STL Viewer: check an STL file before you print it
Drop an STL and see its real size, volume and surface area at once. The viewer reads binary and ASCII files, counts open edges and separate shells, and shows the mesh in a clear neutral colour, since STL stores no colours of its own.
The STL stays on your computer
Drop an STL file here
Binary or ASCII STL, or a ZIP with several parts
No STL at hand? Make one from a library model with the GLB to STL converter.
The mesh is read on this device. No upload, so unreleased designs stay private.
Model library
What is an STL file?
STL comes from stereolithography, the first commercial 3D printing process, and has been the standard exchange format for 3D printers since the late 1980s. A file holds nothing but a list of triangles: three corner points and a facet normal each. There are no units, no materials, no textures and no scene structure.
Binary STL starts with an 80 byte header and a triangle count, followed by 50 bytes per triangle. ASCII STL spells out every facet as text between "solid" and "endsolid", which makes it readable in an editor and several times larger. Slicers, CAD programs and 3D scanners all write STL, which is why it is still everywhere despite its limits.
What the STL tab shows
- Size in three units
- Width, depth and height along X, Y and Z, switchable between millimetres, centimetres and inches. If a part is a thousand times off, change what one file unit means.
- Volume and surface area
- Enclosed volume in cm³ or in³ and the total surface. For a closed mesh the volume is exact, for an open one it is marked as an estimate.
- Watertight check
- Every edge should belong to exactly two triangles. The tab counts open edges, edges shared by three or more faces, degenerate triangles and separate shells.
- File details
- Binary or ASCII, the text of the binary header and whether the file carries face colours, which some programs write into the otherwise unused attribute bytes.
- Display colour
- Grey, white, orange or blue for the surface, so small details read well against the background.
Programs that open STL
- Slicers
- PrusaSlicer, UltiMaker Cura, Bambu Studio and OrcaSlicer turn an STL into G-code for the printer. Several of them offer a repair function for small defects.
- Blender
- Imports and exports STL, for example to sculpt a figure, add a base or split a model into printable parts.
- FreeCAD
- Opens STL as a mesh. To edit it like a CAD part, the mesh has to be converted into a solid first.
- MeshLab
- A free mesh editor for scans: cleaning, decimating and measuring large STL files.
Common STL problems and fixes
- The part is tiny or huge
- STL has no unit. A file meant in inches looks 25.4 times too small when read as millimetres, a file in metres a thousand times. Set the file unit in the STL tab until the size matches the real object, then scale the same way in the slicer.
- The model lies on its side
- CAD tools and slicers use Z as up, many game tools use Y. The viewer opens STL as Z up. If a figure lies down, switch the up axis under View.
- The slicer reports errors or holes
- Open edges mean gaps in the surface, edges with more than two faces mean overlapping geometry. Small defects are often fixed by the repair function of the slicer, larger ones in Blender with the 3D Print Toolbox or in MeshLab.
- Dark or patchy faces
- Some exporters write wrong or zero facet normals. The viewer computes new normals for display when they are missing. Switch the shading to Normals to see which side the faces point to.
Frequently asked questions
How do I measure an STL file online?
Open the file and look at the STL tab: it lists width, depth and height in millimetres, centimetres or inches, plus volume and surface area. The measurements use the file unit you set, millimetres by default.
Is the volume the same as the filament my print needs?
No. The tab shows the volume enclosed by the surface, as if the part were solid. A slicer estimates material from walls, infill and supports, so the real use is usually lower with sparse infill and higher with supports.
What does watertight mean for an STL?
A watertight mesh has no gaps: each edge is shared by exactly two triangles. Only then do inside and outside separate cleanly, which slicers rely on. This check does not test wall thickness or self intersections.
Should I use ASCII or binary STL?
Binary. It holds the same triangles in a fraction of the space and loads faster. ASCII only helps when you want to read or edit coordinates by hand. The 3D Converter writes binary STL.
What about 3MF files?
Use the 3MF Viewer for them. It applies the unit written in the file and shows every object on the plate. To turn a 3MF into STL, there is the 3MF to STL converter.
Convert STL files
Models to print from the Grab3D library
Library models are not offered as STL. The GLB to STL converter turns any of them into a printable file in millimetres.