What STEP stands for
STEP is the international standard ISO 10303, titled Product data representation and exchange, and its first parts were published in 1994. The extensions .step and .stp mark the same format, so a reader that opens one opens the other.
Open one in a text editor and you can read it. The first line is ISO-10303-21;, then a HEADER section with the file name, author and schema, then a DATA section of numbered entities such as #27=CARTESIAN_POINT('',(0.,0.,10.));. That plain-text layout is ISO 10303-21, usually called Part 21.
B-rep: exact geometry instead of triangles
STEP stores a solid by its boundary, a method called B-rep. Faces lie on mathematical surfaces such as planes, cylinders, cones or B-splines; edges and vertices tie them into closed shells. A 6 mm bore stays a true cylinder at any zoom level.
A screen or a printer needs triangles, so a STEP file must be tessellated before you can see it. In our viewer OpenCascade does this inside the browser tab at Coarse, Medium or Fine precision. A rounded test cube came out as 48, 84 and 172 triangles. To keep that mesh, use STEP to STL.
STEP carries only the finished solid. Sketches, constraints and the feature tree stay in the sender's native CAD file, so an import gives a body with faces you can measure and cut, often called a dumb solid. Ask for the native file when you need the history, and for STEP when the part must open anywhere.
Application protocols: AP203, AP214, AP242
ISO 10303 is huge, so each industry uses a slice of it called an application protocol. The protocol name sits in the FILE_SCHEMA line of the header.
| Protocol | Full name | Adds |
|---|---|---|
| AP203 | Configuration controlled 3D design of mechanical parts and assemblies | solids, surfaces, assemblies |
| AP214 | Core data for automotive mechanical design processes | colors, layers, more product data |
| AP242 | Managed model-based 3D engineering | first edition 2014; merges AP203 and AP214, adds tolerances and annotations (PMI) |
For looking at the shape, the protocol rarely matters. It matters when the receiver needs the colors that AP214 adds, or the tolerances and annotations that only AP242 carries. If a supplier promises tolerances in the file, check FILE_SCHEMA before you rely on them.
Units are declared in the file
A STEP model states its length unit, usually millimeters or inches, so a receiver can rebuild the true size. STL and OBJ carry no such field.
Our viewer shows the declared unit and converts the model to millimeters. A 10 mm test cube reads 10 mm, and a 1 inch cube declared in inches reads 25.4 mm. Some exporters label millimeter numbers as inches, and the CAX-IF AS1 assembly then opens at 5080 mm. The viewer then warns Check the units, and one switch, Numbers are millimetres, gives the intended 200 × 87 × 150 mm.
STEP vs IGES and STL
| STEP | IGES | STL | |
|---|---|---|---|
| Geometry | closed solids, exact surfaces | exact surfaces, often loose | triangles |
| Units | declared | a flag in the global section | none |
| Origin | ISO 10303, first parts 1994 | U.S. national standard, 1980 | 3D Systems, 1987 |
| Typical use | CAD exchange today | older CAD and surface data | printing |
IGES, the Initial Graphics Exchange Specification, was first published in 1980, and STEP was developed as its successor. Our IGES file viewer opens both. IGES made of separate surfaces may arrive with mixed face orientation, and Repair fixed that on a 6-surface test cube.
Open a STEP file in your browser
Drop the file on the STEP viewer. The first STEP or IGES file loads a 7.6 MB engine (3.1 MB gzip) from our own domain once; the part itself stays on your machine. Assembly colors are kept, and the mesh check runs per body: AS1 showed 10 of 10 bodies closed. Native part files from desktop CAD (SLDPRT, IPT, F3D) do not open, so export STEP from the source program first. More detail is in how to open STEP files.
Questions and answers
What is a .stp file?
It is a STEP file with a three-letter extension. The content is identical to a .step file, and any STEP reader opens both.
Is a STEP file a mesh?
No. It stores exact faces and edges. Triangles are created only when a program displays or exports it, at the precision you choose.
Can I 3D print a STEP file?
A slicer works with triangles, so convert the STEP to STL or 3MF first. Some slicers import STEP and tessellate it themselves.
Can I make a STEP file from an STL?
Yes, as a faceted solid: every triangle becomes a flat face, about 1 KB per triangle. It opens as a body without true cylinders or fillets. See STL to STEP.
Why is my STEP part 25.4 times too big?
The exporter wrote millimeter numbers but declared inches. Switch on Numbers are millimetres and the size comes back right.
Which STEP protocol should I ask a supplier for?
For a shape to print or check, AP203 or AP214 is enough, and AP214 keeps colors. Ask for AP242 when tolerances and annotations must come with the model; its first edition appeared in 2014.
Does a STEP file keep the feature history?
No. It stores the resulting faces and edges. Most CAD programs can still push and pull those faces with direct editing.