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OBJ to STL converter

Turn an OBJ from Blender, a scanner or a model site into an STL for your slicer. The model is checked for holes and saved standing the way you see it.

60 mm40 mm30 mm

Drop an OBJ file anywhere on this pageChoose an OBJ file to convert it

Nothing is uploaded. Saved in millimeters and standing on Z, as slicers expect. Several OBJ files make one STL.

The OBJ to STL converter with a bracket open: an L-shaped part with three holes, a size of 60 × 30 × 40 mm, a check mark at Watertight, and a Download STL button with links for 3MF, GLB, glTF and PLY.
The sample bracket, read from an OBJ with Y up and ready to save as an STL that stands on Z.

What the STL keeps, and what it loses

In the STL
ShapeEvery triangle. Faces with more corners are cut into triangles, a face with a dent ear by ear
SizeMillimeters, from the unit you choose for the OBJ
Which way upStanding on Z, as the model stands on screen
Objects and groupsJoined into one mesh: an STL has no names
Colors and materialsLost. A 3MF can keep them as parts
Textures and smooth shadingLost. An STL gives each triangle one flat normal

How to convert an OBJ to STL

  1. Drop the OBJ on this page, with its MTL if you have one, or choose it with the button. A ZIP of the folder works too. It is read in your browser; nothing is uploaded.
  2. Look at the model. If it lies on its back, choose Z points up in the Views menu.
  3. Under Units, choose the unit the model was drawn in, so that the size in millimeters is right.
  4. Read the check: Watertight says whether the surface is closed, which a slicer needs.
  5. Choose Download STL. The file is named after the OBJ: chair.obj becomes chair.stl.

Which way up

An OBJ is a list of numbers with no up direction. The convention of modeling programs is Y up, and that of 3D printing is Z up, so an OBJ copied into an STL as it is lies on its back in a slicer. This converter writes the STL the way the model stands in the viewer, which takes an OBJ to be Y up, as Blender and most other modeling programs write it. An OBJ from a CAD program may be Z up already: it then lies on its back here, and Z points up in the Views menu stands it up, for the view and for the STL.

Size

Slicers read the numbers of an STL as millimeters, and an OBJ does not say what its numbers are. Blender writes meters, many other programs centimeters or whatever unit the scene was set to. Choose the unit under Units and the panel shows the size in millimeters; the STL is written at that size. A model under 10 mm across was most likely drawn in inches or meters, and the page asks whether it is in inches.

Holes and flipped faces

A slicer needs a closed surface to know what is inside. The check under the model counts open edges, the holes they make, edges shared by more than two faces, and faces turned the wrong way, and draws the open edges in red. A model made for rendering often has holes a viewer never shows: an open bottom, a separate eye. The STL is saved as it is, holes included, and STL repair closes holes and turns flipped faces when the check finds them.

STL or 3MF

STL is the format every slicer takes. A 3MF holds the same triangles with their unit stated, and a model built of pieces in different colors as a part for each color, which a slicer lets you give a filament each. The 3MF, GLB, glTF and PLY are saved from the links under the button.

Questions

Is my file uploaded?

No. The OBJ is read and the STL is written by your own browser. Nothing is sent to a server.

Why does my model come out lying on its back in other converters?

An OBJ does not say which way is up. Most modeling programs, Blender among them, write it with Y up, while slicers stand a model on Z. This page takes an OBJ to stand on Y, as they write it, and turns it so that the STL stands on Z. If the model lies on its back here, choose Z points up in the Views menu: the STL follows what you see.

What unit is the STL in?

Millimeters, which is what every slicer reads. An OBJ has no unit of its own, so choose the one it was drawn in under Units: millimeters to begin with. A model from Blender is usually in meters, so a part 0.05 long is 50 mm once you choose Meters.

Are the colors kept?

No. An STL holds triangles and nothing else. A 3MF, saved from the link under the button, keeps them as parts where the model is built of pieces in different colors: a slicer lets you give each part its own filament.

My OBJ has several objects. What happens to them?

They all go into the one STL, where they are loose shells, as in any STL. The viewer counts them as bodies. Several OBJ files dropped together become one STL too; hide any you want left out.

Do I need the MTL file?

Not for the STL. Drop it with the OBJ to see the model in its colors, and to keep them in a 3MF, GLB or glTF.