Check an STL before you slice it

STL files from customers and download sites cause most of the "it printed wrong" problems. Four quick checks catch nearly all of them.

1. Is it the right size?

STL files don't store units. Slicers read the numbers as millimetres, so a model drawn in inches comes in 25.4 times too small, and one exported in metres comes in tiny. If a part should be 100 mm and shows as 3.9 mm, that's inches.

2. Does it fit your printer?

Compare the size in X, Y and Z with your bed and height. A part that's too long can sometimes be turned 90° or diagonally. One that's too tall needs splitting.

3. Is the mesh closed?

In a watertight mesh, every edge is shared by exactly two triangles. Gaps (edges used once) and overlapping parts (edges used three or more times) confuse slicers: missing walls, odd infill or layers that skip. Most slicers can repair small problems, but it's better to know before you start.

4. How much will it weigh?

The volume tells you the solid weight: PLA is about 1.24 g/cm³. A real print weighs less, because only the outer 0.8 mm or so is solid and the rest is infill. That's enough for a quick quote; the slicer gives the exact figure.

STL Inspector does all four in your browser, without uploading the file, and can turn the STL into a 3MF set up for your printer. It has sample files to try, including one with a hole.