What is a .step file?

What a STEP file contains

Open a STEP file in a text editor and you will see something like #42 = ADVANCED_FACE('',(#43),#57,.T.); repeated tens of thousands of times. Each line is one entity, and entities reference each other by number. Together they describe a boundary representation: a watertight shell of trimmed surfaces, the curves where those surfaces meet, and the points where the curves meet.

This matters because the surfaces are *exact*. A cylindrical hole in a STEP file is stored as a cylinder with a radius, not as a few hundred flat triangles approximating one. When a machinist opens your STEP file, the hole is round to whatever precision the machine can hold. That is why STEP became the format the manufacturing world settled on.

STEP files also carry an assembly tree. A gearbox exported as STEP arrives as named parts in their correct positions, not as one merged blob.

Which STEP is it? AP203, AP214, AP242

You will see these numbers in file headers and in CAD export dialogs. They are *application protocols*, subsets of ISO 10303 aimed at different industries.

• AP203 is the oldest of the three still in common use. Geometry and

assembly structure, no colour.

• AP214 added colour, layers and materials. It came out of the automotive

industry and is what most CAD packages default to.

• AP242 merges the two and adds product manufacturing information: GD&T

tolerances, surface finish notes and datums carried as data instead of on a drawing.

For sending a part to a machine shop, any of the three works. Choose AP214 or AP242 if colour matters to you, and AP242 if you want tolerances to travel with the model instead of on a separate drawing.

STEP vs IGES for a CAD handoff

Ask the recipient which format they can import reliably. STEP is a practical starting point for a part or assembly exchange. An existing surface workflow may still require IGES. In either case, inspect the imported result in the application that will use it.

STEP can carry product manufacturing information, including dimensions and tolerances, when the export and import support it. Selecting a STEP extension alone does not guarantee those details travelled. Verify the export settings and include the drawing when the recipient needs it. NIST's STEP documentation describes these different kinds of content.

This site's STEP viewer checks the displayed geometry and overall dimensions. It does not validate a complete manufacturing handoff. If your goal is just to compare a received model with an expected shape, open it in the STEP viewer. See the IGES guide for the surface-import issues to check with that format.

What STEP does not store

• No feature tree. Your extrusions, fillets and patterns do not survive.

The recipient gets the finished shape, not the recipe, and cannot change a dimension and rebuild. This is by design: the format is for exchange, not for collaborative editing.

• No sketches, no constraints, no history.
• No mesh optimisation. STEP files of complex parts get large, and they are

slow to open, because exact surfaces cost more to evaluate than triangles.

Units

STEP files declare their unit in the file, in a GLOBAL_UNIT_ASSIGNED_CONTEXT entity. So a STEP file exported in inches and one exported in millimetres both open at the correct physical size, which is emphatically not true of STL. If a part arrives at the wrong scale from a STEP file, something upstream overrode the unit. The format did not lose it.

.step or .stp?

They are the same format. .stp exists because early Windows and DOS tools were limited to three-character extensions. Every CAD package reads both. This site treats them as one format and one page.

One catch with assemblies

STEP assemblies come in two shapes: one self-contained file, or a small index file that references a separate file per part. If you upload only the index file, any viewer will show you nothing, because the geometry is in the other files. If your export produced a folder of files, upload one of the parts, or re-export with "single file" selected.

How to open a .step file

Drop the file on this page. The viewer reads STEP in your browser with the Open CASCADE kernel compiled to WebAssembly, and the file never leaves your computer. You get the overall dimensions and a warning list without installing anything.

To edit the geometry, FreeCAD imports STEP as a proper solid at no cost. SolidWorks, Fusion, Onshape, Inventor and Rhino all import it too, and every one of them can export it back out.

Two things trip people up. A .stp and a .step are the same file, so rename freely. And a STEP file declares its own unit, so if a part arrives at the wrong size the exporter wrote the wrong unit, and rescaling in the importer only hides that. Large assemblies can take a minute to load here while the kernel works through them.