What is inside
- Triangle vertices and normals
- Binary or ASCII encoding
- Nothing else
Free STL Viewer
Drop an STL into the Img3D viewer to see the shape, the bounding box, and an estimated triangle count before you open the slicer. The file stays in your browser and is never uploaded.
Also opens GLB, glTF, OBJ, PLY, FBX, and 3MF files.
What an STL file actually carries
STL is the oldest of the common mesh formats and the most stripped down: a list of triangles and nothing else.
What is missing
- No units and no scale metadata
- No color, material, or texture
- No object names or scene hierarchy
- No print settings
Because the unit is implied rather than stored, a slicer reads the numbers as millimetres by convention. That is the reason a model exported in centimetres arrives ten times too small, and the reason the bounding box is the first thing worth checking.
How to check an STL before slicing
- Open the 3D Viewers workspace, choose the STL viewer, and pick the file.
- Read the bounding box and compare it with the size the print should be. A 20 mm part showing as 200 mm is a unit problem, not a modeling problem.
- Check the estimated triangle count against the detail you need. Very low counts give faceted surfaces, very high counts slow down the slicer for no visible gain.
- Orbit the model to check orientation and the flat face that should sit on the plate.
- Confirm watertight geometry in the slicer. Img3D reports shape and size, not manifold errors, so the repair step belongs there.
Resize an STL for your printer
The 3D Utilities workspace includes a scale calculator: enter the current dimension and the target dimension, and it returns the factor to apply. It is the quickest way to turn a bounding box reading into a number you can type into a slicer or a modeling tool.
- Measure the longest axis in the viewer, since that is the one that hits the build volume first.
- Scale uniformly unless you have a reason not to. Non-uniform scaling changes wall thickness and clearance.
- Re-check the bounding box after scaling, especially when the model carries supports or a base plate.
From an image to a printable part
If the STL does not exist yet, the route through Img3D is: generate a GLB from an image, check the shape in the viewer, use the converter workspace to plan the route to STL, then fix the scale and hand the file to the slicer. A generated mesh is a starting shape, so expect to finish thin features, flat bases, and joining surfaces in a modeling tool.
Image to STL with a hosted pipeline
Photo3D, our sibling site, runs an image to STL flow end to end with its own workspace and viewer. It is the better fit when you want the printable route handled for you rather than planned in the browser and finished by hand.
Questions about the STL viewer
Does an STL file carry units?
No. STL stores triangles without a unit, a scale, or a color. Slicers read the numbers as millimetres by convention, which is why the first print of a model exported in centimetres or inches comes out the wrong size.
Does the viewer check watertight geometry?
No. It shows the shape, the bounding box, and an estimated triangle count. Manifold and shell checks belong in the slicer, which has to repair the mesh anyway.
Can I resize an STL before printing?
Use the scale calculator: enter the current dimension and the target dimension to get the factor, then apply it in your slicer or modeling tool.
Is the STL viewer free?
Yes. It runs in the browser with no account and no upload, and there is no limit on how many files you open.