How to view an STL file
- Drop the STL on the box above, or click Open an STL and choose the file. Binary and text STL files both open.
- Check Units in the file. An STL stores numbers without a unit, so the viewer reads the numbers as millimetres until you choose another unit.
- Drag to turn the part, and scroll or pinch to zoom. Click Fit to see the whole part again.
- Read the size, the volume, the surface area, and the mesh check beside the part.
- Click Download report to save the figures as a text file.
Click Try a sample to see the viewer work first. The sample is a spur gear with 24 teeth and a keyed bore, 52 × 52 × 8 mm, made of 6,044 triangles. The viewer reports a volume of 13.54 cm³ for the gear.
What the viewer measures
- Size. The bounding box along X, Y, and Z, in millimetres and inches. The size shows whether the part fits a printer's bed or a machine's travel.
- Volume. The space the closed surface encloses, worked out from the triangles with the divergence theorem. A cavity inside the part counts as empty.
- Surface area. The total area of the triangles, which matters for painting, plating, and coating.
- Triangles and shells. A shell is one connected piece of surface. A single part usually has one, and a print plate with several parts has one for each part.
An open surface encloses no volume, so the volume line then reads Not measured. The STL repair tool can close the holes first.
The mesh check
A slicer turns the mesh into layers, and the slicer needs to know which side of every face is inside the part. The 3MF core specification states what that takes. Every triangle edge meets exactly one other triangle edge at shared vertices. Neighbouring triangles agree on which way is out, and every face points away from the inside.
The check looks for each way an STL breaks those rules:
- Open edges, where a triangle has no neighbour: a hole or a crack.
- Edges shared by three or more faces, which happen where two bodies touch along an edge or a wall sits inside the part.
- Flipped faces, which point the opposite way from their neighbours, and shells turned inside out, whose faces all point inward.
- Duplicate faces, which lie exactly on top of another face, and degenerate faces, whose corners collapse to a line or a point.
Copper on the part shows the inside of the surface. On a sound mesh only grey shows. Where the mesh is open, or a face is flipped, the copper inside shows through. Open edges are also drawn as copper lines.
Before the check, the viewer joins corners on open edges that sit less than a hundred-thousandth of the part's diagonal apart. An exporter's rounding can leave cracks that narrow. The results say how many pairs were joined. Prusa's help centre notes that modern slicers repair many models without the user noticing, and that not every model can be repaired automatically.
What an STL file holds
STL was first documented in 1988 by 3D Systems, according to the Library of Congress format description. The format describes the surface of an object as a mesh of triangles, with no standard support for colours or textures. There are two kinds. A binary STL has an 80-byte header, a count of triangles, and 50 bytes for each triangle. Those 50 bytes hold a normal, three corners as 32-bit floating-point numbers, and a 2-byte attribute count. A text STL lists the same facets as lines that start with keywords such as facet and vertex.
An STL also has no unit. The Wikipedia article on STL says that the files contain no scale information, and that the units are arbitrary. The same article says the corners of each triangle are listed counter-clockwise when seen from outside. The viewer uses that order to decide which side of a face is out.
What the viewer cannot tell you
- The unit. No STL records one. If a part made in inches looks 25.4 times too small, choose Inches.
- The exact shape of curves. A hole is a ring of flat facets, so the volume and the area are those of the facets. For true diameters, open the STEP file in the STEP file viewer.
- Feature dimensions. You get the overall size, not hole sizes or wall thickness.
- Faces that pass through each other. The check does not look for self-intersections.
- Very large files. The viewer opens files up to 150 MB, about three million triangles, because the whole mesh sits in your browser's memory.
Your file is read in this browser and never uploaded. The page counts each use of the viewer, with the format and the triangle count, and never records the file, its name, or its measurements.
From an STL to a price
A shop that prices a printed part starts from two figures this viewer gives you. The volume sets the material, and the bounding box sets which machine can build the part and how many fit on one build. A NIST review of additive manufacturing costs found that material makes up a major share of the cost of a printed product. Build time is another major driver in that review. The size of the build envelope, and how fully that envelope is used, also affect cost. The 3D printing cost calculator turns the volume into a cost from your own prices.
For a machined part, the STL is a weaker start, because an STL has lost the exact holes and radii. The guide to CNC quoting covers how a shop prices from the model instead. ThriveAI builds quoting systems that read each request for quote and its models. Each system prices the parts from your own rates for an estimator to approve.