A fast library for reading, writing, and modifying STL files, powered by NumPy. Every mesh operation uses vectorized array math for speed.

Stanford Dragon — 871,414 triangles loaded in 0.63s, rendered with matplotlib

Quick Start

pip install numpy-stl

from stl import mesh

# Load an STL file (auto-detects binary/ASCII)
your_mesh = mesh.Mesh.from_file('model.stl')

# Inspect
print(f'{len(your_mesh)} triangles')
print(f'Bounding box: {your_mesh.min_} to {your_mesh.max_}')

# Save
your_mesh.save('output.stl')

Features

• Read and write binary and ASCII STL files
• Read PLY and 3MF files (3MF is experimental, read-only)
• Mesh operations: rotate, translate, transform (4x4 matrix)
• Properties: surface area, volume, center of gravity, inertia tensor, convexity
• Combine multiple meshes by concatenating data arrays
• CLI tools: stl, stl2ascii, stl2bin for format conversion
• Fast: all operations backed by NumPy vectorized math

Supported Formats

Format	Read	Write	Notes
STL (binary)	✅	✅	Auto-detected on load
STL (ASCII)	✅	✅	~5x faster with optional speedups
PLY	✅	✅	Binary and ASCII; from_ply_file / save_ply
3MF	✅	—	Experimental; from_3mf_file

Requirements & Compatibility

• Python: 3.10+
• NumPy: 1.24+ (installed automatically)
• Platforms: Linux, macOS, Windows
• Optional: numpy-stl[fast] for the Cython ASCII speedups (see below)

Performance / Optional Speedups

numpy-stl is fast out of the box. For even faster ASCII STL I/O, install the optional Cython speedups:

pip install numpy-stl[fast]

This installs the speedups package, a compiled C extension for ASCII parsing. The library works identically without it -- pure Python is the default.

Benchmark

ASCII STL read performance — ~5x faster with the speedups C extension, consistent across data sizes (median of 5 runs):

Facets	Pure Python	Speedups	Factor
10,000	36 ms	7 ms	5.1x
100,000	0.36 s	73 ms	4.9x
871,414	3.10 s	0.59 s	5.2x
1,000,000	3.60 s	0.73 s	4.9x

Note: Results will vary by hardware. Run the benchmark yourself: python benchmarks/benchmark_ascii_read.py

Usage Examples

Creating a Mesh from Scratch

import numpy as np
from stl import mesh

# Define vertices and faces of a cube
vertices = np.array([
    [-1, -1, -1], [+1, -1, -1], [+1, +1, -1], [-1, +1, -1],
    [-1, -1, +1], [+1, -1, +1], [+1, +1, +1], [-1, +1, +1],
])
faces = np.array([
    [0, 3, 1], [1, 3, 2], [0, 4, 7], [0, 7, 3],
    [4, 5, 6], [4, 6, 7], [5, 1, 2], [5, 2, 6],
    [2, 3, 6], [3, 7, 6], [0, 1, 5], [0, 5, 4],
])

cube = mesh.Mesh(np.zeros(faces.shape[0], dtype=mesh.Mesh.dtype))
for i, f in enumerate(faces):
    for j in range(3):
        cube.vectors[i][j] = vertices[f[j], :]

cube.save('cube.stl')

Rotating and Translating

import math
from stl import mesh

m = mesh.Mesh.from_file('model.stl')
m.rotate([0, 0, 1], math.radians(90))
m.translate([10, 0, 0])
m.save('transformed.stl')

Mass Properties

from stl import mesh

m = mesh.Mesh.from_file('closed_model.stl')
volume, cog, inertia = m.get_mass_properties()
print(f'Volume: {volume:.4f}')
print(f'Center of gravity: {cog}')

Combining Meshes

import numpy as np
from stl import mesh

m1 = mesh.Mesh.from_file('part1.stl')
m2 = mesh.Mesh.from_file('part2.stl')
combined = mesh.Mesh(np.concatenate([m1.data, m2.data]))
combined.save('combined.stl')

Plotting with Matplotlib

import math
from stl import mesh
from mpl_toolkits import mplot3d
from matplotlib import pyplot

figure = pyplot.figure(figsize=(8, 6))
axes = figure.add_subplot(projection='3d')

dragon = mesh.Mesh.from_ply_file('dragon_vrip.ply')
dragon.rotate([1, 0, 0], math.radians(-90))

axes.add_collection3d(
    mplot3d.art3d.Poly3DCollection(dragon.vectors)
)

scale = dragon.points.flatten()
axes.auto_scale_xyz(scale, scale, scale)
pyplot.show()

API Cheatsheet

Assumes import math, import numpy as np, from stl import mesh, and from stl import Mode (for the ASCII save).

Task	Call
Load (auto-detect)	mesh.Mesh.from_file('m.stl')
Load PLY	mesh.Mesh.from_ply_file('m.ply')
Load 3MF (experimental)	list(mesh.Mesh.from_3mf_file('m.3mf'))
Save (auto/format)	m.save('out.stl')
Save as ASCII	m.save('out.stl', mode=Mode.ASCII)
Save PLY	m.save_ply('out.ply')
Rotate (axis, radians)	m.rotate([0, 0, 1], math.radians(90))
Translate	m.translate([x, y, z])
Transform (4x4 matrix)	m.transform(matrix)
Bounding box	m.min_, m.max_
Mass properties	volume, cog, inertia = m.get_mass_properties()
With density	vol, mass, cog, inertia = m.get_mass_properties_with_density(d)
Combine meshes	mesh.Mesh(np.concatenate([a.data, b.data]))

CLI Tools

# Convert ASCII to binary
stl2bin input.stl output.stl

# Convert binary to ASCII
stl2ascii input.stl output.stl

# Auto-detect and convert
stl input.stl output.stl

Documentation

Full documentation is available at numpy-stl.readthedocs.io.

Contributing

Contributions are welcome! See CONTRIBUTING.md for the development setup guide.

Links

• Source code
• PyPI
• Bug reports
• Documentation
• Changelog

Support

numpy-stl is maintained by Rick van Hattem in his own time. Most of that time goes on the malformed STL files that real scanners and slicers produce.

If it saved you an afternoon, a tip covers an hour of issue triage: Ko-fi or GitHub Sponsors.

If your company funds its dependencies, this package is on thanks.dev.

License

BSD-3-Clause