3D printing file formats: STL vs 3MF vs OBJ vs STEP explained

Use 3MF for slicer projects and multicolor prints, STL for simple universal sharing, and STEP when someone needs to edit the design in CAD. G-code, including Prusa's .bgcode and Bambu's .gcode.3mf, is printer-specific slicer output, not a model. Here's what every common 3D printing file format stores and how to move between them.

3D printing file formats compared

Formats fall into three families: meshes (triangles), CAD solids (exact surfaces and features) and machine code (instructions for one printer).

STL vs 3MF: which should you use?

STL dates back to the late 1980s and stores one thing: a list of triangles, each with three corner points and a normal. It has no units, no colour and no idea which triangles belong to which part. Its superpower is that literally everything opens it.

3MF, maintained by the 3MF Consortium, is a ZIP package of XML files. The core specification declares a unit (millimetres by default, with micron through metre allowed), named base materials, multiple objects and optional thumbnails. Extensions add colour, textures and more. Slicers go further still: a Bambu Studio, OrcaSlicer or PrusaSlicer project 3MF also saves plates, filament assignments, painted regions and print settings.

Use 3MF when the print has several parts or colours, when you want slicer settings to travel with the model, or when you hand a ready-to-slice project to someone.

Fall back to STL when the recipient's software is old or a model site only accepts STL.

STL vs OBJ (and PLY): when colour matters

OBJ is a plain-text mesh format from Wavefront. It can store polygons (not just triangles), texture coordinates and normals, and it points to a companion .mtl file that holds materials and texture image names. Lose the .mtl or the images and the colour goes with them, so share OBJ files as a ZIP.

PLY, the Stanford polygon format, is what many 3D scanners and photogrammetry tools produce. It stores per-vertex colour and comes in ASCII or binary flavours. Neither OBJ nor PLY records units.

For a single-colour print, OBJ and PLY offer nothing STL doesn't. For colour, they're how textured models from scanners and AI generators usually arrive, and Bambu Studio can read OBJ vertex or material colours on import and map them to filaments.

Mesh vs BRep: why STEP vs STL matters

A mesh (STL, 3MF, OBJ, PLY) approximates every surface with flat triangles. A BRep, short for boundary representation (STEP, IGES, native CAD), stores the real geometry: this face is a cylinder of radius 4 mm, this edge is an arc, these two faces meet at a 1 mm fillet. STEP itself is the ISO 10303 family of standards for exchanging product data between CAD systems.

Think of it this way: an STL is a photo of a cake, a STEP file is the recipe, and the native CAD file is the recipe with the chef's notes in the margin. You can print from any of them, but only the recipe lets you bake a slightly bigger cake.

That's why you can't easily turn an STL back into editable CAD. The triangles don't know they used to be a hole or a fillet, and the feature history (sketches, dimensions, the order of operations) was never in the file. If you're still picking a CAD program, see the best free CAD software for 3D printing.

Why mesh-to-STEP gives you a faceted solid

Converters wrap each triangle in a flat STEP face and stitch the faces into a solid. The result is valid STEP that any CAD program opens, but a cylinder made of 2,000 triangles is still 2,000 flat faces, not one round face. You can measure it, use it as a reference or add new features to it. You can't drag its diameter.

How to convert STL to STEP (and what you actually get)

1. Repair the mesh first. Only a watertight mesh converts to a solid; holes give you a surface body or a failed conversion.
2. Reduce the triangle count. Fewer triangles mean fewer STEP faces and a file CAD can handle. Decimate gently so curves keep their shape.
3. Convert in a CAD tool. In FreeCAD, import the STL, run Part → Create shape from mesh, then Convert to solid, optionally Refine shape, and export STEP. In Fusion, insert the mesh and use Convert Mesh: the personal license offers the Faceted method, while Prismatic (which merges flat regions into clean faces) needs a paid license.
4. Decide whether you really need it. For anything you'll keep editing, remodel the part in CAD using the converted solid as a tracing reference. It's usually faster than fighting a body made of triangles.

PrintPal's CAD Agent can also export a faceted STEP of the models it generates (in Advanced mode), which is handy for dropping an AI-designed part into a CAD assembly. Its editable source is the .scad file, and the OpenSCAD tutorial shows how to work with one.

STL units and scale problems

STL coordinates are just numbers. A cube that measures 25.4 could be 25.4 mm or 25.4 inches, and the file can't say which. Nearly every slicer assumes millimetres.

