Filament inventory and print cost tracker

The idea

Anyone six months into 3D printing owns a shelf of twenty-odd part-used spools and no idea what is left on any of them. Mid-print filament runouts waste plastic and hours, and the “what does a print actually cost” question gets answered with a shrug. Slicers estimate grams per print but track nothing across prints; spreadsheets die because updating them means walking to a computer with a scale reading in your head.

The wedge is the slicer handoff. Every slicer already prints a gram estimate on the export screen; pasting that number into a spool’s quick-log page does the decrement, the cost maths, and the inventory update in one motion. QR labels on the shelf make the right spool one scan away. Nobody occupies this gap: slicers stop at estimation, spreadsheets are friction, and existing inventory apps ignore cost entirely.

Who pays

Two buyers. The hobbyist with a spool problem pays GBP 2.50 a month to stop wasting filament. The Etsy or commission print-seller pays GBP 17 a year because pricing work without knowing true cost (filament plus electricity plus machine wear) means guessing their margin. Distribution: r/3Dprinting and r/functionalprint, 3D printing YouTube (creators love workflow tools), and SEO on “3d print cost calculator”, which currently returns one-shot calculators with no inventory behind them.

MVP scope

• Spool CRUD with remaining-weight tracking and scale-reading corrections
• One-paste print logging from slicer gram estimates
• Cost breakdown per print: filament, electricity, wear amortisation
• Printable QR shelf labels deep-linking to quick-log pages
• Free 3-spool tier with Stripe subscription upgrade
• Skip for v1: slicer plugins or file parsing, multi-user workshops, sales/order tracking, and filament price scraping

Build SpoolBook, a web app for 3D printing hobbyists to track filament inventory and true print costs. Stack: Next.js (App Router), Supabase (Postgres + magic-link auth), Stripe.

Core flow: user adds spools (brand, material, colour, purchase price, net weight, spool tare weight) and logs prints against them. Logging a print takes one paste: the user copies the gram estimate from their slicer (PrusaSlicer/Cura/Bambu Studio all show it), picks the spool, and the app decrements remaining weight and computes cost. Cost = filament grams at the spool's per-gram price + electricity (printer wattage x print hours x user's kWh rate) + a configurable wear amortisation per print hour.

Pages: spool inventory (card grid with remaining-weight bars, filterable by material/colour), spool detail with usage history, print log with cost breakdown, settings (printers, kWh rate, wear rate), and a QR label sheet.

Implementation details: remaining weight supports manual correction by weighing the spool; the app subtracts the stored tare weight so users enter the gross scale reading directly. Generate printable QR labels (four per row, standard label sheet) that deep-link to each spool's quick-log page, so logging at the printer is scan, paste grams, done.

Pricing: free for 3 spools, then GBP 2.50/month or GBP 17/year via Stripe. Etsy sellers are the annual-plan buyers; mention margin per print on the cost breakdown screen.