Use the free STL Volume Weight Calculator below. It runs entirely in your browser, with no sign-up and no download.
How much plastic is in that model?
Before you print a model, two questions matter more than any other: how big is it, and how much material will it use? Size decides whether it fits your build plate at all. Material decides the cost of the print, the weight of the finished part, and whether a hollow print will hold together. Both answers are locked inside the STL file, and both can be computed exactly from its triangles without ever opening a slicer.
This calculator reads your STL file entirely in your browser, in binary or ASCII form, and returns its volume, its bounding box and its weight for the material you choose. Nothing is uploaded. The file stays on your device, which is the right behaviour for designs you have not published yet.
How volume is computed from triangles
An STL file describes a surface as a mesh of triangles. The volume enclosed by that surface can be found triangle by triangle, using signed tetrahedra. For each triangle, take the tetrahedron formed by the triangle and the origin, and compute its signed volume from the scalar triple product of its three vertices, divided by six. Add those signed volumes over all triangles, and the parts outside the solid cancel, leaving the enclosed volume. Take the absolute value and you have the volume in cubic millimetres.
The same pass over the triangles gives the bounding box: track the smallest and largest x, y and z coordinates seen, and the differences are the size of the part in each direction. Weight follows from volume and density: convert cubic millimetres to cubic centimetres, multiply by the material density in grams per cubic centimetre, and you have the weight of a solid part. PLA at 1.24, ABS at 1.04, PETG at 1.27, resin at 1.10 and nylon at 1.14 are the densities built into the tool.
A worked example
Try the sample 10 millimetre cube built into the tool. A cube of side 10 millimetres has a volume of 1000 cubic millimetres, which is exactly 1 cubic centimetre, and at PLA density that is 1.24 grams. The calculator reports those figures and a bounding box of 10 by 10 by 10 millimetres, which confirms the parser and the volume maths on a shape whose answer you already know. Whenever you doubt a tool, test it on a cube first.
Now load a real model. A figurine that reports 45 cubic centimetres in PLA weighs about 56 grams solid. But a real print is not solid. At 20 percent infill with a few perimeters, the printed weight is typically 25 to 40 percent of the solid weight, depending on wall count and infill pattern. Multiply the solid weight by your infill fraction and a wall allowance to get a realistic printed weight, and compare with your slicer’s estimate before committing to a long print.
What can go wrong with an STL
The volume formula assumes a closed, watertight mesh with consistent triangle orientation. Many models from the internet are not. Holes, flipped triangles and self-intersecting surfaces all corrupt the total, sometimes badly. If the volume looks absurd, or the bounding box is far larger than the model you see in a viewer, suspect the mesh, not the calculator. Mesh repair tools can fix most such models by closing holes and unifying normals.
Units are the other classic trap. STL files carry no unit information. A model modelled in inches but read as millimetres comes out 16,387 times too large by volume. If the bounding box looks 25.4 times too small or too large, you have a unit mismatch. Scale the model in your slicer before printing, and keep a note of the intended units with the file.
Frequently asked questions
Why does binary STL matter? Most slicers export binary STL, which is compact and fast to parse, while ASCII STL is human readable but much larger. The calculator handles both, so you do not need to convert before checking a file.
Can I use this for resin printing? Yes for volume and size, which are the same problem. Weight uses the resin density option, and the printed weight is close to solid weight because resin prints are typically solid or thick walled.
Does the calculator measure surface area? Not in this version. Volume, bounding box and weight cover the planning questions most makers ask first, and surface area is a slicer statistic for finishing and painting estimates.
Spanish version
Calculadora de Volumen y Peso STL – Volumen y Peso desde tu Archivo STL