OBJ to STL converter

1. Drop your .obj (and .mtl if you want colours in a 3MF).
2. Check the size: pick the unit the model was made in, or type the size you want.
3. Make sure it stands upright; switch between Y up and Z up if it doesn't.
4. Download one merged STL, one STL per object, or a 3MF that keeps the objects and their colours.

OBJ files store plain numbers with no unit. A model built in metres (Blender's default) has numbers like 0.1, which a slicer reads as 0.1 mm, so it arrives 1000× too small. Inch models arrive 25.4× too small. This tool guesses from the numbers (tiny numbers usually mean metres), shows its guess, and lets you pick mm, cm, m or inches, or simply set the size you want.

3D printing uses Z as “up”, but Blender's OBJ export and most 3D apps write Y as up. Files are treated as Y-up by default and turned upright; if your model was made Z-up (common from CAD), switch to Z up.

A mesh is watertight (or “manifold”) when it encloses a solid with no gaps: every edge is shared by exactly two triangles, and all faces point outwards. Slicers need that to know what's inside and what's outside. The check here reports three kinds of problem:

• Open edges: holes in the surface.
• Non-manifold edges: three or more faces meeting at one edge, common where parts overlap.
• Flipped faces: triangles facing the wrong way.

Small problems are common and most slicers repair them automatically when they load a file. For the rest, Repair STL fills holes and turns flipped faces around; overlapping parts and surfaces with no thickness (typical of game assets and some AI-generated models) need a 3D editor. When a mesh isn't closed, its volume is shown as approximate.

If you have the original .blend, Blender can export STL directly (File → Export → STL). Set the scale so one Blender unit becomes the millimetres you want, and keep Z as up. This converter is for when you only have the OBJ, or want to check size and printability first.