STEP file viewer: open STEP files without CAD

STEP (.step or .stp) is the common language of mechanical design. Nearly every CAD program can save it, so suppliers send parts in it and shops ask for it when quoting. The catch is that opening one usually takes a CAD program, and not everyone has one installed.

This STEP viewer displays the file in your browser. The geometry is read by an open-source CAD kernel (Open CASCADE) compiled to WebAssembly. The reader is about 7 MB and downloads only the first time; after that it comes from your browser cache. Everything runs in your browser and the file is never uploaded, which matters for parts under NDA.

Who it's for:

• buyers and estimators who need a quick look at size and volume;
• machine shops and fabricators checking what they received before opening CAD;
• 3D printing users who want to know whether a part fits and need an STL or 3MF;
• engineers on the go looking at a file on a phone or someone else's computer.

CAD formats store exact mathematical surfaces: a hole is a true cylinder, a fillet a true arc. STL only knows triangles and approximates curved surfaces with flat facets.

To display a CAD file, the exact surfaces have to be broken into triangles. This is called tessellation, and the viewer does it on your device. STEP files also record their unit, so dimensions usually come out right in millimeters (or inches, if you switch the display) straight away.

• Size X × Y × Z in millimeters or inches, from the bounding box.
• Volume (cm³ or in³) and surface area from the tessellated mesh.
• Body count: how many separate parts an assembly contains. The assembly tree (parts list, hierarchy) isn't shown; all bodies are displayed together.
• Colors, if the file stores body colors.
• Measure: click two points to get the distance plus ΔX, ΔY and ΔZ, so you can check a hole spacing or a flange thickness in seconds.
• Section: cut the model along Z, X or Y with a slider and flip to the other half. The cut face shows in red, revealing wall thickness, internal channels or hidden cavities.
• Views and screenshot: front, top, side, wireframe, grid, PNG image.

You can't edit the file or save it back as STEP. It's a viewer, not a CAD program.

Volume and surface area are computed from the triangle mesh. Curved surfaces are approximated by flat facets, so the result can differ a little from the exact CAD value. On convex surfaces the facets run just inside the true surface, so the volume typically comes out a hair smaller.

Example: if a circular cross-section is approximated by 64 straight segments, the polygon's area is about 99.84% of the circle's, roughly 0.16% less. That's plenty accurate for quoting and material estimates. For tolerance analysis, use the measured value from your CAD program.

With IGES files the surfaces often don't form a closed solid; they just sit next to each other. The mesh check then shows open edges in red, and the volume is only indicative. That's the nature of the file, not a viewer error.

Some slicers can't read STEP, or read it only partly. The Print tab lets you check everything you need before printing:

• pick from about two dozen preset printers and see whether the part fits, or fits only when rotated 90°;
• see the largest scale that still fits, and scale to 98% of the bed in one click;
• overhangs are shaded orange with an adjustable threshold (45° by default), and the affected area is shown in cm²;
• get a rough filament and time estimate from the material, infill and wall count.

Download the model as STL or 3MF, built from the tessellated mesh. For other formats such as OBJ or GLB, use the 3D file converter. To price the print, try the 3D print cost calculator.

• Large assemblies: tessellating a STEP with hundreds of parts can take several seconds, a minute or more for big files, and needs plenty of memory. A desktop is faster than a phone.
• Units: if the size still looks wrong, set the file unit by hand to mm, cm, m or inches.
• Axis: CAD files usually treat Z as up. If the model lies on its side, toggle Z up.
• Missing parts: if parts of an assembly are missing, the file may reference other STEP files externally. Ask for everything saved into a single file.
• Won't open: ask the sender for an AP214 or AP242 STEP file; these are the most widely supported.