STL format converter
Re-encode an STL mesh either way: paste a binary STL's bytes as base64 or hex to get readable ASCII STL text you can diff and edit, or paste ASCII STL to get a compact binary STL back as a downloadable file. The geometry is carried as the same 32-bit floats binary STL stores — nothing is welded, repaired, scaled or re-ordered — so with scientific notation at 9 decimals a binary → ASCII → binary round trip is byte-identical. Runs locally in your browser.
About this tool
An STL file stores the same thing two ways. Binary STL is an 80-byte header, a 4-byte
triangle count, then exactly 50 bytes per triangle — compact and fast, but not readable in a text
editor. ASCII STL spells every facet out as facet normal / outer loop / vertex lines —
readable, diffable and hand-editable, but roughly four to five times larger.
This converter goes both ways. Binary STL is not text, so paste its bytes as base64 or hex (a
data:model/stl;base64,… URL works too) and you get ASCII STL text back. Paste ASCII STL text and
you get a binary STL back as a downloadable data:model/stl;base64,… URL, or as raw base64/hex if
you are piping it somewhere else. Leave Convert to on the other encoding and the direction is
picked for you.
Only the encoding changes. Coordinates are carried as the same 32-bit floats a binary STL stores, and nothing is welded, repaired, re-ordered, scaled or re-oriented — so the triangle list that goes in is the triangle list that comes out.
Worked example — binary in, ASCII out
Paste this base64 (a one-triangle binary STL named demo):
ZGVtbwAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAAAAAAAAAAAAAAAAAIA/AAAAAAAAAAAAAAAAAAAgQQAAAAAAAAAAAAAAAAAAIEEAAAAAAAA=
and the converter returns the readable form:
solid demo
facet normal 0.000000e+000 0.000000e+000 1.000000e+000
outer loop
vertex 0.000000e+000 0.000000e+000 0.000000e+000
vertex 1.000000e+001 0.000000e+000 0.000000e+000
vertex 0.000000e+000 1.000000e+001 0.000000e+000
endloop
endfacet
endsolid demo
Switch ASCII number format to decimal and the same triangle reads vertex 0 0 0 /
vertex 10 0 0 / vertex 0 10 0 instead. Paste the ASCII form back in and you get the original
134-byte binary file — identical byte for byte, as long as the decimal places are set to 9.
Limits and things worth knowing
- Up to 100,000 triangles. That is a ~5 MB binary STL, or ~6.7 MB of base64 to paste.
- Binary STL cannot be pasted as text. It contains raw float bytes and zero bytes, which no text field survives — encode it as base64 or hex first.
- Six decimals is not lossless. A 32-bit float needs 9 decimals in scientific notation to be reproduced exactly. The default 6 matches what most CAD exporters write and is fine for viewing or printing; use 9 when you intend to convert back to binary.
- Per-triangle colour is lost going to ASCII. Binary STL has 2 spare "attribute byte count" bytes per triangle that VisCAM and SolidView use for 15-bit colour; ASCII STL has nowhere to put them. Choose A conversion summary to see whether your file uses them.
- Auto-detection does not trust the word
solid. Plenty of exporters write "solid" into a binary header, so the input encoding is decided by the 84 + 50 × triangle-count byte length instead. Force it with Input encoding if a file still guesses wrong. - Nothing is repaired. Non-manifold edges, flipped windings, duplicate vertices and holes pass straight through untouched.
FAQ
Why does my binary STL start with the word "solid"?
Because many exporters write a descriptive label into the binary format's 80-byte header, and that
label often begins with "solid". Software that decides the encoding by reading the first five bytes
then misreads the whole file. This tool ignores the keyword and checks the byte length instead — a
binary STL is always exactly 84 + 50 × triangle count bytes. When writing binary output it also
avoids the trap: a solid name starting with "solid" is written into the header as STL <name>.
Will converting to ASCII and back change my model?
Not if you set the decimal places to 9. STL coordinates are 32-bit floats, and 9 decimals in scientific notation is the smallest setting that reproduces every 32-bit float exactly, so the round trip returns a byte-identical file. At the default 6 decimals the values are rounded to about 7 significant digits — invisible for a printed part, but not bit-exact. Facet normals are copied through untouched unless you ask for them to be recomputed.
Why is my ASCII file so much bigger than the binary one?
Binary STL spends exactly 50 bytes on a triangle. ASCII STL spends seven lines of text on the same triangle — around 230–330 bytes depending on the number format and decimal places. A 4× to 6× increase is normal and does not mean anything was added. Choosing decimal number format and fewer decimal places shrinks the text considerably; converting back to binary always returns it to 50 bytes per triangle.
Can I paste a .stl file straight in?
An ASCII .stl is plain text, so yes — open it in a text editor and paste the whole thing. A
binary .stl is not text and cannot be pasted; convert its bytes to base64 or hex first (for
example base64 -w0 model.stl or xxd -p model.stl on Linux and macOS, or
certutil -encode model.stl out.txt on Windows) and paste that. Hex may include spaces, colons or
dashes, and a leading 0x is ignored.
What do the "attribute bytes" in the summary mean?
Every triangle in a binary STL ends with a 2-byte "attribute byte count" field that the original
specification says should be zero. VisCAM and SolidView repurposed it to store a 15-bit RGB colour
(5 bits per channel plus a validity bit), and Materialise Magics puts a part-wide COLOR= tag in
the 80-byte header instead. The summary reports what your file actually contains. These bytes are
preserved when you re-write binary as binary, but ASCII STL has no equivalent field, so converting
to ASCII drops them.
Does this repair, scale or re-orient the mesh?
No, and that is deliberate — a format conversion that quietly changes geometry is impossible to trust. Triangles come out in the same order with the same coordinates. The only optional edits are cosmetic: renaming the solid, and choosing whether facet normals are kept, recomputed from each triangle's winding, or zeroed. For welding, hole filling, winding fixes or unit scaling, use a dedicated repair or mesh-conversion tool.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool stl-format-converter "solid demo
facet normal 0 0 1
outer loop
vertex 0 0 0
vertex 10 0 0
vertex 0 10 0
endloop
endfacet
endsolid demo"New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/stl-format-converter/?stl=solid%20demo%0A%20%20facet%20normal%200%200%201%0A%20%20%20%20outer%20loop%0A%20%20%20%20%20%20vertex%200%200%200%0A%20%20%20%20%20%20vertex%2010%200%200%0A%20%20%20%20%20%20vertex%200%2010%200%0A%20%20%20%20endloop%0A%20%20endfacet%0Aendsolid%20demo&input_format=auto&to=auto&output=stl&solid_name=bracket-v2&normals=keep&number_format=scientific&precision=6&output_encoding=data-urlMachine-readable descriptor: tool.json — title + parameters JSON Schema for agents.