A FILE WITH A KNOWN SIZEOne model.
Clear dimensions.
STL coordinates have no unit field. A number like “1” could mean a millimeter or an inch. Choose the original unit for each file; this converter writes the geometry in millimeters and declares that unit in the 3MF.
For example, a cube measuring 1 × 1 × 1 in inch coordinates becomes 25.4 × 25.4 × 25.4 mm. Different source units can be selected for different files.
When you choose several files, they become parts of one model. Their coordinates are scaled to millimeters without centering or moving them. Use this for parts exported from the same design; assign colors or filaments later in your slicer.
BEFORE YOU PRINTGeometry in.
Geometry out.
Standard STL has no colors, materials or slicer settings to recover. The download is a Core 3MF model, not a configured printer project or G-code. Some slicers ask whether to import it as geometry.
This tool does not repair holes, unite overlapping solids or certify a model for printing. Zero-area triangles, damaged records and unsupported inputs are rejected. Open surfaces are not repaired; inspect the result in your slicer.
Limits: 1–20 files, 20 MiB per file, 100 MiB total source bytes, 200,000 triangles in total and 32 MiB model XML. Larger or more complex files may need desktop software.
Need the other direction? Convert 3MF to STL →