The right shape, at the right size.

3MF declares a unit. STL does not. The numbers in your STL need to be interpreted as millimeters to preserve the dimensions shown by this tool.

The rule used by this converter

We expand the selected build instances, apply their component and build transforms, and convert coordinates to millimeters. The displayed dimensions and exported STL use the same normalized geometry. Moving the preview camera changes only the view.

If the source model omits its unit, the 3MF default is millimeter. The tool explicitly labels this default in the model details and conversion manifest.

Try the 1-inch test

1. Download the original 1-inch cube and open it in the converter.
2. Confirm that the source says “inch” and all three output dimensions read 25.4 mm.
3. Download the STL and import it using millimeters in the receiving application.
4. Compare its measured dimensions with the independently checked STL.

Why is a model 25.4 times too large or too small?

An inch-to-millimeter interpretation mismatch introduces a factor of 25.4. Check the receiving program’s import units before scaling the geometry manually. A different mismatch may come from a source transform, an intentional scaled instance, or the original model itself.

Why did width and depth swap?

Rotation changes an axis-aligned bounding box. A 10 × 20 × 30 mm box rotated 90 degrees around Z has a 20 × 10 × 30 mm bounding box. This is not a unit conversion error. The converter preserves placed coordinates and does not automatically rotate, center, or place objects on the bed.

How precise is the result?

Binary STL stores 32-bit floating-point coordinates. Our original fixtures use an absolute 0.01 mm comparison tolerance; this is an engineering test threshold, not a universal accuracy guarantee. Extremely large coordinates and tiny features can lose precision. Check critical dimensions in your target application.