What Is a STEP File?

Last updated September 26, 2026

A STEP file (.stp or .step) is a 3D model file format used to move CAD designs between different software packages. OpusFab quotes are built from STEP files: you upload one, the platform reads the geometry, and you get an instant price with automated DFM feedback.

What a STEP file contains

STEP stands for Standard for the Exchange of Product model data. It is an ISO standard (ISO 10303), which means almost every CAD program can read and write it.

A STEP file stores your part as precise, solid geometry. It knows that a face is a flat plane, a hole is a true cylinder, and a fillet has an exact radius. This is the information a quoting engine and a machine shop need to calculate material volume, machine time, and manufacturability.

A STEP file does not usually include things like drawings, tolerance callouts, or material definitions. You choose material, finish, and quantity in the OpusFab quote editor after uploading.

STEP vs STL: why manufacturing uses STEP

You may also have STL files, which are common in hobby 3D printing. The two formats are very different:

STEPSTL
GeometryExact solid model with true curvesMesh of flat triangles
A cylinder isA perfect cylinderThousands of small flat facets
Units and scaleDefined in the fileOften ambiguous
Used forMachining and industrial 3D printingHobby 3D printing, rendering

Because an STL approximates every surface with triangles, a quoting engine cannot reliably measure features like holes, threads, or radii from it. STEP files keep the geometry exact, which is why OpusFab (and machine shops generally) ask for STEP.

If your design only exists as an STL, most CAD tools can convert it to a solid, but the result can be messy. It is usually better to go back to the original CAD model and export STEP directly.

How to export a STEP file

The steps below cover the most common CAD tools. Menu names can shift slightly between versions, but the option is always under Export or Save As.

Fusion 360

  1. Open your design.
  2. Go to File > Export.
  3. Set the file type to STEP Files (*.stp, *.step).
  4. Choose a location and click Export.

Alternatively, right-click a component in the browser tree and choose Save As STEP to export just that component.

SolidWorks

  1. Open the part file.
  2. Go to File > Save As.
  3. In the Save as type dropdown, choose STEP AP203/214 (*.step, *.stp).
  4. Click Save.

AP214 preserves color information; AP203 is the more minimal option. Either works for quoting.

Onshape

  1. Open the document containing your part.
  2. Right-click the part in the Part Studio (or right-click the part’s tab).
  3. Choose Export.
  4. Set the format to STEP and click Export to download the file.

FreeCAD

  1. Open your model and select the body or part in the tree view.
  2. Go to File > Export.
  3. Choose **STEP with colors (.step .stp) as the file type.
  4. Click Save.

Tips before you upload

  • Export one file per part. A STEP file containing a full assembly with multiple bodies cannot be priced as an instant quote; it will be routed to a custom quote instead. See multi-part uploads and BOM import for how to quote a whole assembly part by part.
  • Check your units. STEP files carry units, and OpusFab reads them, but it is worth confirming your part measures correctly in the viewer after upload.
  • Use clear filenames. If you plan to use BOM import, the BOM matches parts to rows by filename, so names like bracket-left.step work better than untitled(3).step.

Once you have your STEP file, head to how to upload a part to get your first quote.