STLBenchFree online 3MF to STL converter
3MF to STL, with the losses named
Drop a .3mf and download an .stl — unzipped, read and rewritten in this tab, with nothing uploaded. This direction is a downgrade, which is worth saying plainly: 3MF carries colour, a stated unit and a build plate of separate objects, and STL carries triangles. The geometry crosses untouched at a scale factor of exactly one, everything else is left behind, and the panel underneath lists what that meant for your particular file.
- 100% free
- No signup
- Binary STL out
- Scale stated
- Nothing uploaded
Drag a file anywhere on this page to open it. F fits the view, W switches shading, G the grid, R spins it.
How to convert 3MF to STL
Three steps, and one thing to check in the middle.
Drop the .3mf
It is unzipped and read here. A 3MF from PrusaSlicer, Bambu Studio, Cura or Fusion opens the same way, because they all write the same core specification even when they add their own extras beside it.
Check the part count before you download
A 3MF often holds a whole build plate — several objects, each placed where you arranged them. STL has no concept of a separate object, so everything becomes one body at those same coordinates. If the panel says four parts and you wanted one, go back and export that one.
Download the STL
Written as binary STL, which is a fifth the size of the ASCII flavour and read by everything. The numbers are unchanged: 3MF states millimetres, STL states nothing and is millimetres by convention, so the geometry crosses without being touched.
Technical specifications
| Reads | .3mf — core specification, from any slicer or CAD package |
|---|---|
| Writes | .stl — binary, one body |
| Unit | Millimetres both sides; scale factor 1 |
| Kept | Every triangle, at the coordinates the plate placed it |
| Lost | Colour, materials, the unit statement, the split into objects |
| Also lost | Slicer extensions: painted supports, per-object settings, modifiers |
| File size | No fixed limit — past 100 MB you are told what it will cost this tab, then it opens |
Frequently asked questions
Why would I go from 3MF back to STL?
Because something downstream will not take 3MF. Printer firmware with its own uploader, an older slicer, a print service whose form accepts .stl only, a mesh tool from the 2010s, a client who asked for an STL. The format is better in almost every technical respect and worse in exactly one: STL is accepted absolutely everywhere, and that is a real property of a file.
What do I actually lose?
Colour, materials, the unit statement, and the split into separate objects. A 3MF says outright that it is in millimetres; an STL cannot say anything, so the same numbers arrive as bare coordinates that the next program will assume are millimetres — usually correctly, and silently either way. Anything a slicer stored as its own extension — supports you painted, per-object print settings, modifiers — is not geometry and does not survive.
Does the model change size?
No. 3MF declares millimetres and STL is read as millimetres, so the numbers pass through untouched and the panel shows a scale of 1. It is worth checking all the same, because the neighbouring conversions are not so lucky: a GLB is in metres, and coming from there multiplies every coordinate by a thousand.
My 3MF had four objects on the plate — what happened?
They are all in the STL, at the positions the plate gave them, as one body with four disconnected shells. Slicers handle that: most offer to split a loaded mesh into parts by connectivity. But if you wanted one object on its own, exporting it alone from the slicer is cleaner than splitting it afterwards.
Is the file uploaded?
No. The unzip, the parse and the STL writing all happen in this tab, so nothing crosses the network — and a 3MF from a client project is precisely the kind of file worth not uploading. The download comes from memory in your own browser.
Binary or ASCII STL?
Binary, always. ASCII STL writes every coordinate as text and lands about five times larger for the identical mesh, with rounding decisions in the printing that binary does not have to make. Nothing modern needs the ASCII flavour; it exists because the format is from 1987.
About converting the better format into the older one
3MF was published in 2015 by a consortium that had watched STL cause the same problems for thirty years: no units, no colour, no way to say that these triangles are one object and those are another, and a habit of arriving with holes because nothing in the format requires a closed surface. Every serious slicer now writes 3MF by default. Converting back to STL is therefore rarely about the file being better — it is about the thing at the other end being older than 2015.
The practical advice is to keep the 3MF. Convert a copy, send the copy, and let the original stay the master, because the reverse conversion cannot restore what this one drops — colours do not come back, and neither does the knowledge that the numbers are millimetres. Going the other way adds a unit and a colour because someone decides what they should be, not because the STL remembered.
If what you want is to look inside the file rather than convert it, the 3MF viewer shows the objects, the colours and the declared unit without writing anything.
Where the conversion happens
Every number on this page is worked out by JavaScript running in the tab you are reading it in. The model you open is read straight off your own disk — it is never uploaded, logged or kept, which is also why these tools carry on working after you disconnect from the network.
One clause specific to this page: a 3MF from a slicer often carries the project name, the printer profile and sometimes the client it was made for, all as text inside the zip. None of it is transmitted, because the unzip and the rewrite both run in this tab and there is no server here to receive a file.