Fix
My STL is tiny in the slicer: inches vs millimetres
Published · How we check our guides
Short answer
STL files store numbers without units, and slicers read them as millimetres. A model drawn in inches therefore opens 25.4 times too small. Scale it by 25.4 (2540%), or fix the file once so it opens at the right size everywhere.
Why it happens
An STL file is a list of triangles, and each corner is just three numbers. There’s no field that says whether those numbers are millimetres, inches or metres. Slicers assume millimetres.
So a part drawn in inches, say a 4-inch bracket, is stored as “4”. Your slicer reads 4 mm. Everything is exactly 25.4 times too small (one inch is 25.4 mm). The same thing happens with centimetres (10 times too small) and metres (1000 times).
It’s common with models from US designers, older CAD files, Fusion or SketchUp projects set to inches, and files exported from game or animation tools.
How to tell which unit it was
Look at the size your slicer shows and ask whether the real object would make sense at 25.4 times that:
| Slicer shows | Probably drawn in | Scale by | Real size |
|---|---|---|---|
| 3.5 mm tall vase | inches | 25.4 (2540%) | 88.9 mm |
| 8 mm wide phone stand | centimetres | 10 (1000%) | 80 mm |
| 0.12 mm figure | metres | 1000 (100000%) | 120 mm |
If the model came with a description or a drawing, a known measurement (a screw hole, a bottle neck) settles it.
Fix it once in the file (recommended)
- Open the STL resizer and drop the file. If the size looks like inches or centimetres, it says so and offers the fix.
- Pick “It’s inches (×25.4)”, or type the exact size you need for any one side.
- Download the STL. It now opens at the right size in every slicer, and for anyone you share it with.
Nothing is uploaded: the file is scaled in your browser.
Or scale it in your slicer
- PrusaSlicer, Bambu Studio and OrcaSlicer ask whether to convert from inches when an imported model is tiny (a few millimetres across). Say yes. Bigger inch models don’t trigger the question, so if they don’t ask, set the scale to 2540%.
- Cura: select the model, open the Scale tool, make sure Uniform Scaling is on, and type 2540 in the % field. Cura’s Scale extremely small models preference won’t help: it’s meant for models in metres and scales by 10, 100 or 1000, never by 25.4.
Scaling in the slicer is fine for one print, but you’ll have to do it every time you open the file.
Scaled up and now it’s too big?
A model that’s right in inches can be bigger than your printer. You can turn it, scale it down a little, or split it into parts that fit the bed.