Why an STL opens at the wrong size

STL stores coordinate numbers without recording a length unit. If one application writes numbers that mean inches and another interprets them as millimetres, the same geometry opens at a different physical size. The file alone cannot identify which interpretation the author intended.

One unit, two possible meanings

Our sample spans one coordinate unit on each axis. Interpreting it as inches leaves every coordinate unchanged. Converting that interpreted model to millimetres produces dimensions of 25.4 × 25.4 × 25.4 mm. Exporting it as 3MF records the millimetre unit alongside the geometry.

Choose the operation that matches the problem

If you know the source application's unit, assign that interpretation first. If you need a different unit for the receiving application, convert units while preserving physical size. If the part itself must become larger or smaller, use a uniform factor or one target dimension. Proportions remain locked when setting that dimension.

Check a dimension you know, such as a documented part width. Do not infer the intended unit solely because the object looks too large on screen; a viewer fits its camera to the model, so differently sized parts can occupy the same space in the preview.

Match the output report

When the preparation exporter knows the physical unit, it writes STL coordinates in millimetres and tells you to select millimetres on import. STL still does not store that choice. A unitless source remains uninterpreted until you assign a unit; 3MF export refuses to guess one.

The STL format description documents the triangle representation. The downloadable one-unit STL and one-inch 3MF let you inspect the difference between an interpretation and a recorded unit directly.