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3DAI
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Hollow STL Online

To hollow an STL for resin printing, keep an outer wall of 1.5 to 3 mm, empty the inside, and add at least two drain holes near the build plate. Load your model below, pick the wall thickness, check it in the cut-away view and download the hollow STL.

Loading the hollowing tool...

How to hollow an STL for resin printing

  1. 1

    Load your model

    Drop an STL (or OBJ, GLB, 3MF) and set its print size. It is read on your device, never uploaded.

  2. 2

    Set the wall

    Pick a wall thickness, usually 1.5 to 3 mm for resin, and a resolution. The tool shows the voxel size and a time estimate.

  3. 3

    Add drain holes

    In drain-hole mode, click the model or use Auto to place 2 holes of 2 to 5 mm near the bottom.

  4. 4

    Check and download

    Use the cut-away view to see the walls, read the resin saved, then download the hollow STL for your slicer.

No model yet? Make a printable STL from a photo with AI

Upload one photo and get a 3D model with an STL download, ready to hollow here. The AI generators use credits; this hollowing tool is free.

Image to STL (AI) →or describe it: Text to 3D
3D print cost calculator →Price the print: resin or filament, time and power.STL repair →Close holes and fix inside-out triangles before hollowing.STL volume calculator →Volume, weight and size of any STL.

Hollow STL FAQ

Load the STL above, set the print size and a wall thickness (2 mm is a good start), and press Hollow it. Keep outer surface mode adds an inner shell and leaves your surface untouched; add drain holes in your slicer. Remesh with drain holes mode cuts the holes for you. Check the cut-away view, then download.
Most resin prints use 1.5 to 3 mm. Small figures can go down to about 1.5 mm; larger or heavier parts, and tough or flexible resins, are safer at 2.5 to 3 mm. Walls under about 1 mm can crack, warp or leak while washing and curing.
A closed hollow traps liquid resin, which can leak later or crack the part as it cures, and the sealed pocket creates suction against the resin tank on every layer. Use at least two holes of 2 to 5 mm close to the build plate: one lets resin drain, the other lets air in.
Usually not. FDM slicers already print the inside with sparse infill, set as a percentage, plus a few solid walls, so the part is mostly empty anyway. Hollowing mostly matters for resin (SLA, MSLA, DLP) printers, which would otherwise cure the whole inside solid.
In Keep outer surface mode, no: your original triangles are kept exactly and only an inner shell is added. Remesh with drain holes mode rebuilds the whole surface from a voxel grid so it can cut real holes, which softens details smaller than one voxel. Pick 256 for detailed models.
It depends on the shape. Chunky models save the most: a 40 mm ball with 2 mm walls needs about 9 ml instead of 33.5 ml, a saving of over 70%. Thin parts save little. The tool shows the solid and hollow volume in ml and the money saved at your resin price per litre.
No. Reading, hollowing and writing the STL all happen in your browser on your own device. The file is never sent to our servers, so it also works for private or unreleased models.
Yes. The download is a standard binary STL with the outer wall and the inner shell, so any slicer reads it as one hollow solid. If you used Keep outer surface mode, add drain holes in the slicer before you slice.

PrusaSlicer, Chitubox and Lychee are named only to describe compatibility; The3DAI is not affiliated with or endorsed by them.