The short answer
CAD is the recipe; STL is a photograph of the cake. A CAD model holds features, dimensions, relationships and exact surfaces, and you can still change any of them. An STL holds one frozen approximation of the outer skin as triangles, and nothing else - no features, no dimensions, no units, no history. Every 3D print starts from a mesh, so an export has to happen somewhere. The question worth asking is not "CAD or STL" but who does the exporting, and at what tolerance.
| CAD model | STL | |
|---|---|---|
| Is | The design | An export of its surface |
| Geometry | Exact surfaces and solids | Triangles approximating them |
| Editable | Yes - features and dimensions | Not meaningfully |
| Units | Defined | None stored |
| Design intent | Preserved | Discarded |
| Printable | After export | Directly |
| Typical files | .sldprt, .f3d, .ipt, .step | .stl |
"CAD" is a category, not a format
People say "CAD file" to mean one of two quite different things. There are native CAD files - SolidWorks .sldprt, Fusion .f3d, Inventor .ipt, Onshape documents - which are program-specific and generally unreadable outside their own software. And there is the neutral CAD interchange format, STEP, which any CAD package can read and which exists precisely so that exact geometry can cross between tools.
That distinction matters when someone asks a printing service to "just print my CAD file". A native file usually cannot be opened at all. A STEP file can, and should be the answer whenever exact geometry needs to survive the trip.
What the export step throws away
Exporting to STL is a lossy, one-way operation. Four things go:
- Exactness. Curves become chains of flat facets. A hole is no longer a hole of radius r; it is a polygon that resembles one.
- Units. STL has nowhere to record them. Everyone assumes millimetres; software that assumes inches produces a model 25.4× off with no warning.
- Structure. An assembly of six parts becomes one undifferentiated mesh. Part names, materials and the arrangement are gone.
- Editability. The feature tree does not survive. Changing a fillet later means going back to the CAD file - which is why you keep it.
None of this makes STL a bad format. It makes STL a terminal format: correct at the end of the pipeline, wrong in the middle of it.
The export settings that actually matter
Every CAD package exposes the same two knobs under different names, usually as "chord height"/"deviation" and "angular tolerance".
- Chord deviation - how far a facet may sit from the true surface. For FDM, somewhere around 0.01–0.05 mm is ample. Resin printing can justify the tighter end.
- Angular tolerance - the maximum angle between neighbouring facets, which controls smoothness on tight curves. Around 10 degrees is a sensible default.
Going far tighter than this is the most common mistake: it multiplies the triangle count and the file size while changing nothing a 0.4 mm nozzle can physically reproduce. Going much looser is the second most common: visible faceting on curved surfaces, and holes that measure undersized because the inscribed polygon sits inside the true circle.
So what should you send a printing service?
- Best for scale and units: 3MF. It declares its units inside the file, so nothing about the size is inferred.
- Best for engineered parts: STEP. Exact geometry, units included, and the service can tessellate it appropriately for the process.
- Perfectly fine, and universally accepted: STL - as long as you exported it yourself, at a sane tolerance, in millimetres.
- Do not send: native CAD files. Most services cannot open them.
If you are on the receiving side and deciding what to allow at upload, which 3D file formats to accept covers the intake policy.
Why this matters for quoting
An instant quote is computed from geometry - volume for material, bounding box and surface area for time. Those numbers are only as good as the mesh they come from, and the mesh is only as good as somebody's export dialogue. A part exported at a coarse tolerance quotes slightly light on material and can misreport whether it fits the build plate at all.
Accepting STEP removes that dependency, because the tessellation happens on your side at a resolution you chose rather than on a customer's laptop at whatever the default was. It is one of the few file-format decisions with a direct effect on quote accuracy.
Send the CAD file, get a price anyway
Filaquote accepts STEP and STP alongside STL, 3MF and OBJ. STEP files are converted to a mesh on our servers and priced from their real geometry, so an engineering customer can upload what their CAD tool exports and see a live 3D preview with an instant price - no conversion, no email.
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What is the difference between CAD and STL?
CAD is the design itself - features, dimensions and exact surfaces that can still be edited. An STL is one fixed snapshot of that design's outer surface, approximated with triangles. CAD is the recipe; STL is the photograph of the cake.
Which file format should I export for 3D printing if I want to preserve scale and units?
Export 3MF if your software offers it, because 3MF declares its units inside the file. STL stores no units at all, so scale is inferred rather than stated. If the part is an engineered component, STEP also carries units and keeps the exact geometry.
Can you convert an STL back to CAD?
Not properly. Tools can import an STL and wrap it in a solid body, but you get thousands of flat facets rather than the original curves and features. Real reconstruction means remodelling the part, so keep the CAD file.
What export tolerance should I use for STL?
For FDM printing a chord deviation around 0.01 to 0.05 mm with an angular tolerance near 10 degrees is plenty. Tighter settings mostly grow the file without changing anything the nozzle can reproduce.