Will this file actually print?
Three questions your slicer will ask, answered before you open it: how big is it, does it fit the printer, and is the mesh closed. The file is read on your own machine — nothing is uploaded.
What it checks
A closed mesh is what makes every other number mean something.
An STL is a pile of triangles. In a closed mesh, every edge is shared by exactly two of them. An edge on one triangle is a hole; an edge on three or more is a broken seam. Either way the slicer has to guess what is inside and what is out — and the volume and weight it reports stop being reliable.
The fit check tries, in order: the file's own orientation, then a turn on the plate, then a different face down. It turns the part's bounding box rather than the part itself, so when it says “fits”, it fits; some parts may still fit at an angle it reports as too big.
Each printer's build volume is the maker's own published figure — the same source the machine articles on this site cite — with the date it was read. Makers revise them; check your exact model.
Is my file uploaded anywhere?
No. The file is read by JavaScript inside your browser; no network request carries it anywhere. Close the tab and it is gone.
How do I fix a mesh with open edges?
Most current slicers repair meshes automatically on import. For worse damage, use dedicated repair software — listed under file repair on the tools page — then check it again here.
Why does the weight differ from my slicer?
Because it is a geometry estimate: no support, brim or purge, and no simulation of how your slicer lays the paths. Use it to know the ballpark before slicing, and the slicer's number to quote.