.stl.obj.3mf

STL, OBJ and 3MF files: what they are and how to open them

STL, OBJ and 3MF files are 3D models. Turn one around on Android or Windows in Universal File Opener, measure it and check whether it is watertight, without a 3D program. Viewing is free.

Check your STL, OBJ, 3MF file here

Choose STL, OBJ, 3MF files Choose a folder

Nothing is uploaded. This page names the type and shows its details; to view it, use an app below.

What is an STL, OBJ, 3MF file?

A 3D model file describes a surface as flat faces, usually triangles. STL stores only the triangles, with no units, textures or standard color, and is the usual file for 3D printing. OBJ adds texture coordinates and points to a separate material file. 3MF is a ZIP package from the 3MF Consortium that adds units and colors.

The same viewer also opens PLY, GLB, glTF, OFF, AMF, COLLADA, 3DS and FBX meshes. STL declares no unit, so the apps start by assuming millimeters and let you choose how to read the coordinates.

What is inside

A binary STL has an 80-byte header, a 4-byte triangle count, then 50 bytes per triangle: a normal, three corners and two spare bytes. An ASCII STL starts with the word solid. Some binary files also begin with solid in their header, so a reader checks the size too.

An OBJ is plain text: v lines are points, vt texture coordinates, vn normals and f faces. A mtllib line names the material file. A 3MF is a ZIP whose model part, usually 3D/3dmodel.model, holds the mesh, units and colors in XML.

part.stl (binary)

├─ 80 bytesfree-text header

├─ 4 bytestriangle count

└─ per triangle, 50 bytes:

normal and 3 corners, 12 floats

then 2 spare bytes

A binary STL is a count followed by one fixed record per triangle.

How to open an STL, OBJ, 3MF file

Windows

BlenderFree. Imports STL and OBJ for viewing, sculpting and editing; 3MF needs an add-on.
PrusaSlicerFree slicer that opens STL, OBJ and 3MF for printing.
Universal File Opener for WindowsOursTurns, pans and zooms the model offline and reports measurements and a watertight check. Viewing is free.

Mac

BlenderFree. Imports STL and OBJ; 3MF needs an add-on.

Android

Universal File OpenerOursTurns, pans and zooms the model offline, with measurements and a watertight check. Viewing is free; PNG snapshots need Pro.

In a browser

three.js editorFree. Its File menu imports STL, OBJ and 3MF models into a scene you can look around.

Won’t open?

The model looks tiny or enormous

STL has no unit, and the apps assume millimeters. If that guess is wrong, pick inches in the edit tools; saving the rescaled copy needs Pro.

An OBJ has no color or texture

Colors and pictures live in separate .mtl and image files. Keep them in one folder with the OBJ and grant the folder; a single file picked on Android cannot see its neighbors.

Only dots appear

An OBJ or PLY with points but no faces opens as a point cloud. It has no surface to show.

A big model looks rougher than expected

Large models show a labeled reduced preview of up to 100,000 triangles, and copies are turned off while it is reduced. Measurements still cover the whole model.

A slicer reports holes or bad edges

The watertight check welds matching points and tests every edge. It does not prove a model has no self-intersections, so keep your slicer’s repair step.

The file is cut off

A damaged or truncated STL, OBJ or PLY still shows the complete triangles before the damage.

Convert an STL, OBJ, 3MF file

To STL, OBJ, PLY, GLB or 3MFOpen a full mesh and use Convert; saving the copy needs Pro. Copies keep geometry and colors but not scene structure or textures. A model with only points or lines needs an OBJ copy.
To a pictureSave a PNG snapshot of the current view with Pro. The original file is never replaced.

STL, OBJ, 3MF files in our apps

Universal File Opener opens STL, OBJ and 3MF models offline on Android and Windows. Orbit, pan and zoom, read measurements, run a watertight check and, with Pro, save PNG snapshots and STL, OBJ, PLY, GLB or 3MF copies.

All Android tools and limits for STL, OBJ, 3MF files

Orbit, pan and zoom a shaded model with front, top and side views and a wireframe overlay. Details list triangle counts and, for closed models, the volume.
The same viewer opens PLY, GLB, glTF, OFF, AMF, COLLADA, 3DS and FBX meshes.

Limits

Large models show a labeled reduced preview of up to 100,000 triangles. STL has no unit, so the apps start with a millimeter assumption.
Textures need their image files in the same granted folder. Animation and skinning are not shown.
The watertight check does not prove a model has no self-intersections.