STL & OBJ Viewer with Print-Readiness Check

How to use it

1. Open your file. Drag an STL or OBJ onto the panel, or use the file button. The format is worked out from the contents rather than the extension, so a file someone renamed still opens correctly.
2. Read the verdict. Ready to print, Printable with warnings, or Likely to fail — with the reason next to it, not just a colour.
3. Find the problem. Boundary edges are drawn in red on the model and non-manifold edges in orange. They show through the solid, so a hole on the far side is still visible without rotating to hunt for it.
4. Fix it, then check again. The counts should go to zero.

Nothing is uploaded. The file is read and analysed by your browser inside this page, which you can confirm for yourself: open the network tab, load a model, and watch nothing leave. There’s no share link on this tool on purpose — a model file is often unreleased geometry, and that doesn’t belong in a URL.

What makes an STL printable

An STL is nothing but a list of triangles. It has no units, no notion of an object, and no rule that says the triangles have to add up to a solid. That last part is the whole problem.

A printable model is watertight: a closed surface with a definite inside and a definite outside. There’s a precise way to say that. In a closed surface, every edge is shared by exactly two triangles, and those two triangles walk that edge in opposite directions.

Everything a slicer needs follows from that one rule, and so does every way it goes wrong:

• An edge used by one triangle is a hole. The surface stops there, so at that layer the slicer can’t say which side is material.
• An edge used by three or more is non-manifold. There are several answers to what’s inside, and none of them is right.
• An edge used by two triangles that both walk it the same way means those two faces disagree about which direction is out. That’s a flipped normal.

A viewer that only renders will show you all three of these as a perfectly nice picture. Shading doesn’t care whether a surface is closed. That’s why a model can look completely fine and still refuse to slice.

The seven checks, and what each one means

Every check below runs on every file, and each one reports a count rather than a verdict on its own.

Two limits worth stating plainly. Self-intersection is the only check here that isn’t linear in the triangle count, so above 50,000 triangles it’s skipped and labelled as skipped rather than quietly passed. And it tests faces that cross each other, not faces lying exactly on top of each other — coplanar overlaps get past it.

Using it as an OBJ viewer

OBJ files open the same way and every check runs on them identically. The format is different enough that a few things are worth knowing about how it’s read.

An OBJ stores vertices once and refers to them by number, which is the opposite of STL. Face lines come in four shapes — f a, f a/b, f a/b/c and f a//c — and all four put the vertex index first, so all four work here. Indices are 1-based, and a negative index is relative to the end of the vertex list so far, so -1 means the most recent vertex. That relative form is where a lot of hand-written parsers quietly go wrong.

OBJ also allows faces with more than three sides. A quad or a pentagon is fan-triangulated on the way in, which is correct for convex faces and is what every viewer does. A concave n-gon can triangulate awkwardly, and that shows up as geometry that looks slightly wrong rather than as a check failure.

Materials, textures and vertex normals are ignored. None of them affect whether the geometry will print, and the vertex normals in particular are frequently stale — face normals are derived from the winding here instead, which is both simpler and more reliable.

STL, OBJ and 3MF compared

3MF is the better format and it isn’t close. STL has no units at all, which is why a model sometimes arrives 25.4 times too small — someone’s CAD wrote inches and someone’s slicer read millimetres. STL also can’t say “this is one object”, so anything you know about the model’s structure has to be inferred from the geometry.

None of that has displaced it. Model-sharing sites still run on STL, and it’s still what most slicers export by default. If you’re exporting from your own CAD and both are offered, take 3MF.

A worked example

The sample model is a 20 mm cube with a 10 mm pyramid on top — 20 × 20 × 30 mm overall. It was chosen because every figure it produces can be checked by hand, which is the only way to know a tool like this is telling the truth.

The second button loads the same model with two deliberate faults: one base triangle deleted, and one wall face reversed. The report changes to 3 boundary edges, 3 flipped normals and a verdict of Likely to fail — and the three open edges are drawn in red across the bottom of the model, visible straight through the solid, which is the part no other viewer does.

Both counts are exactly what the arithmetic predicts. Removing one triangle from a closed surface leaves its three edges with a single face each. Reversing one face makes it disagree with each of its three neighbours.

How much filament will it use?

Volume for a closed mesh is exact, not estimated. It’s the signed sum of the tetrahedra formed by each triangle and the origin, which is a standard result and adds up to the enclosed volume for any closed, consistently wound surface. Multiply by the material density and you have the weight of a completely solid part.

The infill column is a rule of thumb and nothing more. Real usage depends on perimeter count, top and bottom layers, and supports — a tall thin part is nearly all perimeter and lands much closer to the solid figure than a chunky one does. Densities also vary by brand and by pigment; these are the commonly published nominal values.

If the mesh isn’t closed, no volume is given. There’s no inside to measure, and a number computed anyway would be arithmetic without meaning.

Will it fit your printer?

The bounding box is checked against about twenty common build volumes, and against a 90° rotation on the bed as well, because that’s the first thing anyone tries and it genuinely changes the answer on a rectangular bed like the Prusa MK4S at 250 × 210 mm.

Twenty, deliberately. One competitor advertises 168 printers, which sounds better and is worse: a build volume that’s wrong is more damaging than one that’s absent, because it gets believed. A short list can be kept correct. Check the figure against your own machine before committing to a fourteen-hour print either way — the numbers here are the manufacturers’ published specifications, and usable area is often slightly less once a clip or a purge line is in the way.

Common problems, and where to fix them

Meshmixer’s Inspector is still the fastest way to fix a genuinely broken mesh, despite Autodesk having retired it — it marks each defect with a coloured sphere and repairs one at a time or all at once. Blender’s 3D Print Toolbox add-on covers the same ground inside a tool that’s still maintained.

What this tool doesn’t do

It doesn’t repair anything. That’s a choice, not a gap. Automatic repair guesses at what you meant, and a guess that closes a hole in the wrong place gives you a model that slices cleanly and prints wrong — which costs a lot more than the file did. Knowing there are three boundary edges and seeing exactly where they are is what lets you fix it properly.

It also doesn’t predict what your slicer will do. Slicers differ in what they repair silently, and a file that fails here may still print if yours is forgiving. These are geometry checks, stated as counts, with the reasoning shown. Treat a clean result as one class of problem eliminated, not a promise.

And it doesn’t need WebGL. If your browser can’t render 3D, every measurement and all seven checks still run and report — the page just says so where the model would be. The diagnostic is the point. The picture is how it’s shown.