Guides / Updated October 20, 2026

How to edit an STL file

You can edit an STL — scale it, cut it, join it, sculpt it. What you can't do is go back and change a dimension you set in CAD, because the file only remembers triangles, not the shape it started as.

First, what an STL can hold

An STL is a list of triangles: three corners and a facing direction each. No history, no sketch, no "hole diameter 4 mm" — just the surface that came out the other end.

That one fact decides the tools. A mesh editor moves those triangles. CAD cannot open the file and show you a feature tree, because there isn't one in there.

What each kind of tool can actually do

You want to	Mesh editor (browser, Blender)	CAD (Fusion, FreeCAD, Onshape)
Change the size, rotate, mirror	Yes, and it keeps proportions if you lock them	Yes, but you convert the mesh first
Cut a hole, add a boss, change wall thickness	Awkward — a boolean and a lot of cleanup	This is the reason to use CAD
Join two prints into one part	Yes, as a union of triangles	Yes, as real solids
Repair holes and flipped faces	Yes — that is what STL repair tools do	Usually worse at it
Keep the change editable next week	No — every edit bakes into the mesh	Yes

If the change has a number attached (a 4 mm pin, a 0.5 mm wall), CAD will save you time by the second edit.

The browser route: resize, repair, combine

Set the exact size first. STL carries no unit, so a model can arrive 25× too small (saved in inches) or 10× too small (centimetres). Type the height you want in millimetres and let the editor scale the rest.
Rotate to the printing orientation before anything else — a part lying flat prints better than one balanced on a corner, fewer supports and less scarring.
Join or overlap parts with a boolean, then check for non-manifold edges. A union is the step that most often leaves three-way edges behind.
Save as 3MF if the file has several parts: STL merges them into one mesh and you lose the ability to move a part afterwards.

The Blender route: sculpt and boolean

Import the STL, then switch to Edit Mode to move vertices. Good for organic shapes — minis, props, repairs on a scan.
For a precise cut use a Boolean modifier: model a cylinder where the hole goes, set the operation to Difference, apply, then delete the cutter.
Booleans multiply triangles and produce long thin ones. Decimate afterwards if the slicer struggles, and run a repair pass to catch the open edges the boolean left.
A Remesh modifier rebuilds the surface from scratch — often faster than patching a scan, at the cost of detail.

The CAD route: when the edit has to be exact

Converting a mesh to a solid is the step that decides everything. In FreeCAD it is Part → Create shape from mesh, followed by Part → Refine. In Fusion 360 you insert the mesh and then convert it to a BREP body.

Both work best on clean, low-poly models. Triangles with long slivers convert into hundreds of tiny faces, and every later cut has to chew through them — a scan of a figurine is a bad candidate, a mechanical bracket is a good one.

Once it is a solid, edit it the way you would edit anything in CAD: change the hole diameter, the wall thickness, the fillet radius. That is the whole point — a mesh edit is a one-off, a CAD edit is a value you can change twice.

Before you print the edited file

Check	Why it bites after editing
Wall thickness	Scaling down a 1.2 mm wall to 0.9 mm is still printable at a 0.4 mm nozzle; below about 0.8 mm the slicer starts dropping perimeters
Non-manifold edges	Booleans and decimation both create them; slicers either warn or silently drop the inner wall
Units	A resize is a guess if you never typed a real measurement — check the bounding box against the part in your hand
Triangle count	A repair or a remesh can turn a 50k model into 500k; it prints the same and slices slower

Run the edited file through the viewer once: the bounding box and the triangle count are on screen, and the cost calculator takes it from there.