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The two things people mean

"Inventory" covers two jobs that have almost nothing in common. One is materials and stock — how much filament you hold, what it cost, what you have made and what is left. The other is orders — who bought what, through which shop, for how much, and whether it has been posted.

They get bundled together because a shop needs both, but they are answered by different tools and trying to do both in one place usually leaves you with two half-finished systems. This page is about the first. The last section says what to do about the second.

Filament, by spool

Track filament as individual spools rather than as a quantity of a material. Two spools of the same black PLA bought four months apart cost different amounts, and the one you actually printed with is the one whose cost belongs in that print.

Per spool, the useful fields are: what it cost, what it weighed when new, how much remains, and when it was opened. From those you get a real cost per gram rather than a price you half-remember, and that number is the input to everything else. If you want the arithmetic in full, it is in what a print actually costs.

Waste, as its own number

Purge, supports and failed prints are filament that went through the nozzle and produced nothing you can sell. If they are not recorded separately they are invisible, and the cost per good print comes out lower than reality — which is one of the most common reasons a maker's prices don't cover their costs.

One kind of waste never goes through the printer: the tail left on a spool you have finished with. You paid for it and never printed it, so it belongs in the waste figure, and in the bottom of the ratio as well as the top, or the percentage exceeds what you actually bought. There is more on that in where your filament actually goes.

Finished stock, as movements

It is tempting to keep one number per product and edit it. That works until the number is wrong and nobody can say why. If you only ever overwrite the count, there is no history: a miscount three months ago looks the same as a sale, and fixing it wipes out what the old figure was.

Record changes instead: a quantity, a date, a one-word reason and a note. Made twelve, sold four, broke one, gave two away. You can then work out the current figure and explain how you got there, and a correction becomes another entry rather than a rubbed-out number.

Closing stock at year end

This is the figure people find they need in January, and it is why recording each change matters. Most tax regimes want to know what stock you were holding at the end of the period, valued at what it cost you to make — not what you hoped to sell it for.

If your stock figure is a number you have been overwriting all year, you cannot produce that, and you certainly cannot explain it to anyone who asks. If it is a list of changes with costs attached, you can work the figure out from it. We are not going to tell you what your obligations are — that is between you and your own accountant — but whatever they are, they will be easier to meet with a record you can rebuild the figure from.

Where the line is

Everything above is materials and stock. The other half — a customer, a sales channel, an order reference, a dispatch status, revenue per sale — is a different kind of product, and good ones already exist.

MakerProof does the first half and deliberately not the second. It tracks spools, waste, finished stock as movements, and what a print costs — alongside the commercial licences behind the models you sell, which is the part nothing else does. It has no order book and is not going to grow one. If you need revenue by channel, that is the report your marketplace already produces: Etsy, Shopify and Stripe all do it better than a second system would.

The main thing is working out which of the two you need. Plenty of people need both, and there is nothing wrong with using two tools that each do one thing properly.