STL Viewer Online
Open an STL file in your browser, orbit it, measure it and cut a section through it before it reaches a slicer. Binary and ASCII both load. The file is read on your own machine and never uploaded — no account, no upload queue, nothing to install.
What an STL file actually contains
STL (stereolithography) was published by 3D Systems in 1987 and has barely changed since, because the specification is deliberately narrow: an STL file is a flat, unordered list of triangles, each one three vertices plus a normal vector. That is the whole format.
Everything a modern CAD model carries is therefore absent. There are no curves — a cylindrical bore is a ring of flat facets, and how round it looks depends entirely on the tessellation tolerance used at export. There is no colour, no material, no layers, no assembly structure, no feature history. There is no topology either: triangles are simply listed, with no record that two of them share an edge, so the format cannot tell you whether a mesh is watertight.
Most importantly for anyone printing, there are no units. Vertex coordinates are bare numbers, so a cube written as 25 units is 25 millimetres or 25 inches depending purely on what the reader assumes. Hand an inch-authored model to a slicer that defaults to millimetres and a 25 inch part arrives at 25 mm; in the other direction a 25 mm part becomes 635 mm and fits on no consumer bed. This omission is behind a large share of prints that come out at the wrong scale, and one measurement catches it in seconds.
Binary and ASCII STL, and why file size varies so much
The same geometry can be written two ways, and the viewer detects which one you have automatically.
Binary STL is fixed width and completely predictable. The file opens with an 80-byte header followed by a 4-byte unsigned integer giving the triangle count — 84 bytes before any geometry. Each triangle then takes exactly 50 bytes: three floats for the normal and three for each of the three vertices (12 floats at 4 bytes each, so 48) plus a 2-byte attribute field that is almost always zero.
The size arithmetic is therefore exact: file size = 84 + (50 × triangles). The 528-triangle spur gear in our sample set is 84 + 26,400 = 26,484 bytes, about 26 KB; a one-million-triangle scan is 50,000,084 bytes, just under 48 MiB. It works backwards too: divide any binary STL by 50 for its triangle count.
ASCII STL writes the same data as text, seven lines per triangle — facet normal, outer loop, three vertex lines, endloop, endfacet. It runs several times larger than the binary equivalent for identical geometry and parses more slowly, but it opens in a text editor when you are chasing an export problem. If a file is enormous for its apparent detail, ASCII is usually why, and re-exporting as binary shrinks it without losing a triangle.
Measuring an STL file
Measurement is why most people open an STL rather than glance at a thumbnail. The viewer reports distance between two picked points and the area of a polygon you trace, with a live readout.
How the picking behaves
- Picked points are placed on the ground plane, not projected onto the mesh surface, so a measurement is effectively taken in plan.
- There is no snapping — not to vertices, not to triangle edges, not to midpoints. Where you click is where the point lands.
- Readings are in the file's own units, whatever those turn out to be, because STL declares none.
So switch to an orthographic preset, zoom in hard, and treat the number as a size confirmation or a unit sanity check rather than inspection metrology.
Worked example: the 25 mm calibration cube
Open the 25 mm calibration cube. Every edge is exactly 25 units, which makes it the ideal file for learning what the tool can and cannot tell you.
- Switch to the top preset and confirm you are in orthographic, not perspective — a perspective camera makes edges converge and your reading drifts.
- Zoom until the cube nearly fills the viewport. With no snapping, precision is limited by how many pixels a millimetre covers.
- Measure corner to corner along one edge. You should read very close to 25, and how close tells you your own picking precision at that zoom — worth knowing before you trust a reading on a part you have never seen.
- Trace the square with the area tool and you should land near 625, an independent check on the same geometry.
Worked example: the spur gear
Now open the 18-tooth spur gear, a 528-triangle mesh. In top view, measure tip to tip across the gear for the outside diameter. The tooth flanks are faceted, so you are picking against a chord of the true involute and the reading shifts with where along the facet you click. Run this check before printing any gear: a plausible millimetre value means the export units were right, a fortieth of the drawing value means the file was authored in inches.
Use the clipping plane on X, Y or Z to slice through the gear and inspect the bore and tooth cross-section, and switch to wireframe or x-ray to judge the tessellation. Coarse facets in the viewer are coarse facets on the printed part.
A pre-slicing checklist
Loading an STL into the viewer before it goes near a slicer takes under a minute and catches the failures that waste a whole print.
- Look at the whole mesh. Orbit right around the model in solid mode. Missing chunks, stray floating shells, a duplicated body offset from the real one and geometry that did not survive export show up immediately. Switch to x-ray for internal geometry you did not expect.
