OpenPrintTag: How to Read & Write Smart 3D Printing Spools with Your Phone

If you 3D print, you’ve probably been there: a shelf full of half-used spools, no idea how much filament is left on any of them, and that one unlabeled spool that might be PETG or might be PLA, with no way to tell without a test print. I’ve been there too, and it’s the kind of small, recurring annoyance that NFC is genuinely good at solving.

That’s what OpenPrintTag does. It’s an open-source NFC standard created by Prusa Research that turns any compatible NFC tag into a smart label for your filament spool. Material type, brand, color, remaining weight: all stored directly on the spool and readable with a quick tap of your phone.

No cloud. No proprietary ecosystem. No internet required. I’ve spent years building NFC.cool, an app for reading and writing NFC tags, and this is exactly the kind of standard I like to see - one that puts the data on the tag and lets it work anywhere. Here’s how it works, and how I read and write OpenPrintTag spools with nothing but a phone.

What Is OpenPrintTag?

OpenPrintTag is a universal, open data format for 3D printing materials. Instead of every manufacturer inventing their own incompatible smart spool system - which is exactly the mess I’ve watched play out in other corners of the NFC world - OpenPrintTag defines a single standard that anyone can adopt, including filament makers, printer manufacturers, slicer software, and apps like NFC.cool.

The key principles, and the reasons I think it’s worth paying attention to:

• Open source: published under MIT license, free to implement, no licensing fees

• Offline by design: all data lives on the tag itself, no cloud service needed

• Rewritable: update remaining filament as you print, reuse tags on new spools

• Universal: works across brands and ecosystems

• Supports both FFF (filament) and SLA (resin)

Over 22 companies and groups have expressed interest, including Prusament, Voron, Fillamentum, 3DXTech, SimplyPrint, and PrintedSolid. The full specification is available at specs.openprinttag.org.

What Data Does an OpenPrintTag Store?

This is the part that won me over. OpenPrintTag isn’t just a label with a name on it. It’s a properly structured data format with fields for almost everything you’d want to know about a spool, and the spec has clearly been written by people who actually print.

Material identification:

• Material class (filament or resin)

• Material type (PLA, PETG, ABS, TPU, ASA, PC, PA6, and 30+ others)

• Material name (e.g. “PLA Galaxy Black”)

• Brand name (e.g. “Prusament”)

• Material property tags: over 68 defined properties like abrasive, conductive, glow-in-dark, food-safe, ESD-safe, flexible, and more

Weight and length tracking:

• Nominal weight (advertised, e.g. 1000g)

• Actual weight (measured for this specific spool)

• Filament length (nominal and actual, in mm)

• Empty container weight (so you can weigh the spool and calculate remaining material)

• Consumed weight (updated as you print; this is the field that makes spools truly “smart”)

Color:

• Primary color in RGBA format

• Up to 5 secondary colors (for multicolor, galaxy, or gradient filaments)

• Transmission distance (opacity value, useful for HueForge projects)

Metadata:

• Manufacturing date and expiration date

• Country of origin

• UUIDs for brand, material, and specific spool instance

• Write protection settings

The spec even covers resin-specific fields like last_stir_time, which records when the resin was last stirred before printing. That’s the kind of detail that tells me the people behind it have actually been burned by un-stirred resin.

The Tag: Not Your Usual NFC Sticker

Here’s a technical detail I’d flag before you buy anything: OpenPrintTag is designed for ISO 15693 (NFC-V) tags, specifically NXP ICODE SLIX and ICODE SLIX2 chips. These are NFC Forum Type 5 tags with a significantly longer read range than standard NFC-A tags, up to 1.5 meters with a dedicated reader. If you’ve only ever bought the cheap NTAG stickers most projects use, this is a different family of tag - I cover the full landscape in NFC tag types for iPhone.

Why NFC-V? A printer’s built-in NFC reader needs to detect the spool regardless of its rotation. The longer range of NFC-V makes this possible without requiring precise tag alignment, which is a smart bit of design.

What about regular NTAG stickers? The OpenPrintTag data format is NDEF-based, so a phone app like NFC.cool can technically read and write OpenPrintTag data on any NFC tag, including NTAG213/215/216. I’ve done it - it works fine for phone-to-phone reading. However, printer hardware and apps like Prusa’s only recognize NFC-V tags. So if you want your tagged spools to work with built-in printer readers, use ICODE SLIX2 tags. Don’t make the mistake I’d expect most people to make and buy a bag of NTAG213s for this.

