Convert BLEND to STL
Getting an STL out of Blender is three clicks. Getting one that prints at the right size and does not fail in the slicer takes about two minutes more, and that is what this page is about.
We cannot open .blend files — only Blender can, because a .blend is a snapshot of its own memory rather than a description of a model. Export the STL from Blender with the steps below. Everything after that, including converting it to 3MF and checking it is watertight, runs here in your browser.
Exporting a printable STL from Blender
Written for Blender 4.x, where the STL exporter is the rewritten one. In 4.1 and earlier there may be two STL entries in the menu — one marked Legacy — and either works; the newer one is faster on large meshes.
Set the scene units to millimetres first
Scene Properties (the printer-and-cone icon) → Units → Unit System: Metric, Unit Scale: 0.001, Length: Millimeters
Do this before anything else. Slicers read STL as millimetres and Blender defaults to metres, and this one setting is the difference between a 60 mm bracket and a 60 metre one.
After changing it, look at the dimensions in the sidebar (press N in the viewport). They should now read in millimetres and match the size you actually want to print.
Apply your modifiers, or check they will be applied
Object → Convert → Mesh
Subdivision, Mirror, Solidify and Boolean modifiers are instructions rather than geometry. The exporter does apply them by default, but converting first means what you see in the viewport is exactly what gets written.
This is also the point to notice that a Subdivision modifier set to 2 in the viewport and 3 at render time will export at 3, which can turn a manageable model into a ten million triangle file.
Check the model is actually solid
Edit Mode → Select → All by Trait → Non Manifold (Shift+Ctrl+Alt+M)
A printable object must be closed, with every edge shared by exactly two faces. This selects everything that is not. If nothing is selected, the mesh is watertight and will slice cleanly.
Blender lets you model things that cannot exist — single-sided planes, faces with no thickness, edges where three faces meet. All of them look fine on screen and all of them confuse a slicer.
Open the export dialog
File → Export → STL (.stl)
Select the object first if you only want one. Then tick Limit to Selected Only in the options panel on the right, otherwise everything in the scene is written into the file, cameras excluded.
Check the scale and axis settings
Transform → Scale: 1.00 · Forward Axis: Y · Up Axis: Z
Up Axis Z is what you want for printing, because slicers are Z-up and so is Blender. This is the one export where the axes need no conversion at all.
Leave Scale at 1.00 if you set the scene units in step one. If you skipped that, set Scale to 1000 here instead — but never do both.
Choose binary
Format: Binary
Binary is the default and it is about five times smaller than ASCII. Every slicer reads it. There is no reason to choose ASCII unless something has specifically asked for a text STL.
Export, then check the size somewhere else
Export STL (the button, top right)
Drop the finished STL into any converter on this site and read the measured width, depth and height. If they match the part you meant to make, the scale is right and you can slice it with confidence.
Why Blender models so often fail in a slicer
Blender is a modelling program for things that will be looked at, and a slicer needs things that can exist. Those are different requirements, and Blender does not stop you from breaking the second one.
The most common problem is a surface with no thickness. A plane, a curved sheet, a face you extruded along an edge — all perfectly good geometry for rendering, and all impossible to print, because a printer makes solid objects and a surface has no inside. The Solidify modifier is the fix: it gives a surface a real thickness, and 1 to 2 mm is usually right.
The second is a mesh that is not closed. Holes where faces were deleted, gaps where two parts nearly meet, vertices that look joined and are not. The non-manifold check in step three finds all of these. Once they are selected, M then By Distance merges vertices that are nearly coincident, which fixes most of them at once.
The third is inverted normals — faces pointing inwards. It is invisible in solid shading. Turn on Overlays, Face Orientation in the viewport and everything blue is correct while everything red is inside out. Select all in Edit Mode and press Shift+N to recalculate.
The open-edge count shown beside the model on every converter page here is the same check, from the other end. Convert your STL and look at it: zero means closed.
The scale problem, and why it hits printing hardest
Blender is in metres by default and slicers read STL as millimetres, and an STL records no unit at all to settle the argument. So the number in the file is taken at face value, and a model built as 0.06 units — 60 mm in metres — arrives in the slicer as a part six hundredths of a millimetre wide, which most slicers show as a dot or reject entirely.
Setting the scene units to millimetres as in step one is the fix that stays fixed. It changes what Blender itself considers a unit to be, so the sidebar dimensions read in millimetres, the modifiers behave sensibly, and the exported numbers come out at the scale a slicer expects.
The alternative — leaving the scene alone and setting the exporter’s Scale field to 1000 — works too and is faster if you are exporting once and never coming back. The danger is that it is invisible: nothing in the scene records that you did it, so the next export from the same file is wrong unless you remember.
Converting the STL to 3MF afterwards is worth considering for anything you will send to someone else. A 3MF states its unit inside the file, so the question cannot be got wrong again by whoever opens it next.
How many triangles you actually need
Blender makes it easy to produce far more triangles than a printer can use. A Subdivision modifier at level 3 multiplies the face count by 256, and at level 4 by 1024. A model that was a comfortable 20,000 triangles becomes five million, and the slicer slows to a crawl for detail the nozzle cannot reproduce.
The useful limit is set by the nozzle. A 0.4 mm nozzle cannot print a feature smaller than about 0.2 mm, so triangles smaller than that are describing something that will never appear in the plastic. For a hand-sized object, a few hundred thousand triangles is generous and a million is excessive.
If your export is enormous, the Decimate modifier with Collapse at a ratio of 0.2 or 0.3 usually loses nothing you can see. Check the result in the viewport before exporting — decimation can flatten a subtle curve you cared about, and it is much easier to notice on screen than in a print.
Where we can actually help
Once you have the STL, converting it to 3MF records the units permanently so nobody downstream can get the size wrong — and the converter tells you whether the mesh is closed while it is at it.
Questions
- Why is my model tiny in the slicer?
- Blender exported metres and the slicer read millimetres. Set the scene units to millimetres in Scene Properties, or set the STL exporter’s Scale field to 1000. Do one, not both.
- The slicer says my model is not watertight.
- Use Select, All by Trait, Non Manifold in Edit Mode to find the problem. Usually it is a surface with no thickness — fix it with the Solidify modifier — or vertices that look joined but are not, which M, By Distance will merge.
- Should I export STL or 3MF from Blender?
- STL from Blender, because Blender’s 3MF support needs an add-on and the exporter here is better tested. Then convert the STL to 3MF on this site if you want the units recorded in the file.
- Do I need to apply modifiers before exporting?
- The exporter applies them for you. Converting to mesh first is still worth it, because it shows you exactly what will be written — in particular whether a Subdivision modifier is set higher for render than for the viewport.
- Binary or ASCII STL?
- Binary. It is the default, it is about five times smaller, and every slicer reads it.
Where to go next
Related guides
Tools that do part of this here
About these formats
Checked against current software versions on 2026-09-11