3MF vs STL: which file format should you use for 3D printing?

Which should I use, 3MF or STL?

• Printing on your own printer with a modern slicer: 3MF. It opens at the right size and keeps your plate, settings and painted colours when you save the project.
• Multi-colour or multi-material printing: 3MF. STL has nowhere to store which part gets which filament.
• Sharing a print that should work for anyone, in any program: STL, or both files. STL is the one format that almost every slicer, CAD program, repair tool and model site accepts.
• A single-colour part for an older slicer, a print service form or a CAD tool: STL is fine. Check the unit when you open it.
• Sharing your exact print setup with a friend on the same slicer: a 3MF project saved from that slicer.

If you only need the shape, both work. As soon as you care about the size being read right, colours, several parts or the settings you tuned, save a 3MF.

3MF vs STL at a glance

What an STL file holds

STL was made for 3D Systems’ first commercial printers in 1987. A file is a list of triangles, and each triangle has three corner points and one normal (the direction it faces). That is all. There is no colour, no texture, no unit, no object name and no link between triangles: a corner shared by six triangles is written six times.

Binary STL, the common kind, has a fixed layout: an 80-byte header, a 4-byte triangle count, then 50 bytes per triangle. So its size is easy to predict. A mesh of 100,000 triangles is 5,000,084 bytes, about 5 MB, whatever the shape. ASCII STL writes the same numbers as text and is several times bigger.

Because STL has no unit, the numbers mean whatever the program assumes. Slicers assume millimetres, so a model designed in inches opens 25.4 times too small. For more on how STL compares with a format that has textures, see STL vs OBJ.

What a 3MF file holds

Rename a .3mf to .zip and you can open it: inside is a small folder tree. The main part is an XML file, usually 3D/3dmodel.model, that lists each object’s vertices and the triangles that use them, then a build section that says which objects to print and where. The 3MF Consortium publishes the specification, and it is also an ISO/IEC standard.

• A unit attribute on the model, so a slicer knows that 20 means 20 mm (or 20 inches).
• Several objects in one file, each with its own position on the build plate.
• Materials and colours. The core spec has base materials with a display colour; the Materials and Properties extension adds colour groups, mixed materials and textures (PNG or JPEG images with UV coordinates).
• Metadata such as title, designer, description, copyright and licence, plus a thumbnail image.
• Rules for printable objects: the mesh of a model object must be closed, with every edge shared by two triangles and the triangles facing outwards.
• Optional extensions for production, slices, beam lattices and secure (encrypted) content, mostly used by industrial printers.

The spec’s closed-mesh rule does not repair anything for you. A 3MF written from a broken mesh is still broken; it only makes it clearer what a valid file should be.

A plain 3MF vs a slicer project 3MF

The same file extension covers two different things, and this is where most “my settings did not come across” questions start.

• A plain (model) 3MF holds the geometry, units, objects and maybe colours. CAD programs and converters write this kind. Any 3MF-aware slicer opens it.
• A project 3MF is what a slicer writes when you save the project. It adds that slicer’s own files to the package: print and filament settings, modifiers, painted supports or colours, and plates. PrusaSlicer, Bambu Studio, OrcaSlicer and Cura all save projects this way.

The extra settings are written in each slicer’s own way, so they mostly stay inside that slicer family. Open a Bambu Studio project in Cura and you usually get the objects, not the settings. Bambu Studio itself warns when a 3MF was not saved by it and loads only the geometry. To share a print with someone on another slicer, a plain 3MF or an STL plus a note of your settings is safer.

How each slicer handles 3MF and STL

If a program is old enough not to list 3MF in its open dialog, convert the file to STL first (see below).

Which file is smaller, 3MF or STL?

Usually the 3MF, for two reasons. It stores each shared corner once and points to it, where STL repeats it for every triangle, and the whole package is ZIP-compressed. A binary STL is never compressed.

There is no fixed ratio, and published comparisons give very different numbers. 3MF writes its coordinates as text, which takes more room per number than binary STL’s 4-byte floats until compression wins it back, and a project 3MF can also carry thumbnails, textures and settings. The honest way to know is to convert your own model: our STL to 3MF converter shows the size of both files.

If an STL is simply too big to upload, reduce the STL file size by lowering the triangle count instead.

When STL is still the right choice

• The receiving tool, print service or old slicer only lists STL.
• You print in one colour and only the shape matters.
• You want the widest reach for a free model download. Many makers upload both an STL and a 3MF.
• You are running the file through a mesh repair or editing tool that reads STL best. Our STL repair tool, for example, takes STL.

Whenever you send an STL, say which unit it was designed in. That one sentence avoids the most common STL problem.

How to convert between 3MF and STL

• STL to 3MF: the mesh is kept as it is, shared corners are joined, and you choose the unit that gets written into the file. Nothing is added: an STL has no colours or settings, so the 3MF has none either until you set them in your slicer. Use our STL to 3MF converter, or open the STL in your slicer and save the project.
• 3MF to STL: the shape survives, but colours, painted areas, plates, supports and print settings are dropped, and the size is written in millimetres. You can export one merged STL or one STL per object with our 3MF to STL converter. In a slicer, use its export (not save) command.

To look inside a 3MF without installing a slicer, use the 3MF viewer. To keep the filament colours when you leave 3MF, convert to OBJ with 3MF to OBJ, which writes each colour as its own material. And if your model is an OBJ, OBJ to 3MF writes it straight into a 3MF with a unit.

Does converting lower the quality?

Not the triangles: the corner positions are copied, not re-made. What you lose going to STL is everything around the mesh (unit, colours, objects, settings), and what you gain going to 3MF is only the unit you pick.

Get a model, then colour it

No model yet? Make one from a photo with image to 3D: a generated model downloads as STL or 3MF (as well as GLB, OBJ and FBX), scaled to 100 mm on its longest side so it opens at a printable size. Or make a print-ready STL directly with image to STL.

An STL has no colour to begin with. The AI 3D Texturing Studio opens STL files (and OBJ, FBX, GLB, glTF and PLY) up to 60 MB and 600,000 triangles, makes UVs for it, and paints a full texture from a text prompt. You download a GLB with the texture inside. To turn colour into a multi-colour print, our guide to adding colour to an STL file covers slicer painting, splitting parts and full-colour services.

Questions