STL Viewer

Open STL files in your browser

This free STL file viewer opens .stl files straight in the browser, so you can check a 3D print before sending it to a slicer, look at a model you downloaded, or see what a customer attached to an email. Drop the file on the page and it appears in a second or two. Nothing is uploaded: the file is read on your own computer.

Both kinds of STL open: ASCII files, which are readable text, and binary files, which are much smaller and are what most programs write today. You do not need to know which one you have.

What an STL file stores, and what it doesn't

STL was created in 1987 by 3D Systems for its first stereolithography printers. The format describes a surface as a list of triangles, each with three corner points and a normal vector that says which way the triangle faces. That is all. It was designed to give a printer a closed shell, and it still does that job well, which is why every CAD program and every slicer supports it.

Because it is so simple, a lot is missing. Knowing this saves time when a model looks wrong.

Units

Not stored. A cube that is 20 units wide could be 20 mm or 20 inches. Most 3D printing files are in millimeters, but the file itself does not say.

Color

Not part of the standard. Some programs (VisCAM, SolidView and others) write per-face colors into the unused bytes of binary STL; this viewer shows those when present. Most STL files are a single color.

Textures and materials

Not supported at all. There are no UV coordinates in STL.

Curves

Not supported. Round surfaces are approximated with flat triangles, which is why a low-resolution export looks faceted.

Parts and assemblies

Usually not. An STL is one bag of triangles; separate bodies are merged into the same list.

Checking a model before you print

The info panel shows the triangle count and the bounding-box size in the file's own units. Read the size first: if a part you expected to be 50 mm long shows as 1.97, it was exported in inches; if it shows 0.05, it was exported in meters. Fix the scale in your slicer or re-export from the CAD program before printing.

Switch on wireframe to see how dense the mesh is. A curved part made of a handful of long, thin triangles will print with visible flats; raise the export resolution (often called chord height, deviation or angle tolerance in CAD software) and export again. A mesh with millions of triangles is the opposite problem: it prints no better and makes slicers slow.

Dark or black patches on the surface often mean flipped normals, where some triangles face inward. Holes and gaps mean the mesh is not watertight. This viewer shows these problems but does not repair them; use your slicer's repair function or a mesh tool such as Meshmixer or Blender's 3D Print Toolbox.

Why an STL file is lying on its side

CAD programs and 3D printers treat Z as the up direction. Many game engines, web viewers and animation tools use Y as up. An STL does not record which convention it was made in, so a model can open lying on its back.

This viewer opens STL files Z-up by default, which is right for most files made for printing. If yours still looks wrong, use the up-axis toggle to switch to Y-up. This only changes how you view it; the file is not modified.

Converting STL to other formats

Use "Download as" to save the model as OBJ, GLB, glTF, PLY or USDZ. Since STL has no colors, textures or UVs, the result contains geometry only. The converter pages turn on a "Fix up axis" option by default when the target is a Y-up format such as GLB or OBJ, so the model stands upright in the target program.

If you need a format that keeps several separate parts, units and colors for printing, 3MF is the better choice. Most current slicers save 3MF project files.

To turn an STL into a mesh you can edit in modeling software, use STL to OBJ.