The short answer
- STL: triangles only, no units, no colour. Can be read by every program and is the most common format on model sites.
- 3MF: triangles plus units, colours and sometimes settings. Send it if you have it.
- STEP: exact geometry from the CAD program. Send it when the dimensions have to be right.
- OBJ: a mesh like STL, common from 3D graphics and scanning software.
All four are shown in your browser as soon as you upload them on Order a custom 3D print online, with dimensions and volume, so you see at once whether the file is as it should be.
STL: triangles only
An STL file describes the model's surface as a mesh of triangles, nothing more. It holds no unit, no colour and no settings, and it does not know whether the model is meant to be 20 mm or 20 inches. That is exactly why it works everywhere: every program can read and write STL, and it is the format most models on MakerWorld, Printables and Thingiverse are downloaded in.
Because STL has no unit, we assume millimetres, as every 3D printing program does. If the model appears to be smaller than 2 mm, the page tells you. If the model is 25 times too small, the file was saved in inches, and if it is 1 000 times too small, it was saved in metres. Export it again in millimetres.
The dimensions are shown under the model as soon as the file is uploaded, so compare them with what you expected before you order. The model is printed at the size the file has.
3MF: a mesh with units and colour
3MF is a newer format that is also built on triangles but carries more: the unit the model was drawn in, colours, and from Bambu Studio and similar programs also settings such as layer height and infill. If you upload such a project, the order page starts from those settings, and you can change them if you like.
If you have painted the model in two colours in Bambu Studio, the colours come along in the file, and we can print it in two colours. A second colour takes longer and costs more than the same model in one.
Many models on MakerWorld are downloaded as 3MF with ready-made settings. Send it as it is. You do not need to turn it into an STL. How to make a 3MF of your own with several parts, or with two colours, is in the guides Several parts in one file and 3D printing in two colours.
STEP: exact geometry from CAD
STEP is the format CAD programs such as Fusion 360, Onshape, FreeCAD and SolidWorks exchange models in. It describes the surface not with triangles but with exact surfaces and curves, so a circle is a circle and a dimension of 40.00 mm is just that. It is the format to choose when the model is a technical part that has to fit something.
When you upload a STEP file, we turn it into a mesh before it is shown and priced. That takes a few seconds the first time, and then you see the dimensions and volume as usual.
OBJ: meshes from graphics and scanning
OBJ is a text format for meshes that often comes from 3D graphics software, such as Blender, and from 3D scanning. We read the geometry and ignore textures and materials, which cannot be printed anyway. OBJ has no unit, just like STL, so the same advice about millimetres applies.
Models made for games and film are often not closed, with open edges and surfaces without thickness. They cannot be printed as they are, and the page tells you if the model has no volume.
Converting between formats
From 3MF, STEP or OBJ to STL you do not need to convert anything to order from us. Upload the file as it is, and the model is shown with its dimensions and volume just as an STL would be. If you still need to convert 3MF to STL, for another program say, open the model in our free STL viewer. It shows the model in 3D in your browser, with dimensions, volume and triangle count, and saves it as STL. The file stays on your computer, and from there you can go on to order the print.
From STL to STEP there is no easy route. An STL holds only triangles, and a CAD program cannot recreate the exact surfaces from them. If you need a STEP file, go back to the original model in the CAD program and export from there. If all you have is an STL, send it as it is.
Whatever the format, the same limits apply: the file may be at most 50 MB and the model at most 256 × 256 × 256 mm in one piece. Which material to choose next is in the guide PLA or PETG.