From Blender to a 3D print

1. Work in millimetres

Blender counts in metres by default, and slicers read STL numbers as millimetres. That's why a model that looks right in Blender can arrive 1,000 times too small.

Fix it once: open Scene properties → Units, set Unit Scale to 0.001 and Length to Millimeters. Now one Blender unit is one millimetre. If you'd rather keep metres, set Scale to 1000 when you export instead.

2. Apply the scale

If you scaled the object, press Ctrl+A → Scale in Object mode. That bakes the size into the mesh, so modifiers like Solidify use real thicknesses.

3. Make it solid

• Find gaps: in Edit mode, Select → Select All by Trait → Non Manifold. Anything that lights up is a hole or a bad join.
• Close holes: select the edges around one and press F to fill it.
• Merge doubled points: Mesh → Clean Up → Merge by Distance.
• Point faces outward: select all and use Mesh → Normals → Recalculate Outside (Shift+N).

4. Check the thickness

A wall thinner than your nozzle can't be printed. On a common 0.4 mm nozzle, aim for at least 0.8 mm, and more for parts that take force. Flat surfaces with no thickness, like a cloak or a leaf, need a Solidify modifier.

The free 3D-Print Toolbox add-on checks this for you. In Blender 4.2 and later, install it from Get Extensions. Its checks flag thin walls, holes and faces that cross each other.

5. Export and test

• File → Export → STL (.stl). Tick Selection Only if you only want one object, and keep Apply Modifiers on.
• Open the STL in our viewer to confirm the size in mm and that it's watertight.
• If it isn't, try the STL repair tool, then slice it and look at the layer preview before you print.