●
3DAI
Free · runs in your browser · files stay on your device

Thicken STL

Drop an open surface, such as a scan shell, a relief, cloth or a terrain sheet, or a part that is too thin. Type a thickness in millimetres and pick the side. The tool offsets the surface, closes every open edge with a side wall and gives you a watertight STL or 3MF, with before and after wall colours.

Loading the thicken tool...
Demo with a sample file, recorded in this tool.

How to thicken an STL

  1. 1

    Drop your surface

    Drag an STL, OBJ, 3MF or GLB onto the tool, or press Choose file. The before view shows grey where the model has no wall at all.

  2. 2

    Set thickness and side

    Type the thickness in millimetres. Inward adds it behind the surface and keeps the front where it is; outward adds it in front; centred puts half on each side.

  3. 3

    Check the after view

    The tool measures the new walls. Green means at least the thickness you set; red marks spots that came out thinner, which is where the surface bends too tightly.

  4. 4

    Download and slice

    Save a watertight STL or 3MF and open it in your slicer. Add a base or find the best print orientation next if you need to.

What happens to your model

Every vertex moves along the averaged normal of the faces around it, corrected at creases and corners so each face moves by the full thickness. What else happens depends on the kind of model:

Open shell (a scan, a dome, a helmet-like shell)Two copies of the surface, the thickness apart, joined by a wall along every open edge. Every edge is then shared by exactly two faces, which is what a slicer needs.
Single-sided surface (a relief, a terrain sheet, cloth)The same: the sheet becomes a slab of the thickness you set. Inward puts the material under a relief so its front stays exactly as it was.
Closed part that is too thinClosed parts can only grow outward: every surface moves out by the thickness, so each wall gets twice the thickness thicker and the part gets bigger by twice the thickness in each direction.
Several parts that now touchParts that overlap after thickening are joined into one solid with a boolean union, so the slicer does not see faces inside faces.

The limit, plainly: where the surface bends more tightly than the thickness (thin spikes, sharp inside corners, deep creases), the offset folds through itself and those spots pinch. The tool counts the folded faces and tells you the largest thickness without any. A closed part that must keep its outside size, or a mechanical part with exact walls, needs a shell or offset command in CAD.

Check, thicken, hollow: one pipeline

Wall thickness checker →Find the walls that are too thin before and after.Hollow STL →The opposite job: remove material from inside a solid to save resin or filament.Add base to STL →Stand a thickened relief or figure on a flat base.

Made with AI and too thin?

Make a 3D model from a photo or a text prompt, thicken any thin shell here, then put it on a base and find the best way to lay it on the print bed. The AI generators use credits; this tool does not.

Image to 3D with AI →Text to STL with AI →Add a baseBest print orientation

More free 3D printing tools

Print orientation optimizerSTL viewerScale STL3D print cost calculatorImage to STL with AI

Thicken STL FAQ

Drop the STL onto the tool at the top of this page, type the thickness in millimetres and pick inward, outward or centred. The tool offsets the surface by that amount, closes the open edges with side walls and shows the result with its measured wall thickness. Download it as STL or 3MF.
A slicer needs a closed volume: an inside and an outside. A single surface, like a scan shell or a relief exported as one sheet, has no inside, so there is nothing to fill and the slicer prints nothing or shows errors. Thickening turns the sheet into a thin solid with two sides and closed edges.
Inward keeps the visible front exactly where it is and adds material behind it, which suits reliefs, terrain and the outside of a shell. Outward keeps the back where it is. Centred puts half the thickness on each side. If the tool guessed the front and back the wrong way round for your file, switch between inward and outward.
Typical starting points from printer and print-service design guides are about 1 mm (two lines of a 0.4 mm nozzle) for FDM, 1.2 mm or more for parts that take load, and about 0.6 mm for resin walls that stand on their own. Your printer or print service gives the real minimum in its design guide. You can check any file with the free wall thickness checker.
A closed part already has an inside, so it can only grow outward here: every surface moves out by the thickness you set, and every wall gets thicker by twice that. The part also gets bigger. If you need the outside to stay the same size and only the walls to change, use a CAD shell command on the original design.
Where the surface bends more tightly than the thickness, for example a thin spike, a sharp inside corner or a deep crease, the offset surface folds through itself. The tool counts those folded faces, tells you the largest thickness without any, and shows thinner spots in red. For such parts a smaller thickness, a smoother mesh or CAD is the honest answer.
For open shells and sheets every edge of the result is checked: it must be shared by exactly two faces, once in each direction. The result is also passed through a solid-geometry check that joins overlapping parts. The panel shows the result of both checks, the volume and the measured wall thickness.
STL (binary and ASCII), OBJ, 3MF and GLB up to 100 MB and 1 million triangles. Units are millimetres unless you change them, because STL and OBJ do not store units. Faces that point different ways are turned to match before thickening.
No upload: the file is read and thickened in your browser and never leaves your device. The files you download are free for personal and commercial use, with no attribution needed. Your own model stays yours.

Thickness is made and measured in your browser. Downloads are free for personal and commercial use, no attribution needed. Wall values in the FAQ are typical starting points, not guarantees; check the design guide of your printer or print service.