STL analyzer and 3D viewer: volume, weight, cost and mesh check

Analyse an STL file: volume, size and printability in seconds

Before you start a 3D print you want to know the volume of the part, its dimensions, its weight in filament and whether the mesh is closed. This STL analyzer reads STL (binary or ASCII), OBJ and 3MF files, computes surface area, volume, centre of mass and shell count, and runs a watertight check: open edges, non-manifold edges, normal orientation, degenerate and duplicate triangles.

The 3D viewer shows the model on a printer bed, paints open edges in red and warns when the part exceeds the build volume. You can then scale, rotate by 90°, convert inches to millimetres, flip normals, drop the part to Z = 0 and export as binary STL, ASCII STL or OBJ. Everything runs in the browser: your file never leaves your device, even a 250 MB model.

1. Load your file

   Click “Choose a file” or drop an STL, OBJ or 3MF onto the dashed area. The sample cube is displayed on load so you can explore the tool. Pick the file unit if your CAD software exported in inches or centimetres.

2. Read the mesh and the printability check

   The “Mesh” block lists triangles, vertices, size, surface area, volume and centre of mass. The “Printability check” block tells you whether the mesh is closed, how the normals are oriented and how many open or non-manifold edges it has.

3. Set the print estimates

   Choose the material, infill, perimeters, line width, top/bottom layers, layer height and price per kg: weight, filament length and cost update instantly, with solid-part and resin variants.

4. Transform if needed

   Use “Scale (%)”, “Scale to size”, “Rotate 90° X/Y/Z”, “Convert units”, “Flip normals”, “Centre on plate” or “Drop to Z = 0”. Every operation re-runs the analysis and the bed check.

5. Export

   Download the transformed model as “Binary STL”, “ASCII STL” or “OBJ”. The name is taken from the original file and the binary STL size is shown before you download.

How the STL analyzer works

A binary STL is recognised by its exact size, 84 + 50 × n bytes for n triangles, not by the word “solid” in the header, which some programs also write into binary files. ASCII STL is read in chunks, handling Windows line endings, scientific notation and solid names with spaces. OBJ files are read from their v and f lines, with polygons split into fans and negative indices resolved. A 3MF is a ZIP archive: the tool reads the central directory, inflates the model part named by the package relationships (usually 3D/3dmodel.model), applies the component and build-item transforms and converts the declared unit to millimetres. Identical vertices are then merged to rebuild the topology.

• Surface area: sum of half the cross-product norm of every triangle.
• Volume: sum of the signed tetrahedra v0 · (v1 × v2) / 6; a negative total reveals inward normals, an open mesh makes the value unreliable.
• Watertight check: every edge must be used by exactly two triangles travelling in opposite directions; one use = open edge, more than two = non-manifold, two in the same direction = inconsistent orientation.
• Shells: connected components over shared vertices (union-find), which reveals cavities and multi-part files.
• Estimates: shell ≈ wall area × perimeters × line width + flat area × layers × layer height, capped at the volume; inner volume × infill; weight = material volume × density; length = volume ÷ filament cross-section.

Typical filament and resin densities

What an STL analyzer is good for

• Quoting a print: weight × price per kg gives the material cost of a quote, for hobbyists and print services alike.
• Checking a downloaded model before slicing: watertightness, multiple shells, inverted normals and the real dimensions of a Printables or Thingiverse file.
• Fixing units: a model exported in inches shows up 25.4 times too small; the inch → mm conversion fixes it before export.
• Shrinking an ASCII STL: the binary export divides the size by four to six without touching the geometry, handy for sharing or archiving.
• Preparing the bed: test whether the part fits in 220 × 220 × 250 mm, 256 × 256 × 256 mm or a custom plate, rotate it by 90° and drop it to Z = 0 before opening the slicer.

Common problems and how to fix them

• The model is 1 mm instead of 25 mm: it was exported in inches. Pick “inches” as the file unit or apply “inch → mm (× 25.4)”.
• Volume marked “unreliable”: the mesh has open edges. Show them in red with “Open edges”, then repair the model in your CAD software or a repair tool before analysing again.
• Orange faces in the 3D view: those triangles are seen from the back, so their normals point inwards. If the whole part is orange, click “Flip normals”.
• “Exceeds the plate”: try a 90° rotation around X or Y, or “Scale to size” on the longest axis; the message updates after every transform.
• Very large file: above 250 MB or 5 million triangles the tool asks for confirmation. Export binary rather than ASCII and reduce the triangle count in your modelling software if the device runs out of memory.

Privacy: the file is read, analysed, displayed and exported by your browser; nothing is sent to or stored on a server. Limits: 600 MB or 12 million triangles at most, depending on device memory; weight, length and cost figures are estimates that only the slicer refines; 3MF is read in its core form (meshes, components, transforms, units) without slicing extensions or external parts, and a compressed 3MF needs a recent browser (DecompressionStream API); ASCII STL export is limited to 1 million triangles; densities are typical values to adjust from your spool’s datasheet.