Key takeaways
Set the scene to Metric with a Unit Scale of 0.001 and Length in millimeters, so one Blender unit is one millimeter in your slicer.
A printable model has to be manifold (watertight): every edge joins exactly two faces, with no holes, loose parts or flipped normals.
Install the free 3D Print Toolbox extension and run Check All before every export. Make Manifold fixes most problems it finds.
Apply scale and modifiers before you export, and give every surface real thickness. About 1 mm is a safe minimum wall for most filament printers.
Export as STL and check the size in your slicer before you print.
What you need
Blender 4.2 or newer. The current release is Blender 5.2.3 LTS, from October 22, 2026, and it's free from blender.org. Our Blender 5.2 LTS review covers what's new.
The 3D Print Toolbox extension. It used to ship with Blender. Since Blender 4.2 it's a free extension on the Blender Extensions site.
A slicer for your printer, such as PrusaSlicer, Bambu Studio, OrcaSlicer or UltiMaker Cura. All four are free.
If you're choosing a modeling program, our roundup of free 3D modeling software compares Blender with CAD tools that suit mechanical parts better.
Step 1: Set the scene to millimeters
STL files don't store units. Slicers read every coordinate as a millimeter, so the easiest setup is one where one Blender unit equals one millimeter.
1. Open the Scene Properties tab and expand Units.
2. Set Unit System to Metric.
3. Set Unit Scale to 0.001.
4. Set Length to Millimeters.
A 20 mm cube now shows as 20 mm in the sidebar and exports at 20 mm. Unit Scale only changes how Blender displays and enters measurements, so set it before you start modeling. Changing it later makes existing objects look 1,000 times bigger or smaller.
Step 2: Install the 3D Print Toolbox
1. Go to Edit > Preferences > Get Extensions and search for "3D Print Toolbox", then click Install. Blender asks you to allow online access the first time. You can also drag the extension from extensions.blender.org into Blender, or download the zip and use Install from Disk.
2. In the 3D viewport, press N to open the sidebar and click the 3D-Print tab.
The toolbox (version 1.5.0) checks the mesh for printing problems, measures volume and surface area, cleans up common faults and exports straight to STL, OBJ or PLY.
Step 3: Model with printing in mind
A model that renders well can still fail to print. Keep these rules in mind as you build:
Keep the mesh closed. A printer needs a solid with an inside and an outside. Open edges, holes and single planes have no volume, so the slicer can't fill them.
Give thin parts thickness. A plane, a leaf or a cape is zero thickness in Blender. Add a Solidify modifier and set its thickness in millimeters.
Respect minimum wall thickness. For a filament (FDM) printer with a standard 0.4 mm nozzle, walls of about 0.8 to 1.2 mm print reliably, which is two to three nozzle widths. Resin printers can go thinner, but check your printer maker's guidance.
Watch overhangs. Most printers handle overhangs up to about 45 degrees from vertical without supports. Steeper parts need supports from the slicer or a redesign.
Avoid needless detail. Every triangle adds to the file size, and a filament printer can't reproduce detail much smaller than its nozzle. Apply a Subdivision Surface modifier at a sensible level instead of the highest one.
Use Booleans carefully. The Boolean modifier works best on closed meshes. Let the shapes overlap slightly rather than sharing a face exactly, and use the Exact solver.
Step 4: Apply scale and modifiers
1. In Object Mode, select the model and press Ctrl+A > Scale, so its scale reads 1, 1, 1. Solidify, Bevel and the toolbox's thickness check all measure in the object's own scale, so an unapplied scale gives wrong results.
2. Apply the modifiers you want in the print (Solidify, Mirror, Boolean, Subdivision) from each modifier's menu, or leave them and tick Apply Modifiers when you export.
3. If the model has several parts that should print as one piece, join them with Ctrl+J and merge them with a Boolean Union, so their insides don't overlap.
Step 5: Check the mesh
In the 3D-Print tab, click Check All. The toolbox lists how many problems it finds in each category:
Check | What it finds |
|---|---|
Solid | Non-manifold edges, where an edge doesn't join exactly two faces, and flipped normals |
Intersections | Faces that pass through each other |
Degenerate | Faces with zero area and edges with zero length |
Distorted | Four-sided and larger faces that aren't flat |
Thickness | Walls thinner than the value you set |
Edge Sharp | Very sharp edges |
Overhang | Faces steeper than the angle you set |
In Edit Mode, click a result to select the faulty geometry so you can see where it is.
