Why STL files get so big
A binary STL spends 50 bytes on every triangle: a normal, three corners and two spare bytes. File size follows the triangle count almost exactly, so a 1M-triangle model weighs about 48 MB. ASCII STL writes the same numbers as text and comes out several times larger; the STL ASCII vs binary guide compares the two.
3D scans, sculpts and models exported at the finest setting carry far more triangles than a printer can resolve. That surplus is what the simplifier removes.
How to reduce STL file size
- Drop the model on the viewer and note the triangle count in Info.
- Press Edit and open Reduce triangles.
- Type how many percent to keep (the field starts at 50) and press Apply.
- Orbit around the part, compare the volume in Info, and press Undo if detail is lost.
- Save with Convert → STL binary.
What 30 % looks like in numbers
The reduction uses meshoptimizer. The test sphere was reduced to 30 % of its triangles and its volume stayed within 2 % of the original.
| Keep | Triangles from 1M | Binary STL, about |
|---|---|---|
| 100 % | 1,000,000 | 48 MB |
| 50 % | 500,000 | 24 MB |
| 30 % | 300,000 | 14 MB |
| 10 % | 100,000 | 5 MB |
If a print service caps uploads at 25 MB, the table tells you roughly where to aim. After Apply, read the triangle count in Info: at 50 bytes each plus an 84-byte header, it gives you the binary file size.
Printing sets its own floor. A 0.4 mm nozzle cannot trace facets much smaller than its line, so a figurine with millions of triangles prints much the same at a fraction of that count. For scale, our slicer handled a 200,000-triangle torus in about 1.3 s in the test suite.
Simplify first when the model is heavy
A 1M-triangle model opens in about 5–12 s and uses about 1.2–1.7 GB of memory. Above roughly 1–2 million triangles the tab may run out of memory, so a lighter copy is easier to edit, repair and slice. Reduce before you run booleans or the cut in the STL editor, so they work on the lighter mesh.
After reducing, run the mesh check again. If it reports open edges, STL repair closes the simple ones.
Limits of the STL simplifier, and other ways to shrink a file
Fine detail goes first: small text, sharp creases and thin fins lose shape before the large surfaces do. A CAD part made of flat faces often has few triangles already, and cutting it further bends its edges. The tool reduces; it does not smooth or rebuild a mesh, which is a job for a remeshing tool.
If the triangles must stay, change the container. 3MF is a ZIP package, so its model data is compressed. Save the mesh with STL to 3MF and compare the size shown in the download bar with your STL. Zipping the STL itself also helps when you only need to send it.
Questions and answers
Will simplifying change the dimensions of my part?
Compare X × Y × Z and the volume in Info before and after Apply. In the test sphere at 30 % the volume moved less than 2 %.
How many percent of the triangles should I keep?
Start at 50 % and look at curves and small text. Dense scans can go lower; CAD parts with few triangles should keep most of them.
Can I go back if I reduced too much?
Yes. Undo restores the previous mesh, up to 20 steps back. Your original file on disk is never changed.
Why is my STL file so large?
Binary STL stores 50 bytes per triangle and no compression, so a dense scan or a fine export grows quickly. ASCII STL is larger still for the same mesh.
Is the mesh sent to a server to compress it?
No. The reduction runs in your browser tab, and the smaller file is saved from there.