Why an STL is harder to modify than a CAD file
An STL holds triangles and nothing about how the part was built. A 10 mm hole is a ring of flat facets, so making it 12 mm means rebuilding the hole, and a mesh editor has no dimension to change. Pick the route by the job:
| Route | Good at | Poor at |
|---|---|---|
| Mesh editor in the browser | size, orientation, cuts, joins, repair, text | changing features, organic shapes |
| CAD with the source model | hole sizes, fillets, wall thickness | files you only have as STL |
| Sculpting app | organic forms, moving single vertices, smoothing | exact millimeter work |
Edits you can make to an STL in the browser
| Edit section | What it does |
|---|---|
| Size | scale by % or to an exact length in mm on one axis; Inches → mm (×25.4) and back |
| Rotate, mirror, move | any angle on X, Y, Z; mirror with normals kept outward; lay on bed; center |
| Cut with a plane | two closed parts with a capped cut, each saved as STL |
| Combine & booleans | add a second file, then A ∪ B, A − B or A ∩ B |
| Reduce triangles | keep a chosen %; a sphere at 30 % kept its volume within 2 % |
| Repair | weld vertices, drop bad triangles, fix normals, close simple holes |
| Add text | raised or engraved, Latin and Cyrillic, Noto Sans Bold |
Undo and Redo go back and forth through 20 steps.
How to edit STL files step by step
- Open the file in the viewer on this page, or in the online STL editor.
- Press Edit and open the section you need.
- Apply the change and check the new numbers in Info.
- Press Convert and save as STL, or as 3MF with millimeters written into the file.
All of it runs in the tab, and the model never leaves your computer. To keep the dimensions under control, scale to a target length with scale STL instead of guessing a percentage.
A typical edit order before printing
- Run the mesh check and Repair if the shell is open.
- Fix the units: Inches → mm if the part came in 25.4 times small.
- Rotate so the largest flat face is down, then lay on bed.
- Cut the model with a plane if it is taller or wider than your printer.
- Reduce triangles on heavy scans so the slicer stays responsive.
Doing the repair first matters, since every later step needs a closed mesh. An open shell that slips through can fail hours into a print, and the filament is gone by then. Save a 3MF at the end if you want the millimeters recorded in the file.
Repair first, then cut or combine
Booleans, cutting and text need a closed mesh. If the shell is open, the tool asks you to run Repair first, and the message lists what it fixed and what is still open. In our test an open box became watertight at 8000 mm³.
Hole filling uses a simple fan of triangles. It suits flat or small holes; a large gap on a curved surface comes out flat. Read more on STL repair, or split an STL into parts that fit your bed.
Jobs that need a different tool
Sculpting, pulling single vertices, smoothing and remeshing belong in a sculpting app. Hollowing with a set wall thickness and support generation also live elsewhere, in a slicer or a dedicated tool. For new holes, fillets or a thicker wall, go back to the CAD source.
If only the STL exists and you want to continue in CAD, STL to STEP writes a faceted solid. Every triangle becomes a flat face, about 1 KB each, so it opens as a body without true cylinders.
Questions and answers
Can I edit an STL file?
Yes, as a mesh: resize, rotate, cut, join, repair or add text. Changing a hole or a fillet as a feature needs the original CAD model.
How do I make an STL bigger?
Open Edit → Size and enter a percentage or the target length of one axis in mm. The other axes follow in proportion.
How do I cut an STL in half?
Use Cut with a plane. Both halves come out closed, with the cut face capped, and each can be saved as its own STL.
Can I add text to an STL?
Yes, raised or engraved on top of the model, in Latin or Cyrillic letters. The mesh must be watertight first.
Can I modify an STL in CAD?
CAD imports it as a mesh body. Simple cuts and joins work there too, while feature edits need a model rebuilt from scratch or the source file.
Will editing change the units of my STL?
STL stores no units, and the editor treats the numbers as millimeters. Inches → mm multiplies them by 25.4; saving as 3MF writes millimeters into the file so the next program reads the size right.
How many steps can I undo?
Twenty. Undo and Redo walk back and forth through the last 20 edits, so you can try a cut and step back if it lands wrong.