Is .stp the same as .step?

Yes. Two extensions for one format, because old systems wanted three letters. Nothing about the contents differs and any program reading one reads the other.

Can I convert an STL back to STEP?

Yes, on this site, and it is worth knowing exactly what you get. The triangles are built into a real solid body, with flat regions merged back into single faces so the file stays small enough to open. Your CAD program can cut it, measure it and assemble with it. What it cannot do is give you back the curves and the feature history, because those are not in the STL to recover.

STEP, for anything new. IGES is from 1980 and frequently delivers unstitched surface patches rather than a solid, which means repair work at the other end. Use IGES only when the receiving software genuinely cannot take STEP.

How fine should the triangles be?

For 3D printing, the Normal preset — it holds every triangle within about a tenth of a millimetre of the real surface on a fist-sized part, which is finer than an FDM printer can place plastic. Move to Fine for resin printing or a visible curved surface, and to Draft when you only want to check a shape or send it to a viewer.

Does the file get uploaded?

No. The CAD kernel is downloaded to your browser and the conversion happens there. The file itself never leaves your computer, which is why this works for confidential parts.

Can it open a whole assembly?

Yes. The parts are listed with the names the CAD program gave them, and you can convert the whole assembly as one model or isolate a single part. Parts that share a name are numbered so no two rows read the same.

STEP files, and how to convert them

STEP is the format CAD programs use to talk to each other. It stores exact curved surfaces rather than triangles, every serious CAD program reads and writes it, and it is the right thing to ask for when someone sends you a file you cannot open. The converters below run a real CAD kernel inside your browser, so nothing is uploaded.

.step .stp · model/step

STEP conversions that work today

Convert a STEP file into something else

Make a STEP file from something else

STEP guides

Exact surfaces, not triangles

Every other 3D format on this site stores triangles. A cylinder becomes sixty flat strips, and the number sixty was chosen when the file was written and cannot be changed afterwards. STEP stores the cylinder: a radius, an axis, a height. It is exactly round at any zoom, and any program can produce triangles from it at whatever fineness it needs.

This is why STEP is the format for anything that will be machined, measured or modified. A mesh has already thrown away the answer; a STEP file still has it.

It also stores an assembly tree of named parts with their placements, the units, and a tolerance describing how accurately the surfaces meet. A mesh format has none of that.

STEP is the file to ask for

When someone sends you a .sldprt, a .ipt or a .f3d, you cannot open it and neither can any free tool, because those formats are closed and undocumented. Asking for a STEP file instead takes them about fifteen seconds and solves the problem completely.

The phrasing that works: "could you export that as a STEP file — File, Save As, and pick STEP." Every CAD program can do it, and the person sending it loses nothing they meant to send you.

Going from STEP to a mesh, and back

STEP to STL is the normal path for 3D printing, and it is a real conversion: the exact surfaces are sampled into triangles at a fineness somebody has to choose. Choose too coarse and a curved part prints faceted; too fine and the file is enormous for no visible gain. Every converter makes that choice, and most of them make it silently.

Ours puts it on the page. Three presets, a custom tolerance you can type in the file’s own units, and the resulting triangle count updating beside the control as you move it — so the trade-off is something you can see rather than something you find out about later.

The other direction is possible and it means something different. Triangles do not become curved surfaces again — that information was destroyed when the mesh was made — but they can be built into a real solid body, and this site does that too. What comes out is a faceted solid: every surface flat, because flat is what a mesh contains. A CAD program will boolean it, measure it, section it and put it in an assembly. It is not a model you can edit parametrically, and nothing anywhere can make it one.

The thing that decides whether such a file is usable is what happens to the flat regions. Written one CAD face per triangle — which is what nearly every free tool does — a printable bracket becomes hundreds of megabytes that CAD software struggles to open. Ours merges triangles sharing a plane into single faces first, so a cube comes out as six faces and a gridded panel as one, and it tells you the ratio it achieved on your file.

Reading one in a browser

Understanding a STEP file means understanding NURBS surfaces, trimming curves and topology, which is why so few free converters handle it and why almost every online STEP converter uploads your file to a server that does the work.

This site runs Open CASCADE — the same CAD kernel FreeCAD is built on — compiled to WebAssembly. It is fetched from a public CDN the first time you open a CAD file — about 3 MB across the network, once — and then it runs inside your browser. Your file never goes anywhere. For anybody converting a part under NDA, that is the difference between being able to use a tool and not.

What STEP cannot store

Textures or rendering materials — it carries colour, not appearance
Animation
The feature history from the CAD program that made it — you get the final shape, not the steps

STEP in detail

STEP compared, row by row
Property	STEP.step .stp
File contents	Plain text in the ISO 10303-21 exchange form. Readable, and enormous.
Geometry	Exact surfaces: planes, cylinders, cones, spheres, NURBS. Not triangles.
Units	Recorded in the file, as a full unit and tolerance context.
Colour and texture	Colour per face or per solid, through the AP214 and AP242 styling entities.
Separate objects	An assembly tree of named parts, each with its own placement.
Published by	ISO 10303, from 1994. AP242 is the current application protocol.
Written by	SolidWorks, Fusion 360, Inventor, CATIA, FreeCAD, Onshape — every serious CAD program.

Questions

Is .stp the same as .step?: Yes. Two extensions for one format, because old systems wanted three letters. Nothing about the contents differs and any program reading one reads the other.
Can I convert an STL back to STEP?: Yes, on this site, and it is worth knowing exactly what you get. The triangles are built into a real solid body, with flat regions merged back into single faces so the file stays small enough to open. Your CAD program can cut it, measure it and assemble with it. What it cannot do is give you back the curves and the feature history, because those are not in the STL to recover.
STEP or IGES?: STEP, for anything new. IGES is from 1980 and frequently delivers unstitched surface patches rather than a solid, which means repair work at the other end. Use IGES only when the receiving software genuinely cannot take STEP.
How fine should the triangles be?: For 3D printing, the Normal preset — it holds every triangle within about a tenth of a millimetre of the real surface on a fist-sized part, which is finer than an FDM printer can place plastic. Move to Fine for resin printing or a visible curved surface, and to Draft when you only want to check a shape or send it to a viewer.
Does the file get uploaded?: No. The CAD kernel is downloaded to your browser and the conversion happens there. The file itself never leaves your computer, which is why this works for confidential parts.
Can it open a whole assembly?: Yes. The parts are listed with the names the CAD program gave them, and you can convert the whole assembly as one model or isolate a single part. Parts that share a name are numbered so no two rows read the same.