, priorities shift, and nobody goes back to update the order. The system still believes the job starts Tuesday. MRP tells purchasing to bring in material for a job that won’t start for three weeks, while a job that was moved up goes without what it needs. Completion dates are wrong too, so any customer promise dates built on them are wrong as well.

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Keeping manufacturing order dates aligned with the real production schedule is less a data cleanup task than a matter of discipline. Whoever owns the schedule needs to treat updating dates in the system as a critical part of the job rather than an afterthought.

Getting the “How Much” Right

BOM Accuracy

If the BOM says a product uses four units of a component and it really uses five, MRP will under-order every time. BOM errors are hard to spot directly, but your inventory data leaves clues. When a specific part or material is regularly adjusted during counts, the BOM often isn’t consuming the right quantity. Reviewing adjustment history by item is one of the simplest ways to find BOMs that need attention.

Raw materials where scrap is a normal part of production deserve extra care. Say you cut 10 feet of material from stock and only 9 feet make it into the finished product. A BOM that lists 9 feet will leave inventory overstated after every job. Most ERP systems let you set a scrap percentage on BOM lines. Using it keeps inventory accurate, and it also applies scrap costs to the specific jobs that generate them, which gives you a clearer view of true product cost.

Inventory Accuracy

MRP starts every calculation from what it believes you have on hand, so if that number is wrong, every suggestion built on it is wrong too. Inventory accuracy depends on regular processes to count and adjust inventory. That might mean full physical counts, cycle counting by item class, or a mix of both. The goal is to catch discrepancies early, while they’re still small and traceable.

Safety stock matters for materials that are adjusted frequently. Even with good counting processes, some items will always carry some uncertainty. Setting appropriate safety levels for those items gives operators a cushion, so a miscount doesn’t turn into a line stoppage. Safety stock shouldn’t be used to paper over bad data, but it’s a sensible protection while you work on the root cause.

Keeping MRP Data Healthy Over Time

None of this is a one-time cleanup. Vendors change, products change, and processes drift, so data that was accurate at go-live won’t stay that way on its own. That’s why we look at MRP data through the Sustain phase of our Process for Transformational Change™, which breaks into three ongoing activities:

• Monitor: Watch for warning signs, such as repeated inventory adjustments on the same items, recurring MRP exception messages, and planners overriding the system’s suggestions.
• Measure: Track a few key metrics, such as inventory record accuracy and how actual vendor lead times compare to the ones in the system.
• Maintain: Set a review cadence, such as quarterly lead time reviews and regular BOM audits, and assign clear ownership. Business process owners (BPOs) in purchasing, production, engineering, and the warehouse should each know which data they’re responsible for keeping current.

MRP Data Health Checklist

☐ Purchase lead times include order placement, confirmation, transit, receiving, and put-away

☐ Lead times are maintained by vendor, not just by item

☐ Lead times are reviewed at least quarterly against actual receipt dates

☐ Routings include buffer time between operations

☐ Routings account for receiving and putting away finished goods

☐ Manufacturing order dates reflect when work will actually be performed

☐ Items with frequent inventory adjustments are reviewed for BOM errors

☐ Scrap percentages are set on BOM lines where waste is expected

☐ Regular count-and-adjust processes are in place

☐ Safety stock is set for frequently adjusted materials

☐ Each data area has a named owner

Is Your MRP Telling You the Truth?

MRP accuracy starts with the processes that feed it. That means how orders get dated, how inventory gets counted, how lead times get reviewed, and who owns each piece. When planners are working around the system instead of with it, those processes are usually where the problem starts. Victoria Fide helps manufacturers redesign the workflows behind their data so their ERP can do what it was built to do. Learn more about our Business Process Improvement services.