How to Make an STL File for 3D Printing

Way 2: Build it from shapes in Tinkercad

Tinkercad is a free 3D design app from Autodesk that runs in your browser. You build objects by stacking simple shapes, like boxes, cylinders and letters. You can also use any shape as a “hole” to cut into another one.

1. Make a free account at Tinkercad and start a new design.
2. Drag shapes onto the work plane. Resize them by dragging the handles or typing sizes.
3. Set shapes you want to cut away to Hole.
4. Select everything and click Group to join the pieces and cut the holes.
5. Click Export, then under For 3D Print, choose .STL.

Tinkercad joins all your shapes into one part when it exports. It can also export OBJ and glTF.

Way 3: Design exact parts in a CAD program

When a part has to fit something real, like a phone, a shelf bracket or a replacement knob, use CAD. CAD programs work with exact sizes and let you change a measurement later.

• FreeCAD is free and open source.
• Fusion from Autodesk is free for personal use, if the project is not commercial and you earn less than $1,000 a year from it.
• OpenSCAD is free and works with code instead of a mouse. It exports STL and 3MF.

The usual steps are the same in all of them. Draw a 2D sketch with exact sizes. Extrude it into 3D. Add holes, rounded edges and details. Then export as STL, or as 3MF if you want the units saved. Our guide on 3MF vs STL explains the difference.

Way 4: Model or sculpt it in Blender

For organic shapes, like creatures, faces and statues, CAD feels stiff. Blender is a free 3D program that can model and sculpt anything.

1. Model or sculpt your object. Keep it as one closed piece if you can.
2. Apply any modifiers you want in the final shape.
3. Check the size. Blender works in metres by default, and STL has no units. A model that is 0.1 units tall may print as 0.1 mm. Scale it so the numbers match the millimetres you want.
4. Select the object and go to File, Export, STL (.stl).
5. Keep the binary format, which is smaller than the text (ASCII) option.

Way 5: Generate it from a picture with AI

AI image to 3D tools turn one picture into a full 3D model, back and sides included. This is the fastest way to get a character, an animal or a figurine you could never model by hand.

In Bake3D:

1. Upload a picture of the subject, up to 12 MB. One clear subject on a plain background works best.
2. Generate the model. It costs 30 to 55 credits, depending on the quality level. New accounts get 70 free credits.
3. Turn the model around and check it from every side.
4. Pose it if you like, then open the Export tab and download an STL of the current pose.

AI models often use their own sizes. Check the size in your slicer and scale it on purpose. A model that is 1.8 units tall becomes 1,800 mm at scale 1.

What about ChatGPT

Chat assistants can make simple STL files. In a test by Tom’s Hardware, ChatGPT wrote the text of an STL cube, which printed after scaling. It also wrote OpenSCAD code for a cup, which the author exported as STL. This works for boxes, brackets and simple geometric parts. It does not work well for faces, animals or figurines.

Check the STL before you print

A file that downloads fine can still fail on the printer. Check these things first.

1. Is it closed? An STL for printing should be watertight, with no holes in the surface. Open it in our free STL checker. It lists open edges, flipped faces and parts that do not connect.
2. Is it the right size? The checker shows the real size in millimetres. Your slicer does too.
3. Are the walls thick enough? Formlabs suggests at least 1 mm for walls on a filament (FDM) printer, and 1.2 mm is safer with a standard 0.4 mm nozzle. Resin printers can go down to about 0.2 mm.
4. Does it have a flat base? A flat bottom sticks to the bed better.
5. Are there steep overhangs? Parts that lean out past about 45 degrees usually need supports.

Common STL problems and quick fixes

• The model is tiny or huge. The file was made in a different unit. Scale it by 1,000 (metres to millimetres) or 25.4 (inches to millimetres).
• Holes or open edges. Close them in your modeling program, or use a repair tool. The checker shows where they are.
• Flipped faces. Some triangles face inward. Most modeling programs can recalculate the normals so they all face out.
• The file is too big. Too many triangles. Reduce them. See how to reduce polygon count.
• Floating parts. Pieces that do not touch the rest will fall over during the print. Join them or add supports.

Now that you have an STL, you can slice it and print. If you want to keep your slicer settings with the model, save the project as 3MF.