• Part is 25.4× too small: it was exported in inches. Scale it by 2540% in the slicer, or re-export in millimetres.
• Part is 10× or 1,000× off: it was exported in centimetres or metres. Blender's default unit is the metre, so set scene units to millimetres before you model.
• Avoid the problem entirely: export 3MF, which declares its unit, or pick millimetres in the export dialog. Fusion's Save As Mesh and Onshape's STL export both ask for a unit.

STL export resolution and tessellation settings

When CAD exports a mesh, it approximates each curve within a tolerance. Too coarse and round holes print as polygons, slightly undersized because the flat edges cut inside the true circle. Too fine and the file swells with no visible gain.

Binary vs ASCII STL

Binary STL stores an 80-byte header, a triangle count, then exactly 50 bytes per triangle. ASCII STL writes the same data as readable text, which makes it several times larger. The geometry is identical, so use binary unless a tool specifically asks for ASCII. Here's one triangle in ASCII form:

solid bracket
  facet normal 0 0 -1
    outer loop
      vertex 0 0 0
      vertex 20 0 0
      vertex 20 10 0
    endloop
  endfacet
endsolid bracket

Watertight and manifold: what slicers need

Slicers find the inside of your part by following its surface, which only works when that surface is closed:

• Watertight: no holes, so every edge is shared by exactly two triangles.
• Manifold: no edge shared by three or more faces, and no surfaces that touch at a single point.
• Consistent normals: every triangle faces outward. Flipped ones confuse inside and outside.
• No self-intersections: no overlapping shells left over from un-merged parts or messy booleans.

Exports from parametric CAD are usually clean. Sculpts, scans and AI-generated meshes are where problems show up; see preparing AI models for 3D printing for the full clean-up routine and the 3D printing design guide for wall thickness and overhang rules.

G-code, .bgcode and .gcode.3mf: printer output, not models

G-code is what the slicer writes after it has made every decision: moves, extrusion amounts, temperatures and fan speeds for one printer, nozzle and filament. It's plain text, so you can read it:

M104 S215          ; set nozzle temperature
M140 S60           ; set bed temperature
G28                ; home all axes
G1 Z0.2 F3000      ; move to first-layer height
G1 X60 Y20 E3.2 F1500 ; extrude a priming line

Two vendor variants are now common:

• .bgcode (Prusa binary G-code): a block-based, compressed format with metadata, thumbnails and checksums. Prusa says it cuts file size by about 70%. PrusaSlicer has exported it by default since 2.7.0 for supported printers (Original Prusa MINI, MK4 and XL on firmware 5.1.0 or newer), and File → Convert switches between binary and text.
• .gcode.3mf (Bambu Lab sliced file): what Bambu Studio's Export plate sliced file writes. It's a ZIP with each plate's G-code at Metadata/plate_1.gcode, plus thumbnails and slice info. It's not an editable project; use Save Project for a .3mf you can reopen and change.

Because G-code is printer-specific, never share it as "the model". A file sliced for one machine can heat, home and move in ways another printer doesn't expect, and you can't recover clean geometry from toolpaths.

To inspect a sliced file, the G-code Viewer opens plain-text G-code (.gcode, .gco, .g and .nc). Convert a .bgcode to text in PrusaSlicer first, or unzip a .gcode.3mf and open Metadata/plate_1.gcode.

How to convert between 3D printing file formats

Most conversions happen in tools you already have. Here's what each route costs you:

The PrintPal 3D Model Converter runs in your browser with no upload. It reads STL, OBJ, 3MF, GLB, glTF and PLY and writes STL, OBJ, GLB, glTF or PLY. It doesn't write 3MF or STEP: for a 3MF, open the STL in your slicer and save a project. Before and after converting, open the file in the 3D Model Viewer to confirm its size and shape.

Before you share a model: checklist

• Export in millimetres, or use 3MF so the unit travels with the file.
• Open the export in a viewer and check the dimensions match your design.
• Run a mesh check: watertight, manifold, no flipped faces.
• Share a 3MF or STL for printing and a STEP (or the .scad source) for people who want to remix.
• For multicolor prints, share a 3MF with each colour as a separate part or painted region.
• Zip OBJ files together with their .mtl file and texture images.
• Don't share G-code as the model; if you include it, name the printer, nozzle and filament it was sliced for.
• Add the license and print notes: orientation, supports and any tolerances that matter.

Frequently asked questions

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3D Model ConverterOpen STL, OBJ, 3MF, GLB, glTF or PLY and save it as STL, OBJ, GLB, glTF or PLY, privately in your browser.

3D Model ViewerOpen STL, 3MF, OBJ, GLB and more in the browser with dimensions and mesh stats.

Mesh Repair & SimplifierFill holes, fix non-manifold edges, remesh and reduce polygons — no upload.

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