SketchUp is a 3D modeling software primarily for architects, designers, builders, makers and engineers. You can create buildings, furniture, interiors, landscapes, and more. Plus, you can upload to Sketchfab directly from the application.
SketchUp Pro
If you're using the Pro version of SketchUp, you have a lot of options for exporting 3D file formats. However, the best way to upload your 3D models from SketchUp Pro to Sketchfab is through the Sketchfab Exporter plugin.
If you decide to export and upload your model manually, we recommend using the OBJ format, which you can do from either the Desktop application or the Cloud application.
Before uploading your 3D models from SketchUp Pro to Sketchfab, we recommend you check out this great tutorial from Alex Schreyer on how to prepare and upload your SketchUp models.
Load times and performance
Some SketchUp Pro models fail to process or perform poorly in the Sketchfab viewer due to the way SketchUp Components and Groups are handled. The biggest culprits here are often trees and plants with lots of leaf components.
If you're using the Exporter or manually exporting as COLLADA (.DAE), the easiest thing to do is DISABLE Preserve component hierarchy.
Another, more manual solution is to Explode all your Components into their basic faces/edges.
Go to Window → Model Info, and go to the Statistics tab. Make sure Show nested components is checked. Here, you can see the number of Component Instances. You'll want to reduce that as much as possible, ideally to zero. This can take some time, and makes the model difficult to edit, so it should be the final step before uploading.
Select All, or just a few Components at time (this process can be quite CPU/memory intensive), and do Edit → X Components or X Entities → Explode. Repeat until there's nothing left to Explode and the whole model is a single mesh.
This will greatly improve upload/processing time and viewer performance.
There is a SketchUp Extension called Bomb that can handle this process for you.
SketchUp Free
SketchUp offers a browser-only version of the software called SketchUp Free.
If you made your model in SketchUp Free, you can only export the model as an STL file. Once you do this, you can upload to Sketchfab.
Note: STL files do not support textures or materials, so SketchUp Free is usually not a good option for uploading complex models to Sketchfab, but it should be ok for simple models like 3D-printable objects.