STL inspector
Paste an ASCII STL, or a binary STL's bytes as base64 or hex, and read the whole mesh at once — triangle and welded-vertex counts, bounding box, bounds and centre, surface area, signed and absolute volume, watertight and manifold status, open and non-manifold edges, disconnected shells, degenerate and duplicate facets, stored-normal mismatches, and an optional weight estimate from material density. Read-only: your mesh is never modified, and it runs locally in your browser.
About this tool
STL is the format almost every slicer and CAD package exports, and it is deliberately minimal: a bare list of triangles, with no units, no vertex sharing, no colours and no declared topology. That makes it easy to write and easy to get subtly wrong — a mesh can look perfect on screen and still be inside-out, cracked along a seam, or made of two objects that were never meant to touch.
This inspector reads one STL and reports everything it can measure, without changing a single byte:
- Input — which encoding was actually found, and the solid name or the 80-byte binary header text.
- Geometry — triangle count, distinct welded vertices, bounding box, explicit min/max bounds, centre, surface area, absolute volume, and signed volume. The sign is the interesting one: on a closed mesh a negative signed volume means the normals point inward, i.e. the model is inside-out.
- Mesh integrity — watertight and manifold status, open (boundary) edges,
non-manifold edges, edges whose two faces disagree on winding, disconnected shells,
degenerate (zero-area) triangles, duplicate triangles, facet normals that disagree with
the geometry by more than 1°, unset
(0,0,0)normals, and non-zero attribute bytes (where some exporters hide per-facet colour). - Verdict — print-ready, or a list of the specific defects that stop it, plus notes for things worth knowing that do not block a print.
Both STL flavours are accepted. ASCII STL is pasted as text. Binary STL is bytes,
so paste them base64- or hex-encoded (base64 model.stl or xxd -p model.stl); hex may
carry spaces, colons or dashes, and base64 may be standard or URL-safe, padded or not.
Auto-detection identifies binary by the 84 + 50 × triangle_count length rule rather than
a leading solid keyword — plenty of exporters write the word "solid" into a binary
header, which is the classic reason a file is mis-read as text.
Everything runs locally in your browser. The mesh you paste never leaves the page.
Worked example
A tetrahedron with corners at the origin and 10 mm along each axis, inspected with
density = 1.24 (PLA):
solid tetra
facet normal 0 0 -1
outer loop
vertex 0 0 0
vertex 0 10 0
vertex 10 0 0
endloop
endfacet
... three more facets ...
endsolid tetra
Report:
STL inspection report
=====================
Input
Encoding ASCII STL (text, auto-detected)
Solid name / header tetra
Units mm
Scale factor 1
Geometry
Triangles 4
Distinct vertices 4
Bounding box 10 x 10 x 10 mm
Bounds min 0 0 0 / max 10 10 10 mm
Center 5 5 5 mm
Surface area 236.60254 mm²
Volume 166.666667 mm³ (0.166667 cm³)
Signed volume 166.666667 mm³ (positive — normals face outward)
Estimated weight 0.206667 g at 1.24 g/cm³
Mesh integrity
Watertight yes
Manifold yes
Open (boundary) edges 0
Non-manifold edges 0
Inconsistent winding 0
Disconnected shells 1
Degenerate triangles 0
Duplicate triangles 0
Normals mismatched 0
Normals unset (0,0,0) 0
Attribute bytes set 0
Verdict
Print-ready yes — closed, manifold, consistently wound, normals outward
The volume checks out by hand: a corner tetrahedron is 10 × 10 × 10 / 6 = 166.67 mm³, which is 0.1667 cm³, and at 1.24 g/cm³ that is 0.2067 g of plastic.
Deleting one facet from the same mesh flips the verdict to
no — 3 open (boundary) edges, with Watertight no and the volume line marked
approximate, the mesh is not closed.
Limits and edge cases
- 100,000 triangles maximum. A binary STL at that cap is about 5 MB of bytes, or 6.7 MB once base64-encoded — already an awkward paste. Larger meshes are rejected with a clear message rather than freezing the tab, and a binary file whose header declares more than the cap is rejected before any bytes are read.
- STL carries no units. The
unitssetting only labels the output and converts the volume to cm³ for the weight estimate; it never rescales the mesh. Usescalefor that —25.4turns a model authored in inches into millimetres,2doubles every dimension and multiplies the volume by 8. - The weight estimate assumes a 100 % solid part. Infill, wall count, supports and material shrinkage are slicer decisions and are not modelled. Treat it as the upper bound.
- Volume needs a closed mesh. The divergence-theorem sum still produces a number for an open mesh, but it is meaningless; the report says so on the volume line instead of quietly printing it.