If you’re buying blank tags, look for ICODE SLIX2 or ISO 15693 specifically. You can find compatible tags on Amazon US or Amazon Europe (affiliate links).

How to Read and Write OpenPrintTag with Your Phone

You don’t need a Prusa printer or any special hardware to work with OpenPrintTag, just your phone. This is the part I was most keen to build, because a phone in your pocket is the most accessible NFC reader there is.

NFC.cool Tools supports OpenPrintTag natively on both iOS and Android, and I made sure the feature is completely free.

Reading a tag:

1. Open NFC.cool Tools

2. Hold your phone near the NFC tag on the spool

3. NFC.cool detects the OpenPrintTag format automatically

4. View the structured data: material, brand, color, weight, length, properties

Writing a tag:

1. Stick a blank ICODE SLIX2 tag on your spool

2. Open NFC.cool → NFC Apps section → OpenPrintTag

3. Fill in the material details: type, brand, color, weight, length

4. Tap to write

Updating remaining material: After a print, update the consumed weight field on the tag. Next time you scan, you’ll know exactly how much filament is left, no guessing, no weighing. This is the bit that turns a smart spool from a novelty into something I’d actually rely on.

If you want to look under the hood, you can use Expert Mode to inspect the raw NDEF records - useful when you need to debug a tag or verify the data structure. New to writing tags in general? I walk through the basics in how to write NFC tags on iPhone.

Why Use Your Phone?

Prusa printers are getting built-in NFC readers, and projects like SpoolSense (an open-source ESP32 reader) are adding dedicated hardware options. So why bother with your phone? Here’s the case I’d make:

• Works with any printer: Voron, Bambu Lab, Creality, Ender, whatever you use

• Write tags for any filament: Prusament comes pre-tagged, but you can tag Fillamentum, eSUN, Hatchbox, or any brand yourself

• Manage inventory away from your printer: scan spools at your desk, in your storage, or at a makerspace

• Debug tags: when a printer can’t read a tag, scan it with your phone to see what’s actually on it - this is the use I’d reach for most

• No extra hardware: your phone already has an NFC reader, and that’s the whole point

Practical Use Cases

Personal inventory: Tag every spool in your collection. When you’re planning a print, scan spools to check material type, remaining length, and color without unboxing anything.

Remaining filament tracking: Weigh your spool before and after a print, update the consumed weight on the tag. No more “will this spool have enough for a 14-hour print?” anxiety.

Makerspace or team use: Tag spools with material details so anyone in the shop can scan and identify them. No more mystery filament.

Filament testing notes: Found the perfect temperature for a specific spool? Update the tag with your notes for next time.

Multi-color and specialty materials: OpenPrintTag supports up to 6 colors per spool and 68+ property tags. Your glow-in-dark, carbon-fiber-filled PETG can finally be properly labeled, abrasive flag and all.

The Ecosystem Is Growing

OpenPrintTag is still young, but the momentum is real:

• Prusament ships with OpenPrintTag NFC tags on every spool

• Prusa printers are adding native NFC readers

• Open-source hardware readers like SpoolSense (ESP32-based) are emerging from the community

• 22+ companies have joined the initiative

• NFC.cool is the only general-purpose NFC app with full OpenPrintTag support on both iOS and Android, and I added it because I wanted to use it myself

I’ve watched the 3D printing industry need an open standard for smart spools for years, and I’ve watched a few proprietary attempts come and go. OpenPrintTag is the most credible one I’ve seen: backed by a major manufacturer, fully open source, and already shipping on real products. That combination is rare enough that I’d bet on it.

Getting Started

What you need:

• iPhone 7 or later, or an Android phone with NFC

• NFC.cool Tools (App Store / Google Play), free, OpenPrintTag included

• Blank ICODE SLIX2 / ISO 15693 NFC tags (Amazon US / Amazon Europe, affiliate links)

• Some filament spools to tag

That’s it. Five minutes from now, your first spool could be smart. If NFC itself is new to you, my beginner’s guide to NFC tags is the place I’d point you first, and the NFC reader/writer feature page covers what NFC.cool Tools can do beyond OpenPrintTag.

OpenPrintTag is an open-source initiative by Prusa Research. NFC.cool is an independent supporter of the standard. Learn more at openprinttag.org.