Running out of filament halfway through an order is one of the most avoidable problems in a shop and one of the costliest: a stopped machine, a customer waiting, and an emergency purchase at whatever price. Filament inventory management does not need a complicated system. It needs you to know which spools you have, how much is left on each, and when to reorder. Here is the method, with numeric examples that are assumptions meant to show the math.
Filament inventory management: what to log for each spool
A spreadsheet works to start, as long as you log the same things for every spool. Five data points are enough for a useful 3D printing materials inventory:
Material and color
PLA, PETG, or another, and the exact color. Two blacks from different suppliers may not be interchangeable for a demanding customer.
Starting weight
What a new spool holds (for example 1 kg). It is your starting point for knowing how much you have used.
Price paid
What that specific spool cost, with shipping if you paid it. Without this you cannot calculate real cost per gram.
Supplier and date
Where you bought it and when. It helps you know who to buy from again and which spools have been stored for a while.
How to track filament stock by weighing spools
Eyeballing how much is left on a spool almost always fails. Weighing it is what works. Weigh the empty spool once (the tare) and write it down. After that, whenever you want to know what is left, weigh the spool and subtract that tare.
Example with assumptions: a spool weighs 620 g on the scale today and you know the empty spool weighs 240 g. You have 620 − 240 = 380 g of filament left. If the next order needs 450 g of that color, you know before starting that it will not be enough.
When to reorder filament: the reorder point
The reorder point is the amount of a material at which you place the order with your supplier. The idea is simple: when you have just enough left to cover what you will use while the purchase arrives, plus a cushion.
Reorder point
Example with assumptions: you use about 150 g a day of black PLA, your supplier takes 6 days to deliver, and you want a 3-day cushion. The reorder point is 150 × (6 + 3) = 1,350 g. When your total black PLA drops below about 1.35 kg, you place the order. If your usage is uneven, use an average of the last few weeks and revisit it every so often.
Restock materials for committed orders first
Not every color weighs the same in your business. If you have an approved order that needs a specific material, that material comes first: reserve what it needs before using it for something else and reorder before the order demands it. A color you rarely sell can wait.
Changing cost per kilo: use the average
Filament prices change between purchases. If you quote with the last price you saw, your costs will be slightly off. A simple approach is the weighted average cost: add up what you paid for all your stock and divide by the kilos you hold.
Average cost per kilo (example with assumptions)
| Purchase | Kilos | Price per kilo | Total cost |
|---|---|---|---|
| Current stock | 2 | 20.00 USD | 40.00 USD |
| New purchase | 3 | 24.00 USD | 72.00 USD |
| Total and average | 5 | 22.40 USD/kg | 112.00 USD |
That 22.40 USD per kilo (112 ÷ 5) is the cost you would use to quote with mixed stock. To see how it becomes a cost per gram and a part price, see the guide on [PLA cost per gram](/en/blog/costo-por-gramo-pla). Important: the prices in the example are made up to show the math, not market prices.
Moisture and storage: protect what you already bought
Filament can absorb moisture from the air, and some materials absorb more than others. Damp filament often prints worse (bubbles, stringing, rough surface) and forces you to waste material or reprint. You do not need an expensive setup to avoid it:
To keep all of this without relying on a spreadsheet that gets messy, it helps when inventory is tied to your orders. That way what you use stops being a guess. If you are still weighing a spreadsheet against a system, [Excel vs software for your shop](/en/blog/excel-vs-software-taller-impresion-3d) compares both paths.
Reserving material for approved orders
The stock you see on the shelf is not all yours: part of it is already committed to approved orders. The useful number is available stock, which is what you hold minus what is already reserved. Example with assumptions: you have 1,200 g of white PLA and an approved order needs 450 g. Real availability is 1,200 − 450 = 750 g. If another order for 900 g comes in, you know it will not fit and you must reorder before accepting it, not after.
When calculating what an order needs, add the waste: purges when changing color, supports, adhesion edges, and the occasional failed part. How much waste is for you to determine from your own weighing, comparing what the slicer said with what the spool actually lost. With a few weeks of data you will have your own percentage, far more reliable than one copied from another shop.
Frequently asked questions
How often should I review my filament inventory?+
How much filament should I keep in reserve?+
How do I know how much filament an order used?+
What do I do with leftover spool remnants?+
Inventory tied to your orders
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