Why is STL 3D model so wrong scale?

STL 3D models often have wrong scale because STL files don’t store unit information, leading to mismatches between design/slicer software or accidental scaling in the original design tool. STL files lack native unit data—if you design in inches but your slicer uses millimeters, the model could be 25.4x too small (or too large if units are swapped). Accidental scaling in CAD software (e.g., unintended resizing during editing) also distorts size. To fix this, check unit settings in both your design and slicer tools, and use the slicer’s measurement feature to confirm dimensions before printing.