ASCII STL to binary STL converter (and back): shrink STL files

In short: drop an STL above. If it's ASCII (text), you get a binary STL that's usually around a fifth of the size; if it's binary, you get an ASCII version you can read in a text editor. The tool detects which you have, converts it the other way and shows the size saved. It runs in your browser.

#How to use it

1. Add an STL, or click Try it with an example (an ASCII STL of a torus knot).
2. The tool converts it straight away, in whichever direction applies.
3. Download the result.

#ASCII and binary STL

Both describe exactly the same thing: a list of triangles, each with three corners and a facing direction. They differ only in how the numbers are written.

• ASCII STL writes every number as text, with words around it (facet normal, vertex). You can open it in a text editor and read it, but each triangle takes roughly 250 bytes.
• Binary STL writes each number as 4 bytes, so each triangle takes exactly 50 bytes.

So binary files are typically a fifth to a sixth of the size, and they load faster.

#Is anything lost?

No triangles are lost either way. Binary STL stores numbers as 32-bit floating point, about 7 significant figures; ASCII is written at that same precision, so converting back and forth doesn't change the shape.

Some exporters store a colour per triangle in binary STL's spare "attribute" bytes. That isn't standard and few slicers read it; it isn't carried over.

#Example

The example ASCII STL converted to binary, with the size saving:

ASCII to binary, with the size saved

#Frequently asked questions

#Is my file uploaded?

No. It's converted in your browser.

#How do I know which kind of STL I have?

Drop it in: the tool tells you. You can also open it in a text editor: ASCII STL starts with the word solid and is readable text; binary STL looks like random characters.

#Which should I use?

Binary, almost always: it's smaller and every slicer reads it. Use ASCII if a program you use only accepts text, or to look inside the file.

#Can I make an STL even smaller?

Not as STL. For sharing, a ZIP of a binary STL is smaller still. For the web, convert it to GLB.