An STL file is a list of triangles with coordinates and nothing else. There is no field that says “these numbers are millimetres”. Every slicer assumes millimetres, so a model designed in inches arrives at 1/25.4 of its intended size, and a model exported from some CAD tools in centimetres or metres arrives 10× too small or 1,000× too large.
Spotting the problem from the bounding box
Open the file in the STL analyzer or your slicer and look at the X, Y, Z size.
| Largest dimension | Probably | Multiply by |
|---|---|---|
| Under about 1.5 | inches | 25.4 |
| 1.5 to 12 | centimetres (or a genuinely tiny part) | 10 |
| 12 to 400 | millimetres | 1 |
| Over 1,000 | micrometres or a scene exported in metres × 1000 | 0.001 |
The analyzer flags the first two cases automatically and offers the multiplier. A 3.5-unit “phone stand” is a 3.5-inch phone stand; a 0.79-unit cube is a 20 mm cube.
Fixing it
- Rescale in the slicer. Bambu Studio and OrcaSlicer: select the object, Scale tool, type 2540% (inches) or 1000% (cm). PrusaSlicer offers “Convert from inches” on import and in the right-click menu. Cura: Scale tool with uniform scaling locked, 2540%.
- Re-export from CAD in millimetres. Fusion 360, Onshape, SolidWorks and FreeCAD all let you set the export unit; Tinkercad exports millimetres only. Fixing it at the source avoids rounding every time.
- Do not rescale to “fit the bed”. Non-uniform scaling changes hole sizes, wall thicknesses and threads. Scale uniformly, and only by the unit factor.
Scaling for fit, not for units
If a model is the right unit but the printed part measures slightly wrong, that is shrinkage or printer calibration, not units. ABS shrinks nearly 1%, nylon up to 2%, PLA about 0.3%. The shrinkage calculator gives the exact X/Y scale percentage from either the material or a measured test print. Holes need separate treatment: FDM holes print 0.15–0.3 mm smaller than modelled, so compensate them in the slicer rather than scaling the whole part.
3MF avoids the problem
3MF files carry a unit declaration and are what Bambu Studio, OrcaSlicer and PrusaSlicer save as projects. If you publish models, export 3MF alongside STL and the import size will be right everywhere.