Make an STL Watertight,
Free & In Your Browser
A slicer needs a sealed shell. MeshTune finds every open hole and boundary edge, closes them, and gives you a watertight, manifold STL — with a verdict that confirms it's closed. No upload, no install, no account.
Nothing is uploaded. Repairs run inside your browser, not on a server — no account, no limits. Your STL only ever leaves your machine if you deliberately attach it to a bug report.
What "Watertight" Actually Means
A watertight STL is a closed, manifold surface: every edge meets exactly two faces, there are no holes, and the shell fully encloses a volume. Pour water in and none leaks out — which is exactly the property a slicer needs to turn your surface into a solid it can print. A single open hole makes the model non-watertight, and most slicers will either refuse it or print it wrong.
No Open Holes
Every boundary loop is filled, so there are no gaps in the surface for the slicer to fall through.
No Boundary Edges
An edge used by only one face is an open border. A watertight mesh has none — every edge is shared by two.
Sealed, Manifold Shell
Coincident vertices are welded and non-manifold seams split, so the closed surface is a valid solid.
How to Make an STL Watertight
Three steps, entirely in the browser — nothing to install, nothing uploaded.
Load the STL
Drag your STL into the viewer. It's read straight from your disk and rendered instantly.
Find the Open Edges
Run Analysis — every open hole and boundary edge is highlighted in 3D, so you can see where the surface isn't sealed.
Seal It & Confirm
Click Repair to fill the holes and weld the gaps, then read the verdict — Closed means watertight. Download the sealed STL.
How to Tell If Your STL Is Watertight
Before you seal anything, it helps to know what's open. MeshTune's analysis gives you a plain answer and shows you exactly where — so you can decide whether a quick seal is enough or the model needs more work.
See Every Gap in 3D
Open holes are outlined on the model. Click one to fly the camera straight to it and judge the size of the gap.
A Plain Verdict
A summary tells you whether the model is printable and how many problems block it — no jargon to decode.
Closed / Manifold / Oriented
After sealing, the repair verdict confirms all three — the exact properties a slicer checks for a valid solid.
Watertight STL — Common Questions
What does watertight mean for an STL?
A watertight STL is a closed, manifold surface with no holes and no boundary edges — a sealed shell that fully encloses a volume. If you poured water into it, none would leak out. Slicers need this to compute a solid to print.
How do I know if my STL is watertight?
Open it in MeshTune and run Analysis. If Open holes is zero and there are no boundary edges, it's watertight. Any open hole means it isn't — and it's highlighted in 3D so you can see exactly where the gap is.
Will making it watertight change my model's shape?
Only where there were gaps. Hole filling spans the open boundaries with new triangles and welding merges coincident vertices; the rest of the surface is untouched. The change is limited to sealing what was already broken.
Is a manifold mesh the same as watertight?
Related, but not identical. Manifold means every edge is shared by exactly two faces; watertight additionally means the surface is fully closed with no boundary. MeshTune's repair produces a mesh that is both — and the verdict reports each separately.
Do I need to make my STL watertight before slicing?
Almost always, yes. Cura, PrusaSlicer and most other slicers assume a closed solid; a non-watertight model can slice with missing walls, filled cavities or skipped regions. Sealing it first avoids surprises on the print bed. For broader problems beyond holes, see the full STL repair guide.
Seal Your STL Now — Free
Drop a file into the viewer and have a watertight, printable STL in under a minute. Nothing uploaded, nothing installed.