- Welding drives almost everything. STL stores each triangle's three corners
independently, so vertex count, edge topology, shells and the watertight verdict all
depend on
weld_tolerance(default0.000001). Raise it to0.001to find out whether hairline export cracks are the only thing keeping a mesh open; set0to merge only bit-identical coordinates. - Binary coordinates are 32-bit floats, so a value like
0.1round-trips as0.100000001. Measurements of a binary mesh differ from the same mesh in ASCII in the seventh significant digit — expected, not a bug. - It only reports. Nothing is welded, re-wound, hole-filled or simplified; use a mesh repair tool for that and re-run this check afterwards.
- No 3D viewer. This page returns text and JSON, not a rendered preview — you cannot rotate or zoom the model here. Load it in a slicer or mesh editor for that.
- Only STL is read. OBJ, PLY, 3MF, STEP and glTF are different formats and are rejected.
FAQ
How do I paste a binary STL?
Encode the file's bytes first, then paste the encoded text. On macOS or Linux,
base64 model.stl | pbcopy (or | xclip) gives you base64; xxd -p model.stl gives you
hex. Either is accepted, and the encoding is auto-detected — hex is tried before base64,
because every hex string is also valid base64 and would otherwise be mis-read. If
auto-detection guesses wrong, set the input encoding explicitly to "Binary STL bytes,
base64" or "Binary STL bytes, hex".
My mesh says it is not watertight, but it looks closed. What is wrong?
Almost always the corners do not match exactly. STL writes each triangle's vertices
separately, so two faces only share an edge if their corner coordinates land on the same
welded vertex. Rounding in the exporter can leave neighbours a fraction of a micron apart,
which shows up as thousands of open boundary edges along seams that look perfectly closed.
Raise the weld tolerance to something like 0.001 (in the mesh's own units) and re-run: if
the open-edge count drops to zero, the geometry is fine and only the file's precision was
the problem. If it stays high, there are real holes.
What does a negative signed volume mean?
The volume is computed by summing signed tetrahedra from the origin to each triangle, so its sign follows the winding order. On a closed mesh, a positive result means the facet normals point outward — the convention every slicer expects. A negative result means the mesh is inside-out: it will often still slice, but supports, wall ordering and boolean operations can come out backwards. The absolute volume is reported either way, and the verdict calls out an inverted mesh explicitly. On an open mesh the sign is not conclusive and the report says so instead.
Why do the stored facet normals not have to match the geometry?
Every STL facet stores a normal vector alongside its three corners, but the corner winding
already implies one. When the two disagree by more than 1°, the file is internally
inconsistent — usually because a mesh was mirrored or scaled by a negative factor without
recomputing the normals. Most slicers ignore the stored value and recompute from the
winding, so a mismatch is reported as a note rather than a defect. A (0,0,0) normal is
counted separately: it is explicitly allowed by the format and simply means "derive it".
Are several disconnected shells a problem?
Not by itself. A shell is a group of triangles connected through shared welded vertices, so a plate with three separate parts on it legitimately reports three shells and still prints. The count is worth checking anyway: an unexpected extra shell is usually a stray fragment left behind by a boolean operation, or a duplicate object sitting exactly on top of the first one. If each shell is individually closed, the whole file is still watertight and the verdict stays print-ready, with the shell count listed as a note.
Can I use this in a script or in CI?
Yes — switch the report format to JSON. You get one flat object with triangles,
distinct_vertices, bbox_min, bbox_max, size, center, surface_area, volume,
signed_volume, volume_cm3, weight_grams, watertight, manifold,
outward_normals, boundary_edges, non_manifold_edges,
inconsistent_winding_edges, shells, degenerate_triangles, duplicate_triangles,
normals_mismatched, normals_unset, attribute_bytes_set, print_ready, plus issues
and notes arrays. Gate a build on print_ready, or assert a bounding box before sending
a part to a printer. The same inspection is available from the command line, which is
usually the easier CI path.
Will it repair the problems it finds?
No. This tool is read-only on purpose, so the report can be trusted as a description of the file you actually have — nothing is welded, re-wound, hole-filled, decimated or re-exported, and no output mesh is produced. Fixing holes, flipping inverted normals and dropping degenerate facets are separate, destructive operations; run a mesh repair tool for those, then paste the result back here to confirm the numbers moved the way you expected.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool stl-inspector "solid demo
facet normal 0 0 -1
outer loop
vertex 0 0 0
vertex 0 10 0
vertex 10 0 0
endloop
endfacet
endsolid demo"New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/stl-inspector/?stl=solid%20demo%0A%20%20facet%20normal%200%200%20-1%0A%20%20%20%20outer%20loop%0A%20%20%20%20%20%20vertex%200%200%200%0A%20%20%20%20%20%20vertex%200%2010%200%0A%20%20%20%20%20%20vertex%2010%200%200%0A%20%20%20%20endloop%0A%20%20endfacet%0Aendsolid%20demo&input_format=auto&output=report&units=mm&scale=1&density=1.24&weld_tolerance=0.000001Machine-readable descriptor: tool.json — title + parameters JSON Schema for agents.