Open an STL file to rotate, zoom and measure it. See the size in X, Y and Z, the triangle count, volume and surface area – in millimetres or inches.
Could not open this file
Drop an STL file hereRelease to open
or
Binary and ASCII STL · opened on your device, never sent anywhere
No file at hand?
Model info
How it works
Open an STL file
Drop it on the page or choose it. Binary and ASCII STL both work, and the file is read by your browser.
Look around
The model sits on a grid with Z pointing up, as in most slicers. Use the preset views (front, top, right, iso) to line the part up, and wireframe to see the triangles.
Read the numbers
The info panel shows the bounding-box size along each axis, the number of triangles, the enclosed volume and the surface area. Measure gives the distance between two points on the surface.
Check an STL before printing
- Size: compare Size X, Y and Z with your printer’s build plate. A part that is about 25 times too small was probably exported in inches; one that is 1000 times too small, in metres. A part that is 25 times too large was probably converted from inches twice.
- Orientation: the viewer puts Z up, like most slicers. Use Front and Top view to see which face will sit on the bed.
- Wall thickness: turn on Measure and snap to two opposite edges. Walls thinner than about two nozzle widths (0.8 mm for a 0.4 mm nozzle) often print badly.
- Volume: multiply the volume by your filament’s density (about 1.24 g/cm³ for PLA) for a rough weight before infill and supports.
- Mesh: volume is only correct for a closed (watertight) mesh. If it looks far off, the mesh probably has holes. Most slicers repair small gaps, but large holes are worth fixing in the CAD program.
Settings explained
Units
STL files store plain numbers without units. The viewer reads them as millimetres, which almost every 3D printing and CAD tool exports. Switch to inch in the info panel to convert the values.
Snap to corners & edges
Turn on Measure and click two points to get the distance between them. With Snap to corners & edges on, a click near a corner or edge lands exactly on it, so you can measure edge lengths precisely. Only the part’s real corners and sharp edges count, not the triangles of the mesh.
Controls & shortcuts
Press ? in the viewer to see all mouse, touch and keyboard controls.
Binary vs ASCII STL
STL is the most common file format for 3D printing. It describes the surface of a part as a list of triangles, and nothing else: no colours, no materials and no units. STL files come in two flavours, binary (compact, the usual choice) and ASCII (plain text, larger). This viewer opens both, including binary files whose header starts with the word “solid”.
Limitations
Volume is only correct when the mesh is closed. Snapping looks for the part’s real edges and corners, where faces meet at more than 30°, and ignores the triangles in between. Smooth, rounded edges have no sharp edge to snap to, so a click there gives a free point. On phones, snapping is off for models over 1.5 million triangles to save memory. The viewer does not repair or convert files.
| Property | Binary STL | ASCII STL |
|---|---|---|
| Extension | .stl | .stl |
| MIME type | model/stl | model/stl |
| Encoding | Binary | Text |
| Size | 84-byte header + 50 bytes per triangle | Seven lines of text per triangle |
| Colours and materials | No, not in the standard | No |
| Units | Not stored | Not stored |
| Opens in this viewer | Yes | Yes |
STL vs OBJ vs 3MF
STL stores only triangles. OBJ can also store colours and textures through a separate MTL file, and is common in 3D graphics. 3MF is a newer format for 3D printing that keeps units, colours, materials and several objects in one file. Bambu Studio and PrusaSlicer save projects as 3MF.
FAQ
Is my STL file uploaded?
No. The file is read and drawn by your own browser. Nothing is sent to a server, so confidential parts stay on your device. You can check this in the Network tab of your browser’s developer tools.
Does it open both binary and ASCII STL files?
Yes. The format is detected automatically, including binary files whose header starts with the word "solid", which confuses some viewers.
What units are the dimensions in?
STL files store plain numbers without units. Almost all 3D printing and CAD tools export millimetres, so the viewer assumes millimetres. Switch to inches in the info panel to convert the values.
How large a file can I open?
Files over 100 MB (millions of triangles) work on a normal computer. The file is parsed in the background, so the page stays responsive. Phones and tablets have less memory and may struggle with the largest files.
Why does the volume look wrong?
Volume can only be calculated for a closed (watertight) mesh. If the model has holes, gaps or overlapping parts, the volume is an estimate. Size and surface area are not affected.
Does it work on a phone?
Yes. Drag with one finger to rotate, pinch to zoom and drag with two fingers to pan. Turn on Measure and tap two points to measure the distance between them. Full screen gives the model the whole screen.