What is a .fbx file?

What FBX is for

FBX began at Kaydara, went to Alias, and has been Autodesk's since 2006. It is the interchange format of the animation, games and VFX world: Maya, 3ds Max, MotionBuilder, Blender, Unity and Unreal all speak it, and a rigged, animated character moves between them with more of its setup intact than any other format manages.

That purpose explains what it is good and bad at. FBX carries skeletons, skinning weights, blend shapes, animation curves, cameras, lights and material graphs. It carries no exact geometry, no tolerances and no manufacturing information at all.

Binary and ASCII

Both exist and both are common. Binary FBX begins with the bytes Kaydara FBX Binary and is a tree of length-prefixed nodes with optionally deflate-compressed arrays. ASCII FBX is an indented text version of the same tree.

Neither has a published grammar. Autodesk provides an SDK rather than a specification, which is why third-party FBX support varies so much in quality and why files from one exporter sometimes fail in another tool.

This site reads both forms directly and takes only what it can carry across: geometry, the node transforms and material colour. Rigs and animation are not read, because no format this site writes could store them.

Units, which FBX gets right and wrong at once

FBX stores a UnitScaleFactor in its global settings, expressed in centimetres per file unit. So the file does tell you its scale, which puts it ahead of STL, OBJ and glTF.

The wrongness is cultural rather than technical: 3ds Max, Maya and Unity all have different default working units, and exporters frequently write a scale factor that is technically correct and practically surprising. A model that arrives 100 times too large has almost always crossed a centimetre/metre boundary.

Versions

FBX version numbers appear in the header: 6100, 7100, 7400, 7500 and so on. The 7500 series moved to 64-bit offsets in the binary layout, so older readers fail on newer files. When an FBX will not open, exporting as FBX 2018 binary or FBX 2014/2015 is the usual fix.

Should you use FBX for engineering?

No. If a supplier sends you an FBX of a part, they have sent you a visual asset, not a manufacturable model. Ask for STEP.

FBX is the right format when the destination is a game engine or an animation package, and when rigs or animation matter.

Converting FBX

• GLB is the natural target: same scene model, an open specification, and

far better support outside Autodesk's tools

• STL or 3MF to print the shape
• OBJ for a plain, universally readable mesh
• STEP only as a faceted reconstruction, with the limits stated on that

page, since there is no exact geometry in an FBX to recover