STL File Viewer

Three steps from a file on disk to a checked model. Nothing is installed, and nothing is uploaded.

1. 1

   Open the file

   Drop an STL onto the panel above, or press Open 3D file(s) and choose it. Opening an OBJ or glTF model works the same way; add its MTL, BIN and texture files to the same selection so the materials resolve.

2. 2

   Look from a few angles

   The model is framed to fit the moment it loads. Drag to orbit, scroll to zoom, right-drag to pan. Fit, Front, Top and Right repeat the same views from buttons, which is steadier than a trackpad when the question is whether an edge is straight.

3. 3

   Read the numbers, then save

   Overview lists the bounding box, triangle count and mesh count. Wireframe shows how the surface is built. Export PNG saves the current view at 1× or 2×, on the stage colour or with a transparent background.

The gap between downloading a file and dropping it into a slicer is where most surprises appear. Four checks cover the usual ones.

• Is the whole part there?

  Fit puts every triangle in frame. A flat edge where a curve belongs usually means the download stopped early: the file is truncated, not modelled that way.

• How dense is the mesh?

  Wireframe turns the surface into its triangles. A dense mesh is not automatically wrong, but a million triangles on a bracket gives the slicer more work than the printer.

• Which way is up?

  STL stores no up axis. Top, Front and Right view the part from known directions, and the Z-up button stands up a model exported from a Z-up tool. The file itself is not changed.

• Are the dimensions plausible?

  Overview reports the size written in the file, measured before any fitting. A part that should be 100 mm and reads 3.937 is in inches; the slicer will ask which unit to apply.

These checks describe the geometry. They do not test wall thickness, overhangs, supports or whether a part will print. A viewer is not a slicer, and no readout replaces a slice preview.

An STL file describes a surface as a list of triangles. Each triangle carries three vertices and a normal, and nothing else: no colour, no material, no unit, no scene graph. Binary is the compact, usual variant; ASCII is plain text and easier to read by hand.

Two things follow from that format, and both explain what the viewer shows. A number in an STL has no unit until a program assigns one, so the size is reported as source units and the millimetre-or-inch decision stays with the slicer. And a solid STL has no surface material, so a neutral grey model is the correct rendering of the file rather than a failed load.

Grey is the shape of the format

The format has no colour, texture or material to display. The viewer shades one neutral surface from a light at the upper left so edges and curvature stay readable. The same model will be grey in a slicer.

The part is lying on its side

STL records no up axis. A part exported from a Z-up tool arrives on its back in a Y-up scene. Press Z-up in the toolbar to stand it up; the geometry, the dimensions and the file are untouched.

The size is 25.4 times off

Nothing in an STL says whether one unit is a millimetre or an inch. Dimensions that are 25.4 times smaller or larger than expected mean the file was written in the other unit. The viewer does not rescale, because a silent conversion would put the readout at odds with the slicer.

A file opened in the viewer is read by the browser and held in memory for as long as it is open. There is no upload endpoint in the application, so the bytes have nowhere to go even when a network is available.

• No account, no cloud library, no share link.
• No file name, path or mesh data is included in any request.
• Clear drops the model and releases its memory; closing the tab releases the rest.
• The one request made for a model is the sample file, fetched from this site when Try a sample is pressed.

Page views are counted by two analytics scripts, and neither is given a file. The privacy notes list what they receive and what never leaves the tab.