How do you calculate the volume of an STL file?

For each triangle, take the tetrahedron it forms with the origin and compute its signed volume, a · (b × c) / 6. Summing those over every triangle gives the enclosed volume of a closed mesh; parts outside the solid cancel out. Mes3D does exactly this, in one pass over the mesh.

Is the STL volume in mm³ or cm³?

STL has no unit, so the volume is in the cube of whatever unit the model was exported in. Mes3D assumes millimetres and reports cm³ (1 cm³ = 1000 mm³). If the model was exported in inches, the true volume is 25.4³ ≈ 16,387 times larger.

How much resin will my print use?

Roughly the model's volume: 1 cm³ is 1 ml, so a 42 cm³ solid model uses about 42 ml of resin plus supports and waste. Hollowing the model reduces it a lot.

Why is the volume wrong for my model?

The calculation is only meaningful for a closed (watertight) mesh. Holes make the result unreliable, and overlapping or intersecting shells are counted twice. The calculator shows a watertightness check next to the volume so you can tell.

STL Volume Calculator — Volume & Surface Area of an STL Online

How to use the calculator

Drag your .stl file onto the drop zone above, or click Choose 3D file. 3MF, OBJ, STEP, PLY and the other supported formats work too.
The model opens and the analysis panel appears with:
- volume in cm³,
- surface area,
- estimated weight for PLA, PETG, ABS or resin,
- a watertightness check (open edges, non-manifold edges, flipped and degenerate triangles),
- the overhang share of the surface.
The dimensions card shows width, height and depth in millimetres and the triangle count.

It works the same on a phone. Nothing is uploaded: the mesh is read and measured by your browser.

How the volume is calculated

The method is the divergence theorem applied to a triangle mesh. Each triangle with corners a, b, c forms a tetrahedron with the origin, whose signed volume is

V = a · (b × c) / 6

Triangles facing away from the origin contribute negative volume, those facing towards it positive volume, and everything outside the solid cancels. The sum over all triangles is the enclosed volume, computed in a single pass over the mesh.

Surface area is simpler: each triangle contributes half the length of the cross product of two of its edges.

Units: the number-one source of wrong answers

STL stores bare numbers with no unit. The calculator assumes millimetres, which is what almost every slicer and CAD export uses for printing. Watch for these signs:

What you see	Likely export unit	Correct the volume by
A phone case that is 6 mm long	Inches	× 16,387 (25.4³)
A figurine that is 0.12 mm tall	Metres	× 1,000,000,000
A part 10 times too small	Centimetres	× 1,000

The full story, including how to rescale, is in STL units: mm vs inches. 3MF and STEP files declare their unit, and Mes3D converts them to millimetres automatically.

mm³, cm³, ml — conversions

1 cm³ = 1,000 mm³
1 cm³ = 1 ml, so the volume in cm³ is the volume in millilitres
1 in³ = 16.387 cm³
1 litre = 1,000 cm³

That makes the calculator directly useful for resin printing, where resin is sold by volume: a solid 42 cm³ model needs about 42 ml of resin, plus supports and what is left on the vat and build plate. Hollowing the model with drain holes can cut that by half or more.

When the volume cannot be trusted

The tetrahedron sum assumes a closed surface. Three things break it:

Holes. A missing triangle leaves the solid open, and the cancellation no longer works. The watertightness line will show open edges.
Overlapping shells. A model made of several bodies that intersect — common in game assets and quick Boolean-free modelling — counts the shared space twice. The real printed volume is smaller.
Inside-out parts. Flipped normals make a shell subtract instead of add.

If the check reports problems, repair the mesh in your slicer or a mesh tool first; the guide on watertight and manifold files walks through it.

From volume to grams and cost

Volume × density = weight at 100 % infill. That is an upper bound for FDM printing, where parts are mostly hollow with a few walls and sparse infill. For grams by material and a realistic infill estimate, use the 3D print weight calculator.

Want more on the phone?

The Mes3D app for iPhone, iPad and Android does the same analysis and adds a wall-thickness view, highlights problem areas on the model, cuts cross-sections, and can resize the model to exact dimensions — including a one-tap inch-to-millimetre fix.

STL volume calculator