STL file

An STL file is a 3D data format that describes the surface of an object as a polygon mesh of connected triangles (a triangulation). The name comes from stereolithography, the process the format was created for in the late 1980s; the expansions Standard Triangle Language and Standard Tessellation Language were added later. It is still the most widely used input format for 3D printing, and the files have the .stl extension.

Each triangle is stored as the coordinates of its three vertices plus a normal vector that defines the outer side of the surface. The file can be text (ASCII) or binary. The binary version is several times smaller and therefore more common. A standard STL carries no units, colors, material, tolerances or assembly structure: it contains only the bare surface geometry.

Curved surfaces are replaced by flat facets, so a cylinder in an STL is really a polyhedron. The accuracy of this approximation is set when exporting from CAD, usually as the maximum chordal deviation from the true surface and the maximum angle between adjacent triangles. For 3D printing, a deviation of a few hundredths of a millimeter is normally enough. A finer setting only makes the file bigger without improving the print, while a coarser one shows up as visible facets on rounded surfaces.

For printing, the mesh must be closed (watertight, or manifold): no holes, no intersecting or duplicate triangles, and consistently oriented normals. STL is also a common output of 3D scanning, but there it typically contains millions of triangles and needs cleanup. A more modern alternative for 3D printing is the 3MF format, which can carry units, colors, materials and several objects in one file.

Use STL to send data for 3D printing (FDM, SLA, SLS and other processes), for quick visualization, or to share a shape when the recipient does not need to change anything. Practically every slicer, 3D viewer and scan processing program can open it.

It is not suitable for CNC machining, mold making or further design work. A machine shop needs exact surfaces in STEP format, because triangles cannot reliably tell you the diameter of a hole or the flatness of a face. Turning an STL back into an editable CAD model is not a simple format conversion: it means remodeling the geometry (scan to CAD).

The most common mistake involves units. Because STL does not store them, a part exported in inches opens 25.4 times smaller in software set to millimeters, and vice versa. Check the dimensions in a viewer before sending the file, or state them in the order. The second common mistake is an export that is too coarse: holes and pins with visible facets then do not match the required diameter.

Have the mesh checked and repaired before printing (holes, flipped normals, non-manifold edges, walls that are too thin). If the part has critical dimensions, send a STEP file or a drawing with tolerances alongside the STL, because the supplier cannot read them from the mesh. When archiving a design, do not treat STL as source data: keep the native model, or at least a STEP file.