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What the STL viewer shows the moment the file opens
A downloaded model rarely tells you what you are about to print. The Info panel on the right answers the first questions before you open a slicer.
- Size as X × Y × Z in mm or inches, with a units switch in the status bar.
- Volume and surface area in cm³ or in³, plus triangles, vertices and the number of separate parts.
- Weight for 17 materials, from PLA at 1.24 g/cm³ to steel and brass, with a % solid field for infill.
- Mesh check: watertight yes or no, holes, open edges, non-manifold edges, flipped, degenerate and inside-out triangles.
STL stores no units, so the viewer reads the numbers as millimetres. A part drawn in inches shows up 25.4 times too small, and it is cheaper to see that here than after six hours on the printer. Edit → Size → Inches → mm fixes it in one click.
Look around the model: views, section, measure
Orbit, zoom and pan with the mouse, or with your fingers on a touch screen. The left strip has the standard views (isometric, top, front, right and the opposite sides), a grid, wireframe, model colour and a PNG screenshot.
The section plane cuts the model along X, Y or Z with a slider and paints the inside red. Use it to see a wall thickness or a hidden cavity without cutting the file. The measure tool snaps to vertices and gives the distance with ΔX, ΔY and ΔZ; on our 20 mm test cube two corners read exactly 20.00 mm. Hold Alt to measure without snapping.
Four next steps from the same tab
Most people open an STL because something has to happen next. Each of these starts with the file you already have open.
- Convert to 3MF for a modern slicer, STEP for CAD, OBJ or GLB for other 3D tools: see the STL converter.
- Fix holes and inside-out normals with STL repair, then scale, cut or combine parts in the STL editor.
- Price the job from weight, material and print hours with the 3D print cost calculator.
- Slice it into G-code with the STL to G-code slicer, then step through the layers in the G-code viewer.
Files the viewer opens, and the ones it does not
| Group | Opens (tested) |
|---|---|
| 3D printing | STL binary and ASCII, 3MF, AMF (plain and zipped), G-code |
| CAD exchange | STEP / STP, IGES / IGS, tessellated in the tab with the unit from the file |
| Mesh and scene | OBJ, PLY, OFF, GLB, glTF (with .bin or embedded), Draco-compressed GLB, FBX 7.x, DAE, 3DS, WRL (VRML 2.0) |
Native CAD part files such as SLDPRT, IPT or F3D do not open: export STEP from the program that made them, and the STEP viewer takes it from there. Project files of modelling programs (SKP, BLEND, 3DM), USDZ, IFC and FBX 6.x are also outside what the engine reads, and so are binary .bgcode and .gcode.3mf print projects. The 3D file viewer overview lists what each format keeps.
How big a model the tab can handle
There is no file-size cap; your computer's memory sets the limit. We measured it on a laptop CPU in headless Chrome. A 250,000-triangle STL (12 MB) opens in about 1.5 s. A 1 million triangle STL (48 MB) takes about 5–12 s and roughly 1.2–1.7 GB of memory. Above about 1–2 million triangles the tab may run out of memory. For scans of that size, reduce the triangle count first or open them on a desktop machine with more RAM.
The viewer is tested in Chromium browsers (Chrome 155) and at a 390 px phone width. Firefox and Safari should work, as they have WebGL2 and WebAssembly, but they have not been tested yet.
Your STL never leaves the computer
JavaScript in this tab reads the file from your disk; viewing, converting, editing and slicing run on your device. We ran a full test that opened every tested format and ran every conversion and edit. Every network request in it went to stlview.app itself, all were GET, and nothing was posted or sent to another domain. The engines, including the 7.6 MB OpenCascade module for STEP, come from our own domain. Recent files are remembered only in this browser.
Prefer opening models straight from a web page? The Chrome extension counts the 3D file links on a page and opens the one you right-click.
Questions and answers
How do I open an STL file online?
Drag the .stl onto the drop area at the top of this page or press Open File. In our tests a 250,000-triangle model opens in about 1.5 s. You need no account and nothing to install.
Is this an STL reader for ASCII and binary files?
Yes. Both STL variants open, and the converter can save either one. The ASCII vs binary guide explains the difference.
Why does my model look tiny or huge?
STL has no units. The viewer assumes millimetres, so an inch model reads 25.4 times too small. Use Edit → Size → Inches → mm, or set an exact size on one axis.
Can I preview STL files on a phone?
The layout is tested at 390 px width and touch orbit works. Large models need memory, so a phone handles small and medium files better than million-triangle scans.
Does the viewer change my file?
No. Your original stays as it is on disk. Edits and conversions create a new file only when you press Download.