STL units: millimeters, inches and scale errors
An STL file usually gives you numbers for vertex coordinates but not a dependable declaration that those numbers are millimeters or inches. The exporting and importing applications must agree on the convention.
Why this creates 25.4× mistakes
One inch equals 25.4 millimeters. If software exports geometry using inch values and another program assumes those values are millimeters, the model becomes 25.4 times too small. The reverse assumption makes it 25.4 times too large. These are among the easiest scale problems to diagnose because the ratio is distinctive.
Other scale patterns
Not every problem is 25.4×. Meter-to-millimeter mismatches can create a 1,000× difference. Centimeters introduce a factor of 10. Some CAD systems also apply document-unit or export settings separately, so the same source can produce different coordinate values depending on the export dialog.
A reliable check before printing
Open the exported STL in an independent viewer and compare one known feature. Do this before slicing, especially when a file came from another person or another CAD system. Record the unit convention with the file if it will move between teams.
- Check a known dimension.
- Look for 25.4×, 10× or 1,000× patterns.
- Confirm both export and import settings.
- Prefer a format that stores units, such as 3MF, when the receiving workflow supports it.
What a viewer should and should not claim
A viewer can report the coordinate extents accurately. It should not label them as millimeters unless the format contains a trustworthy unit or the user explicitly selects one. Transparent handling of uncertainty is better than silently guessing.