A STEP file (.stp or .step) is a neutral 3D CAD format defined by ISO 10303, the international standard for exchanging product data between design, manufacturing and inspection software. Unlike a triangle mesh, it can carry the exact solid geometry of parts and assemblies, which is why machine shops and mold makers ask for it.
In short: STEP is the file format that lets one CAD system hand a precise 3D model to another.
What Does STEP Stand For?
STEP stands for STandard for the Exchange of Product model data. It is the common name for ISO 10303, a family of standards maintained by ISO technical committee TC 184/SC 4. The file itself follows Part 21 of the standard, so it is also called a Part 21 file, and it uses the extension .stp, .step or .p21.
A STEP file is plain text. It opens with the line "ISO-10303-21;", followed by a header section describing the file and a data section listing every entity in the model. You can open one in a text editor, but the numbered entities only make sense in software that understands the schema.
Because the format belongs to no single CAD vendor, it is used to pass models between CAD, CAM, analysis and coordinate-measuring (CMM) software, and between companies using different systems.
What Are AP203, AP214 and AP242?
STEP is organized into application protocols (APs), each defining what data a file can hold for a particular industry or task. Three widely supported protocols are AP203, AP214 and AP242.
AP242, "managed model-based 3D engineering," is the newest of the three; its current edition is ISO 10303-242:2025. Its scope covers parts and assemblies, several kinds of solid and surface geometry, and dimensional and geometrical tolerance data. That tolerance data, known as product and manufacturing information (PMI), can include full GD&T, so an AP242 file can carry some of what used to live only on a 2D drawing.
STEP vs STL: What Is the Difference?
| STEP | STL | |
|---|---|---|
| What it stores | Solid CAD geometry, assemblies and, in AP242, tolerances | Triangle mesh of the outer surface |
| Curves | Exact, as in the original CAD | Approximated by flat facets |
| Standard | International standard ISO 10303 | De facto format, 3D Systems, 1988 |
| Editing | Imports into CAD as solid bodies | Hard to modify precisely |
| Best for | CNC machining, tooling, quoting | 3D printing, 3D scans |
How to Send a STEP File for a Quote
- Export from your native CAD rather than converting a mesh to STEP. A mesh converted to STEP is still a mesh underneath.
- Choose AP242 if your CAD offers it. It is the newest protocol and can also carry tolerances.
- Send one file per part, or the full assembly if parts must fit together, and name files by part number.
- Add a 2D drawing for critical tolerances, threads and surface finish, even if the STEP file carries PMI.
- Open the export once in a STEP viewer to confirm no bodies or faces are missing.
NICE Rapid's quote form lists STEP or IGS as the preferred 3D formats, alongside quantity, material, color, finish and target timeline. You can upload files up to 100 MB through the request-a-quote page.
Frequently Asked Questions
What is a STEP file used for?
It is used to move a 3D CAD model between different software and companies without losing accuracy. Manufacturers use STEP files to quote parts, program CNC toolpaths, design molds and set up inspection, because the file keeps the exact solid geometry.
What does STEP stand for?
STEP stands for STandard for the Exchange of Product model data, the common name for ISO 10303. STEP files use the .stp, .step or .p21 extension and follow Part 21 of that standard.
Should I send a STEP or an STL file for a quote?
For CNC machining, molding or tooling, send a STEP file: it keeps exact curves and hole sizes that an STL can only approximate. For 3D printing, an STL is fine, though a STEP file works too.
Related Services at NICE Rapid
- CNC Machining — accepts STEP, IGES, DWG or DXF files, and CNC programs are generated in CAM from your design file.
- Plastic Injection Molding — every project starts with a DFM review of your production design files.
- Request a Quote — STEP or IGS files preferred, with a response within 24 hours.
Sources
- NIST, STEP File Analyzer and Viewer — "STandard for Exchange of Product model data"; also known as Part 21 with extension .stp, .step or .p21; represents parts, assemblies and PMI for exchange between CAD, CAM, CAE and CMM software; PMI includes GD&T; AP242, AP203, AP214 — nist.gov
- Library of Congress, Sustainability of Digital Formats — STEP-file, ISO 10303-21 — plain-text exchange format; first record "ISO-10303-21;"; mandatory HEADER and DATA sections; maintained by ISO TC 184/SC 4; application protocols in parts 2xx of ISO 10303 — loc.gov
- ISO 10303-242:2025, Application protocol: Managed model-based 3D engineering — current edition (published 2025-08); scope includes parts and assemblies, boundary, surface and tessellated geometry models, and dimensional and geometrical tolerance data — iso.org
- Library of Congress, Sustainability of Digital Formats — STL (STereoLithography) File Format Family — STL triangle mesh, 3D Systems 1988, curves approximated by many triangles (for the comparison table) — loc.gov
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