STL viewer online
Open STL files instantly — measure in mm, cut a section, spot overhangs and check if the model is ready to print. Nothing is uploaded.
What is a STL file?
STL was created by 3D Systems in the late 1980s for its stereolithography machines, and it became the default exchange format for 3D printing. A file describes a surface as a list of triangles, each with three vertices and a facet normal. That is all it contains. There is no header field for units, no colour, no texture and no notion of separate parts. Two flavours exist: ASCII, which is readable text but large, and binary, which spends exactly 50 bytes per triangle after an 80-byte header and a 4-byte triangle count. Binary is what almost every modern tool writes. Because the format is so small and so old, every slicer, CAD package and mesh editor can read it, which is why hobbyists still download most printable models as STL from sites like Printables and Thingiverse.
What a STL file can store
An STL stores one unstructured soup of triangles. Vertices are not shared between triangles, so the file repeats every corner several times and relies on the reader to weld them. The standard defines no units, so a model drawn in inches and one drawn in millimetres look identical on disk; slicers simply assume millimetres. There is no colour in the standard, although some unofficial binary variants hide colour bits in the two spare attribute bytes. Texture coordinates, materials, multiple named objects and metadata cannot be stored at all.
Apps that open STL
Bambu Studio, PrusaSlicer, OrcaSlicer, UltiMaker Cura, Chitubox, Lychee Slicer, Blender, FreeCAD, Fusion, Meshmixer, MeshLab.
Strengths
- Accepted by every slicer and most CAD and mesh tools
- Binary files are compact and fast to parse
- Simple enough to write by hand or from a few lines of code
- Millimetre convention is universal in printing
Limits
- No colour, texture or material data in the standard
- No unit information, so scale mistakes are common
- Cannot hold several separate objects or part names
- Triangles only, so smooth CAD surfaces are approximated
- Nothing enforces a closed mesh; holes are easy to ship
STL and 3D printing
STL is still the safest file to hand to a slicer. Bambu Studio, PrusaSlicer, OrcaSlicer, Cura and the resin tools Chitubox and Lychee all import it, and they all treat the numbers as millimetres. The usual problems are scale (a model exported from a metre-based tool arrives 1000 times too small) and meshes that are not watertight, which makes slicers fill or skip regions. MeshShift shows the real dimensions before you convert and can scale from metres automatically. Its print check lists holes, non-manifold edges and flipped normals, and one click rebuilds a closed solid at the same size. If you need colours or several plates in one file, use 3MF instead.
Frequently asked questions
How do I open a STL file without installing software?
Drop the STL file on this page. It opens in your browser with studio lighting and real dimensions; nothing is uploaded. You can rotate, zoom, measure between two points, cut through the model and check whether it is watertight for printing.
Does an STL file contain units or colour?
No. The STL standard has no field for units and none for colour, so slicers assume millimetres and print in whatever filament you choose. A few programs squeeze colour into the two spare bytes of each binary triangle, but those files are non-standard and many slicers ignore the data.
What is the difference between ASCII and binary STL?
Both hold the same triangles. ASCII writes every number as text, so files are roughly five to six times larger and slower to load. Binary stores 50 bytes per triangle after an 80-byte header. Use binary unless a tool specifically requires readable text.
Can MeshShift open or convert STL files in the browser?
Yes. MeshShift reads STL and can write it as binary or ASCII, all inside your browser, so the file is not uploaded. You can check dimensions, run the print check and repair, and convert to formats such as 3MF, OBJ, GLB, PLY or STEP.