How do I open an STL file?

Any slicer (Cura, PrusaSlicer, Bambu Studio, OrcaSlicer) opens an STL, as does most CAD and 3D software. To just look at one without installing anything, use a browser STL viewer — it opens the file locally and shows the model, its size and triangle count.

Does an STL file contain color?

No. STL stores triangle geometry only — no color, texture or material. A multi-color model saved as STL loses its color and prints as a single solid piece. If color needs to survive, use 3MF or OBJ instead.

What's the difference between binary and ASCII STL?

They store identical geometry. Binary is compact — about 50 bytes per triangle — and is what most tools write. ASCII spells the triangles out as readable text and is several times larger. Any STL tool reads both, so you rarely need to care which you have.

Why does my STL open at the wrong size?

STL files carry no units, so a part exported in inches shows up about 25.4× too small when a slicer reads it as millimetres. Scale it in your slicer, or re-export from CAD with the units set to millimetres.

STL or 3MF — which is better for printing?

If your slicer supports 3MF (all current ones do), 3MF is the better file: it keeps units, color and multiple parts in one place. STL is the safe fallback for maximum compatibility with older tools and services that only accept triangles.

What is an STL file?

STL, short for stereolithography

STL is the format 3D printing runs on. It has been the standard exchange format for four decades, and almost every model you download from Printables or Thingiverse arrives as a .stl. Its whole design is a single idea taken to the extreme: describe the surface as triangles, and nothing else.

Open a .stl in the STL viewer →

What's inside an STL

An STL file describes an object as a mesh of flat triangles. Each triangle — a “facet” — is stored as a surface normal plus its three corner points, and that is the entire format. There is no color, no material, no texture, no scene structure, and, famously, no unit. That last omission is the one that catches people: the numbers in an STL are just numbers, so the same file can be read as millimetres in one tool and inches in another. Slicers resolve it by assuming millimetres.

STL comes in two encodings that store identical geometry. Binary STL packs each triangle into about 50 bytes after an 80-byte header and is what nearly every tool writes today. ASCII STL spells the same triangles out as readable text (“facet normal … vertex …”) and runs several times larger.

Why it's still everywhere

STL persists precisely because it does so little. A format that is only triangles is trivial for any slicer, printer or service to read, which is why STL became — and remains — the lowest common denominator of 3D printing. Newer formats like 3MF carry far more (color, units, multiple parts), but STL's bare simplicity is the reason it never went away.

Its limits are the flip side of that simplicity. Because an STL is only a surface, it has no idea whether that surface is closed; a mesh with holes or flipped faces looks fine until a slicer refuses it as “not watertight.” And because it stores no color, an STL of a multi-color model prints as one solid piece.

STL files — FAQ

How do I open an STL file?: Any slicer (Cura, PrusaSlicer, Bambu Studio, OrcaSlicer) opens an STL, as does most CAD and 3D software. To just look at one without installing anything, use a browser STL viewer — it opens the file locally and shows the model, its size and triangle count.
Does an STL file contain color?: No. STL stores triangle geometry only — no color, texture or material. A multi-color model saved as STL loses its color and prints as a single solid piece. If color needs to survive, use 3MF or OBJ instead.
What's the difference between binary and ASCII STL?: They store identical geometry. Binary is compact — about 50 bytes per triangle — and is what most tools write. ASCII spells the triangles out as readable text and is several times larger. Any STL tool reads both, so you rarely need to care which you have.
Why does my STL open at the wrong size?: STL files carry no units, so a part exported in inches shows up about 25.4× too small when a slicer reads it as millimetres. Scale it in your slicer, or re-export from CAD with the units set to millimetres.
STL or 3MF — which is better for printing?: If your slicer supports 3MF (all current ones do), 3MF is the better file: it keeps units, color and multiple parts in one place. STL is the safe fallback for maximum compatibility with older tools and services that only accept triangles.

Working with STL files

STL viewer STL → OBJ GLB → STL STL repair Make an STL watertight All 3D formats

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