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STL or 3MF: which file should you export?

Printing it · 4 min read · Updated 2026-08-12

The practical difference between the two common 3D print formats, what each one keeps, what each one loses, and when to use which.

When a design is finished it has to leave the design tool as a file the slicer can open. Two formats cover almost everything you will meet.

STL

The old standard, supported by absolutely everything. An STL is a bare list of triangles describing the surface. That is all it is. It records no units, no colour and no information about the object beyond its shape.

Because it records no units, a program opening an STL has to assume what the numbers meant. Most assume millimetres and get it right. Occasionally one assumes inches, and a 40 mm bracket arrives as a metre-wide bracket — a confusing failure with an easy fix once you know to look for it.

3MF

The modern replacement. A 3MF records the same geometry plus the things STL throws away: the unit, so millimetres stay millimetres, and colour, so a two-colour design arrives as a two-colour design. It is a smaller file and better defined.

Which to choose

• Use 3MF by default. Every current slicer opens it.
• Use STL when sending a file to an older tool, an online print service that asks for it, or someone whose software you do not know.
• If you send an STL, say in the message that it is in millimetres.

Neither format is a picture or a document. It cannot be opened by a photo viewer, and a slicer or a 3D viewer is needed to look at it. That confuses a lot of people the first time.