First decide what you actually need
Almost every wrong answer to this question comes from skipping it. "Open the file" means four different jobs, and the tool that does one well is often useless for another.
- Look at it and take a measurement
- You need a viewer. This is the easiest case and several free ones do it properly, including one from the vendor.
- Print it, mesh it, or run it through a mesh tool
- You need a triangle mesh — STL or 3MF. Any conversion route will do, because tessellation is the destination anyway.
- Keep modelling in a different CAD system
- You need a solid, which in practice means STEP. Expect an imported body rather than an editable feature tree; no format carries SOLIDWORKS history into another system.
- Quote it, machine it, or send it on
- STEP again. A machine shop asked for "the STEP" because that is what their CAM system reads.
If you were asked for "the CAD file" and you have a SLDPRT, the thing to send back is usually a STEP. It is the neutral format everyone can open, and sending the SLDPRT itself just moves the problem to the next person.
The six options, honestly compared
| Option | Cost | View | Convert | The catch |
|---|---|---|---|---|
| eDrawings Viewer | Free | Yes | Limited | The vendor's own viewer, so fidelity is perfect. Install required, and export is restricted unless the sender enabled it when saving. |
| Online converter | Free tier | Varies | Yes | Nothing to install and works on any machine. You are uploading the file, so it matters who runs the service and what they do with it. |
| Autodesk Viewer | Free account | Yes | Some | Solid viewer. Requires an account and an upload, and conversion targets are limited. |
| Onshape | Free tier | Yes | Yes | Genuinely good, and it will import the part properly. The free tier makes your documents public, which rules it out for anything confidential. |
| FreeCAD | Free | Partly | Partly | Open source and offline. SOLIDWORKS support is indirect and version-dependent; expect it to fail on a fair share of real files. |
| Ask the sender for a STEP | Free | n/a | n/a | Genuinely the best option when it is available. Thirty seconds of their time removes the problem entirely. |
The last row is not a joke. If the person who sent the file still has SOLIDWORKS open, asking for a STEP export is faster and more faithful than anything else on this list. Everything above it exists for when that is not possible — an archived file, a supplier who has moved on, a client who has gone quiet, a part you found in a folder.
The fastest route: convert it in a browser tab
If you do not have a CAD seat and do not want to install one, converting is the shortest path from "I cannot open this" to "I have the geometry". Drop the file in and choose what you need: STEP to carry on in CAD, STL or 3MF to print.
On our servers. No account: deleted within 24 hours. Signed in: kept until you delete them. Never shared or used for training.
Two things worth knowing before you download. A multibody part keeps its bodies separate with their names intact, so it does not arrive as one anonymous shell. And a STEP produced this way usually carries the part's own surfaces — planes stay planes and holes stay cylinders — so it measures, mates and quotes like modelled geometry, but there is no feature history in it.
No converter can give you back an editable SOLIDWORKS feature tree in another CAD system. Anything that claims otherwise is either rebuilding the model by algorithm and guessing, or quietly handing you an imported body like everyone else.
What about SLDASM assemblies?
An assembly is a different kind of document. A SLDASM records which components sit where and how they are constrained; the component shapes themselves live in the SLDPRT files it references. This is the single most common reason an assembly "will not open" anywhere: it arrived on its own, and there is no geometry in it to show.
If you have been sent a lone SLDASM, ask for the parts as well, or better, ask for one STEP of the whole assembly. When the components do travel with it, the assembly converts as a whole and the component names are worth preserving so the result still reads as a product.
Before you upload anything
A part file is often commercially sensitive, and "free online converter" covers a very wide range of operators. Three questions are worth asking of any of them, including this one.
- Where does the file actually go?
- Some services forward your upload to a third-party conversion API. Your geometry then sits with a company you never chose. MeshWeave converts on its own servers and sends files to no third party.
- How long is it kept?
- Without an account, a file converted here is deleted within 24 hours — in practice about an hour later. With an account, the original and the converted file are kept in your files until you delete them, and a deleted file leaves the servers within 24 hours. Work you save into a project stays until you delete it.
- Is it used for anything else?
- No ads, no analytics trackers, and your files are never used to train AI models.
If the part is under NDA and you cannot answer those questions for a given service, use the offline route: ask the sender for a STEP, or open it in eDrawings on a machine you control.