Every small plant's parts room tells the same story: three shelves of stuff you'll never use, and the one bearing you actually needed is on backorder while the line sits down. Both problems have the same root cause — there's no system deciding what to stock and when to reorder. It's all in someone's memory.
Min/max levels and reorder points fix that. They're simpler than they sound, and you can run the whole thing in a spreadsheet.
The three numbers that run a parts room
For every part you stock, you only need three numbers:
- Minimum (Min) — the safety floor. Hit this, and you reorder. It covers demand during the time it takes a new order to arrive.
- Maximum (Max) — the ceiling. Don't stock more than this; it's cash sitting on a shelf.
- Reorder Point (ROP) — the trigger. When on-hand quantity drops to here, it's time to buy.
For a lot of small operations, ROP and Min are effectively the same number, and that's fine. Don't overthink it.
How to actually set the minimum
The Min has to cover you during the lead time — the gap between placing an order and the part being on the shelf. The plain-English formula:
Min = (average usage per week) × (lead time in weeks) + a little safety buffer
Example: you use about 2 drive belts a month (≈0.5/week), and your supplier takes 3 weeks. That's 0.5 × 3 = 1.5 belts of expected demand during lead time. Round up and add a buffer for a critical part → Min of 3. Drop to 3 on the shelf, reorder.
The buffer is judgment, not math. A cheap, fast-moving, easy-to-get part needs almost none. A long-lead, single-source part on critical equipment deserves a generous one — the cost of an extra unit on the shelf is nothing next to a day of downtime.
How to set the maximum
Max is about not tying up cash and shelf space. A practical starting point:
Max = Min + one normal order quantity
If you typically buy belts a half-dozen at a time, and your Min is 3, your Max is around 9. This keeps you between "about to run out" and "drowning in inventory" without much fuss.
Let criticality override the math
The formulas give you a starting point. Criticality decides where you bend the rules:
- Critical + long lead + single source → stock it, even if you use it twice a year. The math says "don't bother"; the downtime risk says otherwise.
- Non-critical + cheap + next-day delivery → stock little or nothing. Buy it when you need it.
This is the judgment a spreadsheet can't make for you — but the spreadsheet makes it easy to act on once you've decided.
Make the shelf tell you what to do
Here's where it comes together. In your parts list, put on-hand quantity next to Min, and add a status column that compares them:
- On-hand > Min →
IN STOCK - On-hand at/near Min →
LOW STOCK - On-hand = 0 →
OUT OF STOCK
Color-code it — green, amber, red — and your parts inventory becomes a reorder list you can read at a glance. No memory required. Add a bin/location column and a part-to-equipment link, and a new technician can find the right part for the right machine without hunting.
Start with the parts that hurt
Don't try to set Min/Max for 800 line items on day one. Start with:
- Parts for your High-criticality equipment
- Parts with long or single-source lead times
- Parts you've run out of in the last year (your team remembers every one)
That short list covers most of your real risk. Expand from there.
A ready-made starting point
You can build this in Excel with a parts table, a few formulas, and conditional formatting. If you'd rather not, our Spare Parts & Inventory template in the CMMS Lite set has the min/max logic, low-stock and out-of-stock flags, bin locations, and equipment linking already built in.
Either way, the win is the same: stop relying on memory, let the shelf tell you when to reorder, and keep cash off the shelf you don't need there.
Want help organizing your parts room and setting levels? Reach out for a hand.