watertight

Free, open-source terminal app that repairs STL files for 3D printing.

Pick files in a built-in file explorer, queue as many as you like, and watch them repair in parallel. Each fixed file is saved next to the original as <name>_repaired.stl. Files that are already watertight are skipped. Your originals are never changed.

 watertight 0.1.0 │ workers 4/7 │ selected 0 │ overwrite off │ trimesh✔ manifold3d✔ pymeshfix✔
╭──────────────────────────────────────╮╭──────────────────────────────────────────────────────────╮
│ EXPLORER                             ││ QUEUE                                                    │
│📂 prints                             ││ #    File              Status                  Progress  │
│├── [ ] 📄 bracket.stl  634 B         ││ 1    bracket.stl       ✔ Done                  ██████████│
│├── [ ] 📄 broken.stl  48 B           ││ 2    broken.stl        ✘ Failed                ██████████│
│├── [ ] 📄 edge.stl  1.3 KB           ││ 3    edge.stl          ! Needs review          ██████████│
│├── [ ] 📄 gear.stl  684 B            ││ 4    gear.stl          ✔ Done                  ██████████│
│├── [ ] 📄 hinge.stl  734 B           ││ 5    hinge.stl         ✔ Done                  ██████████│
│└── [ ] 📄 vase.stl  684 B            ││ 6    vase.stl          ✔ Skipped (watertight)  ██████████│
╰──────────────────────────────────────╯╰──────────────────────────────────────────────────────────╯
 Overall  ━━━━━━━━━━━━━ 6/6 finished · 3 repaired · 1 skipped · 1 review · 1 failed · elapsed 00:02

Install

Needs Python 3.10 or newer.

pipx install watertight          # or: uv tool install watertight

From a clone of this repo:

pipx install .                   # or: uv tool install .   or: pip install .

Stronger repair (optional)

pipx install "watertight[meshfix]"

This adds pymeshfix, which fixes harder problems (for example two holes that touch at one point). It is installed only if you ask for it, because pymeshfix is GPL-3.0 (its PyPI metadata also lists AGPL-3.0), while watertight itself is MIT. The app detects it at run time and uses it as the last repair step. The header line shows pymeshfix✔ or pymeshfix✘.

Use

watertight                       # open the TUI in the current folder
watertight ~/prints              # open the TUI in ~/prints
watertight a.stl b.stl           # open the TUI and queue these files right away
watertight --headless a.stl b.stl --jobs 2     # no UI, text report, exit codes

Keys

Network drives

Press l to see what your computer already has mounted. watertight does not mount shares and never asks for passwords: connect the drive with your system first, then press l.

A slow or offline drive cannot freeze the app: folders open with an 8 second limit, and listing files runs in the background. Repairing over the network reads the whole file and writes the result next to it, so a slow link means slower jobs (there is no time limit by default; add --timeout SEC if you want one). Only the macOS and Linux lists were tested on real mount output formats; the Windows drive list follows the Win32 API and has not been run on Windows.

Headless exit codes

How it works

A file is OK (skipped) when it is watertight, has consistent winding, and has positive volume. Otherwise the repair runs in steps and stops at the first step that gives an OK mesh:

The result is checked against three limits. If any one is broken, the result is not saved (the next-best result that stays inside the limits is used instead, or nothing is written):

The tool never rescales, moves or rotates your part. Select a row to see what was wrong, what each step did, and each limit with its measured value.

Keep trying (--refine, key f)

After a good repair, the tool can try other settings and keep the best result. It stops when no nearby setting is better (or after --refine-tries N, default 8) and tells you so. It takes longer, so it is off by default. The file is written once, at the end, so Ctrl+C still saves nothing.

Memory

Big files need memory (about 14 times the file size). The queue starts a job only if the running jobs plus this one fit in 60 % of your installed memory. At least one job always runs.

Limits (please read)

• Some meshes cannot be fixed automatically. Those end as Needs review with the reason.
• Two bodies that only touch along an edge cannot become manifold without moving geometry. They end as Needs review. pymeshfix may drop one of the bodies; the size check flags it.
• Big, curved holes are closed with a simple fan. Check the patch before you print.
• STL only (binary and ASCII in, binary out). No 3D preview.

Benchmark

python scripts/benchmark.py generates a synthetic defect set (holes, flipped faces, inside-out, duplicates, degenerate faces, floating specks, internal fins) and runs it. Result on one 8-core Mac, with pymeshfix installed:

These are not real-world numbers. For those, run the script on your own folder of imperfect files: python scripts/benchmark.py --folder ~/stl.

Develop

uv venv && uv pip install -e ".[dev,meshfix]"
.venv/bin/pytest
.venv/bin/ruff check src tests

See CONTRIBUTING.md. Library use: from watertight import check_file, repair.

Privacy

No network calls. No telemetry. The app only reads STL files you point it at.

License

MIT. See LICENSE and THIRD_PARTY_LICENSES.md.