Convert STP to SAT in your application
3D InterOp is a CAD interoperability SDK that lets your software read STEP files and write ACIS (SAT, SAB), with exact B-Rep geometry carried across — no tessellation, no CAD license.
Spatial does not offer an online converter or a desktop conversion app. 3D InterOp is a component that software vendors embed in their own products.
- Reads STEP · Writes ACIS
- Exact geometry → exact geometry
- C++ / C# APIs
STP to SAT at a glance
Both formats store exact geometry, so the model stays exact. The differences are in the product data each one can carry.
| STEP (source) | ACIS (target) | |
|---|---|---|
| Full name | Standard for the Exchange of Product model data | ACIS model file (text SAT, binary SAB) |
| Standard / owner | ISO 10303 — AP203, AP214, AP242 | Proprietary — Spatial (Dassault Systèmes) |
| File extensions | .stp, .step | .sat, .sab |
| Geometry | Exact B-Rep | Exact B-Rep |
| Assemblies / structure | Yes | Parts in SAT/SAB, assemblies in ASAT/ASAB |
| PMI | Graphical and semantic (AP242) | PMI annotation classes |
| 3D InterOp support | Read and write | Read and write · ACIS R18 to 2026 1.0 |
What's preserved, what changes
Here is what your application can expect to carry across from STEP to ACIS.
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Preserved
Exact geometrySTEP solids and surfaces are written to ACIS as exact B-Rep.
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Preserved
PMIPMI annotations such as text, flag notes, datums, geometric tolerances, roughness and dimensions are translated to ACIS annotation classes.
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Improved
Spline surfacesThe ACIS writer can repair erroneous spline surfaces and extend short surfaces to the face limits (DoSplineResurface option, on by default).
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Changes
Assembly structureBy default, the ACIS writer flattens a STEP assembly into a single SAT file. Your application can instead write an ACIS assembly (ASAT/ASAB) or an XML E-BOM with one SAT file per part.
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Not carried
Some assembly propertiesAn ACIS assembly (ASAT/ASAB) does not support layers or colors at instance level, suppressed or hidden instances, or units at instance level.
Why software vendors add STP to SAT conversion
Hand models to kernel-based applications
Many CAD, CAM and CAE applications are built on the ACIS kernel. Writing ACIS gives them exact geometry they can load without another translation step.
Downstream modeling and analysis
An exact ACIS B-Rep can still be edited, meshed or machined by downstream tools, which a tessellated export would not allow.
Exchange within one ecosystem
Partners whose tools share the ACIS kernel often prefer to exchange ACIS files directly.
Things to check before you ship
- Choose how assemblies are written: flattened into one SAT (default), an ACIS assembly (ASAT/ASAB), or XML plus one SAT per part.
- Pick the ACIS version to write: the writer saves from R18 up to the current release, and entities that older versions do not support are not saved.
- ACIS does not support B-spline geometry with a degree above 25: such geometry in a STEP file may be missing in the output.
- Reading semantic PMI from STEP AP242 requires a license in addition to the STEP Reader.
- Validate a sample of translated files in the target system your users actually work with.
Working with Spatial was a real milestone for us: Thanks to the powerful 3DInterOp library, we were able to successfully bring our complex CAD-based solutions to the cloud while remaining flexible to meet individual customer requirements. For us, Spatial is not just a technology partner, but a true enabler of our product strategy.
Dr. Arno Michelis, Founder, Simus Systems
Read the Case StudyAbout the formats
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Source format
What is a STEP (.stp) file?
STEP is the ISO 10303 standard for exchanging 3D product data between CAD systems. Its application protocols range from exact geometry (AP203, AP214) to full model-based definition with PMI (AP242).
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Target format
What is an ACIS SAT file?
SAT is the native file format of 3D ACIS Modeler, Spatial's geometric modeling kernel used in CAD, CAM and CAE applications. It stores exact B-Rep models as text (.sat) or binary (.sab); ACIS assemblies use the .asat and .asab extensions.
Frequently asked questions
By default, the ACIS writer flattens the assembly into a single SAT file. Your application can instead write an ACIS assembly file (ASAT or ASAB, ACIS R19 onwards) or an XML E-BOM with one SAT file per part.
Yes. 3D InterOp translates PMI annotations such as text, datums, geometric tolerances, roughness and dimensions to ACIS annotation classes.
Yes. STEP solids and surfaces are written to ACIS as exact B-Rep, with no tessellation.
The ACIS writer saves from R18 to 2026 1.0, as SAT or SAB. Entities that the chosen version doesn’t support aren’t saved.
Not above degree 25: ACIS doesn’t support B-spline geometry with a higher degree, so such geometry in a STEP file may be missing from the output.
AP203, AP214 and AP242, from .stp, .step and .stpx files. Compressed .stpZ and .stpxZ files and .stpa files are read too.
Yes. 3D InterOp reads and writes both formats.
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Building software that needs to convert STP to SAT?
3D InterOp is an SDK that lets your application read STEP and write ACIS alongside 30+ CAD and BIM formats.