Convert OFF to STL

Why convert OFF to STL?

OFF, the Object File Format, is a minimal text format that lists vertices and polygon faces. It is common in academic work: the ModelNet and Princeton Shape Benchmark datasets, computational geometry libraries such as CGAL, and MeshLab exports use it. Printers do not read it, so OFF to STL makes a research mesh printable. The conversion is simple because OFF holds exactly what STL needs, vertices and faces; polygons with more than three corners are split into triangles. OFF has no unit and files from datasets are often normalised to a unit cube or sphere, so the model will be tiny until you set a real size. Use the dimension labels and type in the height or width you want in millimetres.

How to convert OFF to STL

1. Drop your OFF file onto the converter — it opens in your browser, nothing is uploaded.
2. Check the preview and the dimensions in mm; fix units or the up axis in “Units & orientation” if the size or orientation looks wrong.
3. Choose STL as the output (binary is smaller; ASCII is human-readable) and click Convert. Several files download as one ZIP.

What carries over from OFF to STL

What changes

• Polygons larger than triangles are split into triangles.
• No unit is stored; the model is often normalised and needs rescaling.
• Any per-face colours (COFF) are dropped, as STL has no colour.
• Geometry is otherwise unchanged.

Tips

• Set the target size in millimetres, since dataset models are often about one unit wide.
• Dataset meshes can be open or self-intersecting; run the print check and repair.
• Check the up axis, because the orientation in datasets is arbitrary.
• Thin or low-resolution shapes may need thickening before they print well.

About OFF files

OFF, the Object File Format, was defined for Geomview, an interactive geometry viewer developed at the Geometry Center at the University of Minnesota in the 1990s. A file is plain text. The first line holds the keyword OFF, the second gives the counts of vertices, faces and edges, then each vertex appears as three coordinates and each face as a vertex count followed by index numbers. Variants add colour (COFF), normals (NOFF) or texture coordinates (STOFF) through prefixes, and colour is optional at the end of each line. The simplicity made it a favourite in computational geometry, and benchmark sets such as ModelNet used it for machine-learning research on shape classification. CGAL and libigl read and write it directly. It has no units, materials or scene structure, so it is rarely a printing format, but converting it is common.

About STL files

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.

3D printing notes

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.

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