- Confirm the overall size. Measure the largest dimension in top view against the source drawing. This is the units check, and the highest-value thing you can do with an STL you did not author.
- Inspect wall thickness visually. Drive the clipping plane through the part and look at the cut. Thin walls, ribs narrower than your nozzle can lay down and shells meant to be solid become obvious in section, and you can measure across the cut for a rough number.
- Check orientation. The top, front and right presets show how the part sits in the file's own coordinate system, and where the face that ought to be on the bed actually is. A part lying on a curved surface needs support the designer never intended.
- Look at the tessellation. In wireframe, a curved surface built from a dozen facets was exported at too loose a tolerance. No slicer setting recovers detail that is not in the file — the fix is a re-export from CAD, not a finer layer height.
The viewer does not repair meshes, does not run manifold or normal-orientation checks, and does not slice. It is a fast visual and dimensional gate in front of those tools, not a replacement for them.
How to open an STL file
- Open the viewer — go to dwgvieweronline.com in any modern browser with WebGL, on desktop or mobile.
- Load your file — drag the .stl onto the viewer or use Browse Files. Parsing happens in the page; there is no upload step.
- Inspect — orbit with left-click drag, pan with right-click, zoom with the scroll wheel, and use the view presets and orthographic/perspective toggle to get square on to a feature.
- Measure, section, mark up — take distances and areas, cut a clipping plane on any axis, add arrows, shapes, freehand marks or text notes.
- Export — save a PNG or an A4 landscape PDF of the viewport. Both are raster snapshots: right for an email or a print log, wrong for anything needing selectable text.
The viewer installs as a PWA and works offline once cached, which helps on a shop floor with unreliable connectivity.
What the STL viewer gives you
Distance and area
Measure across a part or trace a polygon, with a live readout in the file's own units. No snapping — zoom in and pick carefully.
Section planes
Cut through the mesh on X, Y or Z to inspect wall thickness, bores and internal cavities.
Solid, wireframe, x-ray
Three render modes. Wireframe exposes tessellation quality; x-ray reveals geometry hidden inside the shell.
Nothing leaves your device
The file is read locally with FileReader and rendered on your GPU. No server processing, no account, no retention.
Converting STL to other formats
When you need colour, texture coordinates or named groups, STL cannot carry them and the file has to change format. STL to OBJ runs in the browser like everything else here and produces a text-based mesh most CAD and DCC tools ingest happily; OBJ to STL covers the return trip. Converting never adds information, though — an STL that reached you with 200 coarse facets on a cylinder still has 200 afterwards.
If you handle 2D drawings alongside print files, the same viewer opens DWG and DXF, and side-by-side compare holds two files in sync for revision checks.
Try it with a sample file
These open straight in the viewer, no download needed:
Frequently Asked Questions
Can I open an STL file online without installing software?
Yes. Your STL is read locally with the browser FileReader API and rendered on your own device, so nothing is uploaded. No account, no signup, no file limit, no watermark.
Can I measure dimensions in the STL viewer?
Yes. It reports distance between two picked points and the area of a polygon you trace. Picked points land on the ground plane rather than the mesh surface, and there is no snapping to vertices, edges or midpoints, so use an orthographic preset view and zoom in before clicking. Good enough to confirm an overall size or catch a unit error, not a substitute for inspection metrology.
Why does my STL print at the wrong size?
An STL stores bare numbers with no unit declaration. If the CAD program wrote inches and the slicer assumes millimetres, a 25 unit part becomes 25 mm instead of 635 mm, or the reverse. Measure a feature whose size you know and compare it with the drawing before you slice.
Does the viewer read both binary and ASCII STL files?
Yes. Both encodings are supported and the format is detected automatically, so you do not need to know which one you were sent. Most files exported by CAD and mesh tools are binary.
How large an STL file can I open?
There is no imposed limit. The ceiling is your device: the whole mesh is decoded and held in browser memory, so several million triangles orbit less smoothly on a phone than on a desktop. Meshes in the hundreds of thousands of triangles are routine.
Can I edit, repair or slice an STL file here?
No. This is a viewer: you cannot move geometry, close holes, flip normals or write a modified STL back out, and it does not slice. It converts STL to OBJ and exports PNG and PDF snapshots of the viewport. Use a mesh repair tool or your slicer for topology fixes.
Other Supported Formats
Check your model before you print it
Open an STL, measure it, section it and confirm the units. It takes a minute and runs entirely in your browser.
Open STL Viewer