STL Volume Calculator: dimensions, surface area and watertight check, no upload

The bounding box is the smallest axis-aligned box that holds every vertex: width, depth and height as the slicer will see them. Volume is computed by summing the signed volume of the tetrahedron formed by each triangle and the origin, which is the standard divergence-theorem method; it is exact for a closed mesh and meaningless for an open one, which is why the watertight check sits next to it. Surface area is the sum of the triangle areas, always valid. Triangle count is what the file holds; vertex count is the number of distinct corner positions after merging, which is the number a modelling tool would report. Watertight means every edge is shared by exactly two triangles; the report gives the number of open edges (one triangle) and non-manifold edges (three or more).

Units are whatever the file uses. STL, OBJ and PLY carry no unit, so the tool shows the raw numbers as millimetres and, next to them, the same values in inches; the toggle swaps which one is primary. 3MF declares its unit and is converted to mm. If the largest dimension is under 10 or over 1,000 the unit guess warning appears, because a 1.5 mm part or a 4 m part is usually an inch or metre export read as millimetres.

Like every tool on this site, the work happens in your browser. The file is read from your device, parsed in memory and drawn with WebGL; no copy is sent anywhere. Open the network panel of your browser's developer tools while you load a model and you will see no upload. Close the tab and the model is gone.

Drop a 40 mm cube STL. The panel reads:

Bounding box   40.00 x 40.00 x 40.00 mm   (1.575 x 1.575 x 1.575 in)
Volume         64,000 mm3 = 64.00 cm3      (3.905 in3)
Surface area   9,600 mm2 = 96.00 cm2       (14.88 in2)
Triangles      12
Vertices       8
Watertight     yes (0 open edges, 0 non-manifold)

The volume is 40 cubed, the area is six faces of 1,600 mm2, twelve triangles is two per face, and eight vertices is the corners. If the same file came from a CAD package set to inches, the box would read 1.575 x 1.575 x 1.575 mm and the warning would tell you so; the fix is to scale by 25.4 in your slicer.

Now delete one face of that cube in a mesh editor and drop it again. Triangles drop to 10, open edges rise to 4 (the four edges of the missing face), watertight becomes no, and the volume line is shown with a warning, because the tetrahedron sum still produces a number (it will happen to be close to 64 cm3 for this shape) but the number is not trustworthy for an open mesh in general. A slicer would close that face on its own with a flat layer, but a print-cost estimate from an open mesh can be off by any amount.

• Will it fit on the bed? Compare the bounding box with your printer's build volume. Remember that the box is axis-aligned; a diagonal part may fit after rotation.
• How much filament or resin? Volume times material density, adjusted for infill. The print cost calculator does the arithmetic with presets for PLA, PETG, ABS, TPU and resin.
• Is it printable at all? Watertight yes, no non-manifold edges, and a bounding box that makes sense in mm. Anything else, see STL repair.
• Surface finish and post-processing. Surface area tells you how much sanding, painting or plating you are signing up for; paint and plating suppliers quote per cm2.
• Mesh density. Triangles per cm2 of area is a quick measure of how faceted a curved surface will look. Under 10 per cm2 on a curved part and you will see the facets.

• Volume is only reliable for a watertight mesh. Open meshes get a number with a warning, and self-intersecting shells double count the overlap.
• The bounding box is axis-aligned. It does not find the smallest rotated box; rotate in your slicer to check a tight fit.
• Units are inferred, not known. STL, OBJ and PLY cannot say whether they are mm, inches or metres; the tool assumes mm and warns on implausible sizes.
• Point clouds (PLY without faces) have a bounding box and vertex count only.
• The whole file is read into memory; hundreds of MB may exceed a phone.