STL stores three complete vertices for every triangle and shares nothing, so a typical model repeats each vertex about six times. Removing that costs no shape at all.
—
80
Resize
50
Encoder detail
6
fastzopfli
256
28
0 lossless51 tiny
The largest saving available.
50
Speed, audio and advanced
8
0 smallest13 fastest
128
96
Advanced
8
Nearly all of the saving comes from the images inside.
150
75
Advanced
50
aggressivegentle
Advanced
14
coarsefine
0 merges only exactly equal vertices. A small value also closes hairline cracks left by CAD exporters.
Geometry only — texture coordinates,
materials and vertex colours are not carried through. Decimating a
UV-mapped mesh correctly is a much harder problem, and mangling your
UVs quietly would be worse than leaving them alone.
12
6
Advanced
ZIP and 7z. 7z encrypts the file list too.
What actually shrinks STL files
Welding merges identical vertices into an indexed mesh. It is completely lossless and routinely removes 80% of the vertex data on its own.
ASCII STL is roughly five times the size of the identical binary STL. Converting between them is free.
Reducing the triangle count uses quadric error metrics, which collapse the edges that distort the surface least — so the silhouette survives far better than naive decimation.
Quantising coordinates onto a grid finer than any printer resolves makes the file much more compressible with no visible effect.