Set Thickness to your printer's minimum wall, such as 0.8 mm, before running the check. Edge Sharp and Overhang are warnings, not errors; a model can print with them.
Step 6: Fix the problems
Work through the results from the top:
1. Merge doubles. In Edit Mode, select everything with A and press M > By Distance. This joins duplicate vertices, the most common cause of non-manifold edges.
2. Fix normals. With everything selected, press Shift+N (Recalculate Outside) so every face points outward.
3. Fill holes. Select the open edges with Select > Select All by Trait > Non Manifold and press F to fill them, or Alt+F to fill with triangles.
4. Run Make Manifold. In the toolbox's Clean Up section, Make Manifold fills holes, fixes bad normals and removes stray edges and faces. Distorted triangulates faces that aren't flat.
5. Remesh as a last resort. If the mesh is a tangle of intersections, add a Remesh modifier set to Voxel, or use the Voxel Remesh in the Object Data tab, and set a voxel size such as 0.2 mm. It rebuilds the model as one clean closed shell but softens sharp edges and fine detail.
Run Check All again until Solid, Intersections and Degenerate read zero.
Step 7: Confirm the size and volume
Open the sidebar's Item tab and check the Dimensions match the size you want in millimeters. In the toolbox, Volume shows how much material the model will use, before the slicer's infill and supports, which helps when you estimate resin or filament. Scale To resizes the model to a target volume or bounding size.
To save material, the toolbox's Hollow tool shells out a solid model with an even wall. Resin prints that are hollowed also need drain holes so uncured resin can escape.
Step 8: Export the STL
1. Select the model.
2. Go to File > Export > STL (.stl).
3. Tick Selection Only, so cameras and other objects stay out.
4. Leave Scale at 1.0 and Scene Unit off. With the 0.001 unit scale from Step 1, the coordinates are already in millimeters, and turning Scene Unit on would shrink the model 1,000 times.
5. Tick Apply Modifiers if you didn't apply them in Step 4.
6. Leave the axes at Y forward and Z up, which most slicers expect.
7. Click Export STL.
You can also export from the toolbox's Export section, which uses the same exporter.
STL or 3MF? STL is the format every slicer opens, but it holds only geometry. 3MF also stores units, colors and several objects in one file. Blender doesn't export 3MF on its own; a community extension adds it. For a single-color print, STL is fine.
Step 9: Preview it in your slicer
Open the STL in your slicer and confirm the size matches what you modeled. If it's 1,000 times too big or too small, the unit scale and the exporter's Scale or Scene Unit setting don't match; go back to Step 1 and Step 8.
Most slicers repair small mesh errors on import and say so. If yours reports a lot of repairs, go back to Step 6 rather than relying on the fix, because an automatic repair can fill a hole you meant to keep. Then slice, check the layer preview for missing walls or floating parts and print.
Frequently asked questions
Is Blender good for 3D printing?
Yes, especially for sculpted and artistic models such as figures, miniatures and props. It's free and its 3D Print Toolbox checks models for printing problems. For mechanical parts with exact dimensions, a parametric CAD program is easier to edit precisely.
How do I make a model watertight in Blender?
Merge duplicate vertices with M > By Distance, recalculate normals with Shift+N, fill any holes with F and run Make Manifold in the 3D Print Toolbox. If the mesh is still broken, rebuild it with a Voxel Remesh.
What units should Blender use for 3D printing?
Use Metric with a Unit Scale of 0.001 and Length in millimeters, so one Blender unit is one millimeter. Then export the STL at a Scale of 1.0 with Scene Unit off.
Why is my model the wrong size in the slicer?
STL files have no units, and slicers read them as millimeters. A model that's 1,000 times too big or small means the scene's unit scale doesn't match the export settings. Set the scene as in Step 1 and export with Scale 1.0.
Do I need to apply modifiers before exporting?
Either apply them in Blender or tick Apply Modifiers in the STL exporter. Otherwise the file contains only the base mesh, without Solidify thickness, Mirror halves or Boolean cuts.
Should I export STL or 3MF?
Export STL for a simple single-color print, since every slicer opens it. 3MF stores units, colors and several objects in one file, but Blender needs a community extension to export it.