Why your 3D model prints at the wrong size
You slice a downloaded model and it lands on the plate as a speck, or fills the whole bed. Nothing is damaged. The file simply never said what its numbers meant.
Most 3D formats do not store a unit
An STL file contains a list of triangles, and each corner is three numbers. That is the entire format. There is no field anywhere saying whether a 10 means ten millimetres, ten centimetres or ten inches — and OBJ is the same.
Slicers resolve the ambiguity by assuming millimetres. That convention is why STL works at all for printing, and it is also why it breaks the moment the file was authored under a different assumption.
glTF and GLB are the exception, and that is the trap
The glTF 2.0 specification does define a unit: every linear distance is in metres. It is the sensible choice for a format built for the web and for AR, where a chair should be about one unit tall rather than nine hundred.
So a GLB and an STL can hold the same geometry and the same numbers and mean things a thousand times apart. Convert one to the other without touching the numbers and a 0.1 m figurine becomes 0.1 mm — too small for the slicer to render. A one metre desk becomes a thousand millimetres and swallows the build plate.
This is the single most common reason a converted model prints wrong, and neither file says a word about it.
Check before you slice, not after
Open the model and read its bounding box. A phone case should be around 150 by 75; a tabletop miniature around 28 tall; a benchy 60 by 31. If the numbers are three decimal places, the file is almost certainly in metres. If they are in the tens of thousands, someone worked in microns.
Doing this takes ten seconds and costs nothing. Discovering it after a four-hour print costs four hours.
The three numbers worth knowing
- Metres to millimetres: multiply by 1000. This is the GLB and glTF case, and the one you will hit most.
- Inches to millimetres: multiply by 25.4. Common in models exported from SketchUp or older US CAD software.
- Centimetres to millimetres: multiply by 10. Turns up in older scanner output.
Scaling in the slicer and scaling the file both work. Scaling the file is worth doing when you will print it more than once, or send it to someone else — otherwise the next person repeats your discovery.
What looks like a scale problem but is not
- A model lying flat that should stand up is an orientation problem, not a scale one. Rotate it in the slicer.
- A model slightly too large for the bed may just be too large. Not everything is a units error.
- Walls that come out thinner than designed are usually nozzle width, not scale.
- A model that slices as nothing at all is more often non-manifold geometry than a tiny one — check the bounding box first, because that distinguishes the two immediately.