The short answer
- Use STEP to send a design to someone who will keep engineering it: another CAD system, a machinist, a supplier or a CNC shop.
- Use STL for 3D printing and for tools that only need the surface: slicers, mesh editors, quick visual checks.
- Keep the STEP (or native CAD file) as your master. You can always make an STL from it, but not the other way round.
Side by side
| STEP (.step, .stp) | STL (.stl) | |
|---|---|---|
| Geometry | Exact surfaces and solids (B-rep): a hole is a true cylinder | Triangles only: a hole is a many-sided polygon |
| Units | Stored in the file | Not stored; usually assumed to be millimetres |
| Assemblies | Parts, names and structure | One surface, no parts |
| Colour and metadata | Colours, and in AP242 manufacturing annotations (PMI) | None in the standard |
| Editable in CAD | Yes, as solids | Only as a mesh |
| Typical use | CAD exchange, machining, supplier hand-off | 3D printing, mesh tools, quick previews |
Converting STEP to STL
Going from STEP to STL is routine: the exact surfaces are tessellated into triangles. Finer tessellation gives smoother curves and bigger files. The free STEP to STL converter does it in your browser without uploading the design, and the same tool writes OBJ (keeping assembly parts as groups) or GLB for web and AR viewers.
Why STL to STEP doesn't give you a real CAD model
Tools that "convert STL to STEP" wrap the triangles in a STEP file. The result is a faceted solid: every triangle becomes a flat face, curves stay polygonal, and you can't select a hole's axis or edit a fillet. Getting a true parametric model back means remodelling it, often using the mesh as a reference. Ask for the original STEP whenever you can.
STEP application protocols: AP203, AP214, AP242
STEP comes in application protocols. AP203 covers configuration-controlled 3D design; AP214 adds automotive-oriented data such as colours and layers; AP242 merges both and adds model-based definition (PMI) and tessellated geometry. Most CAD exports today are AP214 or AP242. The version checker reads a STEP file's header and tells you which protocol and which program wrote it.
Free, local-first CAD viewer
Open DWG, DXF, STEP and STL files without uploading them.
Frequently asked questions
Is STEP or STL better for 3D printing?
STL (or 3MF) is what slicers expect. If you're given a STEP, convert it to STL first. Many modern slicers also import STEP directly.
Is STEP the same as STP?
Yes. .step and .stp are two extensions for the same ISO 10303-21 format.
Can I view a STEP file without CAD software?
Yes. The free Caddis viewer opens STEP and STP files in the browser and on iPhone without uploading them.