To prepare an STL for printing, check four things before you slice: its size, its orientation, the state of the mesh, and the wall thickness. Catching problems at this stage saves failed prints. The STL viewer lets you check the first three in your browser in a minute.
Work through the checklist below.
1. Check the size
Open the STL and read its dimensions in the info bar. STLs are almost always in millimetres, so confirm the model fits your printer’s build volume and is actually the size you intended. A model that exported in the wrong units is the single most common surprise, and it is obvious the moment you see the dimensions.
2. Check the orientation
Rotate the model and think about how it will sit on the print bed. The orientation affects how much support material you need, how strong the part is along its layers, and how good the surface finish looks. A pose with fewer steep overhangs usually prints more cleanly. You set the final orientation in your slicer, but it helps to plan it while viewing.
3. Inspect the mesh
Switch to wireframe and look the model over. A very low triangle count makes curved surfaces look faceted; an extremely high count can slow slicing without adding visible detail. Obvious gaps or strange shading can hint at mesh problems such as holes or flipped faces, which you would then repair in your slicer or a mesh-fixing tool.
4. Think about wall thickness
Very thin walls may be thinner than your nozzle can reliably print, so they come out weak or fail. While a viewer cannot measure wall thickness directly, seeing the model at true scale helps you judge whether delicate features are realistic for your printer, or need thickening in CAD first.
Then slice
Once the size, orientation and mesh look right, take the STL into your slicer to set layer height, infill and supports, and generate the print. To start the checks, open the STL viewer, it is free, and your file never leaves your device.