# 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](/tools/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.