MRP system, right-sized.
Also called: Material requirements planning · Procurement plan · Order schedule · Inventory planning
Working out which components and materials to order, how many, and when, so you neither stock out nor tie up cash in excess.
Drive every order off the production schedule and each part’s lead time. Order the right quantity to arrive just before it is needed. Watch the long-lead items first. For a lean team a spreadsheet does the whole job. There is a worked plan below.
What an MRP system is
An MRP system answers three questions for every part in the product: what to order, how much, and when. It reads the production schedule (how many units, by when) and the bill of materials (what goes into one unit), multiplies them out, then works backwards from each part’s lead time to a date you have to place the order. That is the whole idea. The fancy software is optional; the logic is not.
The trap it exists to defeat is cash. Order too early or too much and you have a garage full of stock that has eaten your working capital before a single unit sells. Order too late and the line stops while you wait on a part. Material requirements planning is the discipline that keeps you off both rocks. For a lean team running a first production batch, that discipline lives perfectly well in one well-built spreadsheet, and I would actively resist anything heavier until the volume genuinely demands it.
How an MRP system works
The worked plan does two jobs at once: it sizes every order to the schedule, and it flags the part whose lead time controls the whole run. Here is the proofing box’s first-run plan, the one we worked through in the pilot, so you can see the shape of a good answer rather than a generic template.
Notice nothing in the plan is bought before its lead time demands it. The schedule pulls the orders; the orders do not push the schedule. Here is the same idea set against the way it usually goes wrong.
- Over-order “to be safe” and tie up cash in a garage full of stock.
- Buy everything at once, on day one, regardless of lead time.
- Miss the long-lead ceramic and stock out mid-run.
- Plan in your head, not on paper, and lose track of dates.
- Order to the schedule and the lead time, no earlier, no more.
- Place each order at its required date minus its lead time.
- Track the long-lead item first; it sets the whole critical path.
- Run the plan in one spreadsheet the whole team can see.
The weak column feels prudent. “Buy it all now, then we never run short” is a comforting sentence right up until the working capital is gone and the first batch has not shipped. The strong column trades that false comfort for dates you can defend.
How it fits the bigger picture
MRP System is activity 09.20.04, the last activity of Stage 09 Manufacture. It runs off the production schedule and supplier choices made earlier in Manufacture, and it closes the stage. Once the parts are ordered to a plan, the product is ready to move into Stage 10 Deliver, where the finished units reach buyers.
What it can do
It turns a production schedule into a dated list of orders, sized to the run, so the right parts arrive just before they are needed. It protects working capital by keeping cash in the bank until a lead time genuinely demands it leaves, and it stops the long-lead ceramic from blindsiding the whole batch.
What it can’t do
It can’t fix a schedule it doesn’t trust; rubbish demand numbers in, rubbish order dates out. And it can’t absorb a supplier who slips. The plan is only as good as the lead times you feed it, which is why I keep a little slack on the part that would stop the line.
See the full 10-stage process →
Try it yourself
Take your bill of materials and your first-run quantity. For each part, write three things: the quantity you need, the lead time, and the required date. Subtract the lead time from the required date and you have the order date. Sort by order date. The part at the top is your long-lead item; it gets watched. Keep the whole thing in one spreadsheet, not your head.
Want a structured first pass at the production thinking behind it? Start the Free Sprint → and the GPT will help you frame the build.
Your MRP checklist
Project notes: keeping cash out of the garage
▸ From the notebook · optional reading
Planning the proofing box’s first run with Dan and Anna Hartley in Stockport, the long-lead ceramic, and why we refused to fill the garage.
3 min read · click to open
Dan wanted to “just order everything and be done with it” the week the first run was signed off. I understood the instinct. Five suppliers, a launch date in the diary, and the urge to clear the procurement off the list in one afternoon. We pushed back: “Not everything, and not yet. Lead times first.”
What the plan looked like
One spreadsheet, a row per part. Ceramic blanks, the PCBs from the Manchester assembler, the heating elements, the wood cladding, and the retail packaging. Against each one: quantity for a 500–1,000 unit run, the lead time, and the date we needed it on the bench. The order date fell straight out of the subtraction.
Ceramic. Ten to twelve weeks, far and away the longest. It set the critical path, so it was the only thing we ordered early. Everything else hung off it.
PCBs. The Manchester assembler quoted around six weeks. Ordered a month after the ceramic, to land in the same window.
Wood, elements, packaging. Days to a fortnight. We deliberately held these back, because every week their cash stayed in the account was a week we hadn’t bet on the run selling.
Where the discipline paid
The temptation was always to round the ceramic order up “in case it sells fast”. A garage full of unsold ceramic is the most expensive kind of optimism, so we sized it to 1,000 and no more, with the scrap allowance built in. When Dan got twitchy about a possible stockout, I pointed at the long-lead row: that was the only part worth a reorder trigger, and we set one. The rest could be topped up inside a fortnight if the run sold out.
It is not a glamorous artefact. One spreadsheet, no software licence, BS EN 61010 and UKCA paperwork filed alongside it. But it kept the working capital where a lean team needs it, in the bank rather than the garage, and the first batch landed on time. That, for this stage, is the whole job done.
— Manufacture stage, project notes, 2026
— Next stage → Stage 10 · Deliver