Convert an STL from Inches to Millimetres

STL files store bare numbers with no unit, and every common slicer reads them as millimetres. A part designed in inches, which is the default in many American CAD set-ups, therefore opens 25.4 times too small: a 4-inch bracket appears as a 4 mm speck on the plate. The fix is a single multiplication by 25.4, and this page does it with the unit fix already selected.

The same happens with other units: a file exported in centimetres is 10 times too small, one in metres (game engines, many AI 3D generators) 1000 times too small, and the opposite case, a millimetre model loaded by software that expects inches, needs division by 25.4. All four conversions are one click, and the new size is shown before you download.

Nothing about the model changes except its size: the file keeps the same triangles and is written back as a binary STL. You also see whether it now fits your printer and its solid weight in common materials. Everything runs in your browser and the file is never uploaded.

Slicers catch this only in narrow cases. PrusaSlicer asks whether a model was made in inches only when its volume is under 9 cubic millimetres, about a 2 mm cube, so a 4-inch box loads without a question and simply looks small. Cura's Scale extremely small models option, on by default, multiplies by a power of ten when a model is over 100 times smaller than the build volume; that rescues files in metres, never files in inches, because 25.4 is not a power of ten.

To tell which unit you have, divide the size you expected by the size the slicer shows: about 25.4 means inches, 10 centimetres, 1000 metres. If the model has no real unit, as with many AI-generated or scanned models, use the STL Resizer instead and type the exact height or width you want.