Tessellation quality you can see before you download
A STEP file holds exact surfaces; an STL holds triangles. How many triangles is a choice, and CADverter makes it visible rather than fixed. Four tiers set the linear deflection as a fraction of the model’s bounding-box diagonal — Coarse at one part in a hundred, Normal at one in a thousand, Fine at three in ten thousand, Ultra at one in ten thousand — with the angular deflection stepping down from 0.5 to 0.1 radians alongside. Because the deflection scales with the model, a 5 mm pin and a 2 m frame get the same visual smoothness at the same tier instead of one being blocky and the other absurdly dense.
The viewer shows the same tessellation the writer will emit, so what you see orbiting on screen is what the slicer receives. Raise the tier until the fillets stop showing facets, then convert.
Binary by default, ASCII a click away
Binary STL is what every slicer expects and roughly a fifth the size of the ASCII form of the same mesh. CADverter writes binary unless you switch the encoding option to ASCII, which is the right choice only when you intend to read or diff the file by hand or feed it to an old tool that predates the binary form.
STL carries no unit — here is what that means for your part
STEP declares its length unit; STL cannot. CADverter reads the STEP’s declared unit, writes the STL in millimetres — the convention every slicer assumes — and tells you so in the panel, because a file that cannot state its unit is the single most common cause of a part arriving at 1/25.4 scale. If your downstream tool assumes inches, scale on import. If you need the unit carried in the file, choose 3MF instead: same panel, same geometry, and it records millimetres explicitly.
Assemblies merge into one mesh, and how to avoid that
STL has no notion of parts, so a multi-body STEP becomes one triangle soup. When that is not what you want, either select the parts you need before converting — Ctrl+click in the viewer or the parts panel, and only those are written — or turn on separate files to receive one STL per part in a single ZIP, each named after its part. For a printer that handles multi-material plates, 3MF keeps the parts apart in one file.
Z stays up
STEP and STL are both Z-up, so no rotation is applied and the part lands on the build plate the way it stood in CAD. That is worth stating because glTF, GLB and USDZ are Y-up and CADverter does rotate for those targets; if you convert the same model to GLB and to STL and one appears lying down, the rotation is the target format’s convention, not an error.
A folder of parts to one ZIP
Drop every STEP in a folder at once, choose STL, and download one archive with an STL per source file. A file that cannot be read is named in the report rather than silently omitted from the ZIP — you should never discover a missing part on the print bed. The run happens sequentially in the browser, so a 40-file batch takes about 40 times one file; there is no server queue to wait in and no per-day quota.
How to prove the file stayed on your machine
Keep the browser’s Network panel open while you drop, convert and download. The only traffic is this page’s own assets and, once, the WebAssembly kernel; the STEP file and the STL that comes out never appear in a request. The production build is tested for exactly this: the end-to-end suite opens a model, converts it and downloads the result while asserting that the list of external requests is empty.
Where a desktop tool still wins
FreeCAD opens STEP and exports STL with a per-export deviation setting, and it is the right tool if you need to repair or modify the solid first. CAD packages export STL natively with their own quality dialog. CADverter is the quicker path when you only need the mesh, the machine is not yours, the file must not leave it, or you have a folder of parts and want one ZIP back without opening each file.
STEP to STL questions
Which quality tier should I use for 3D printing?
Normal for most parts. Fine when the model has small fillets or threads you can see faceting on at print scale; Ultra rarely earns its file size. Coarse is for previews and very large assemblies.
Why is my STL 25 times too small or too large?
STL has no unit, so the receiving program assumes one. CADverter writes millimetres; a tool that assumes inches will show the part at the wrong scale. Set the import unit there, or export 3MF, which carries the unit.
Can I convert a STEP assembly into separate STL files?
Yes. Turn on the separate-files option and each part becomes its own STL inside one ZIP, named after the part. Or Ctrl+click parts to convert only some of them.
Does the conversion keep colours?
No — STL cannot hold colour. The panel lists that loss before you download. 3MF, OBJ and GLB keep per-part colour if you need it.