What is a .3ds file?

An old format you mostly receive

.3ds is the native format of 3D Studio for DOS, superseded by 3ds Max and its .max format in 1996. Nobody chooses 3DS for new work. You encounter it because a model library, a supplier's CAD download page or an architectural asset pack has not been updated in twenty-five years, and there are a great many of those.

For that reason 3DS support is still nearly universal, and the format is a perfectly reasonable way to get an old model into a modern tool.

The chunked layout

A 3DS file is a tree of chunks. Each chunk is a 16-bit identifier, a 32-bit length covering the chunk and everything inside it, then the body. The outermost chunk is 0x4D4D and its length equals the file size, which is how this site identifies the format without trusting the extension.

The structure means a reader can skip anything it does not recognise, which is why the format aged as well as it did.

The limits

• 65,536 vertices per mesh. Vertex indices are 16-bit. A detailed model

exported to 3DS is either split into many meshes or silently truncated.

• 32-bit float coordinates, so large models lose precision.
• No unit. As with STL, the numbers are just numbers.
• 8-character object names, in the original specification. Later writers

ignored this, but some readers did not.

• Materials are limited: a diffuse colour, a specular colour, a shininess

value and a texture map name.

Axis convention

3DS is Z-up, while most modern tools and every format this site writes are Y-up. A 3DS model imported without that conversion lies on its side. This site converts on import, and reverses the triangle winding as it does, because mirroring the coordinate system otherwise turns every surface inside out.

When you receive one

• Check the scale first. There is no unit, and 3D Studio scenes were often

modelled in arbitrary units.

• Expect the model to be split into several meshes if it is detailed, because

of the vertex cap.

• Textures are referenced by filename only, and are almost never included.

What to convert it to

• GLB or OBJ for general use
• STL or 3MF to print it
• STEP only as a faceted reconstruction, with the caveats on that page

There is no reason to convert *to* 3DS unless a specific old tool demands it, and this site does not write it.