OBJ to STL

OBJ to STL for 3D printing

This page converts OBJ to STL in your browser so you can send a model to a slicer. Drop the .obj, check it in the preview, and download a binary STL. The file stays on your device.

Most slicers now open OBJ directly, so the conversion isn't always required. It still comes up often: a printer's software or a print service only takes STL, an older slicer mishandles OBJ groups, or you simply want a single, predictable file with nothing but triangles in it. OBJ files are common on model-sharing sites, from 3D scans and from sculpting programs, and STL is the format almost every 3D printing workflow ends in.

How to convert OBJ to STL

1. Drop the .obj file onto the converter. The .mtl and texture files are not needed for STL.

2. Check the preview and the bounding-box size in the info panel.

3. Leave “Fix up axis” on for models from modeling or game software; turn it off for files that were already Z-up.

4. Click Convert and download the STL. It is binary by default, which is smaller than ASCII and read by every slicer; choose ASCII only if a tool specifically needs it.

Need the opposite direction, to edit a printable part in a modeling program? See STL to OBJ.

What happens to your OBJ

STL is a much smaller format than OBJ. It knows about triangles and nothing else, so the conversion is mostly about what gets left behind:

Geometry

Kept. Quads and larger polygons in the OBJ are split into triangles, which is what STL requires.

Groups and objects

Merged. Every group in the OBJ ends up in one combined mesh. Parts keep their positions relative to each other.

Materials and textures

Dropped. STL has no way to store them. You don't need to load the .mtl file for this conversion.

UVs and smoothing

Dropped. STL stores one normal per triangle, so smooth shading from the OBJ is not kept. This does not affect the print; the slicer only cares about the surface.

Size

Unchanged. The numbers in the OBJ are copied as-is. Slicers read them as millimeters.

Getting the size right

OBJ has no units. A file exported from Blender with default settings is usually in meters, a file from CAD software is usually in millimeters, and a game asset can be anything. The converter doesn't rescale, so a character modeled 1.8 units tall becomes a 1.8 mm speck in the slicer, and a part modeled in centimeters comes in ten times too small.

Fix it in the slicer rather than guessing beforehand. Read the dimensions it shows, then scale by the factor you need (100,000% turns meters into millimeters; 1000% turns centimeters into millimeters) or type the target height directly. Some slicers notice a suspiciously small model and offer to convert it from meters or inches for you; if yours asks, accept.

Before converting, the info panel shows the model's bounding box in the file's own units. That number tells you what the slicer will see.

Up axis: why “Fix up axis” is on

OBJ is a Y-up format: height runs along Y. STL and slicers are Z-up. Without a correction, a standing figure exported from a modeling program would arrive in the slicer lying face down or on its back. With “Fix up axis” on, which is the default, the converter rotates the model 90 degrees so it stands on the bed the way it stood in the modeling program.

If your OBJ came from a CAD program or another Z-up source, it may already be oriented for printing. In that case turn the checkbox off. Either way, slicers let you lay a part flat on a chosen face, so a wrong guess costs a few seconds, not a failed print.

Will it print? Common OBJ problems

Converting to STL doesn't make a model printable. Many OBJ files are made for rendering, where a surface only has to look right, not enclose a solid. The converter copies the mesh as it is and does not repair it. Problems to look for:

Holes and open edges

Rendering models often leave out faces nobody sees, like the bottom of a statue. The slicer cannot tell inside from outside. Most slicers offer an automatic repair; for bigger holes, close them in a mesh editor.

Zero-thickness surfaces

Leaves, hair cards, cloth and flags are often single planes. They have no volume and will slice to nothing. Give them thickness in a modeling program.

Overlapping parts

Separate pieces that intersect, like an arm pushed into a body, are usually fine. Most slicers treat them as one solid. Odd gaps in the preview are a sign they aren't.

Flipped normals

Faces pointing inward can confuse the slicer about what is solid. Repair tools in slicers and mesh editors can recalculate them.

Fine detail

Details smaller than your nozzle width will vanish once the model is scaled to print size.