Compare two STL files.
See every geometric change.

Runs entirely in your browser. Your files never leave your device.

100% In-Browser
Powered by WebAssembly + Web Workers.
No Uploads
Files never leave your device.
Private by Default
No tracking, no analytics, no signup.

How it works

01Drop two STL files
Add your Original (A) and Modified (B). Files stay in your browser — nothing is uploaded.
02Auto-align & compute diff
B is aligned against A via a multi-scale voxel correlation search, then the volumetric diff is computed locally.
03Inspect changes
Green for added geometry, red for removed. Toggle layers, switch modes, and read live volume stats.
Advanced — modes, alignment, sensitivity
Three display modes
Voxels (volumetric blocks), Heatmap (signed-distance field on the real mesh) or Overlay (two transparent meshes side-by-side). Switch instantly — no recompute.
Voxel resolution 64³ → 384³
Trade speed for fidelity. The Sensitivity preset controls how aggressively sub-voxel noise is ignored when computing the boolean masks.
Multiscale auto-align
Coarse 24³ → refine 48³ → polish 96³ correlation search reaches sub-voxel precision relative to your final diff grid. Or pick Floor / Center / None alignment manually.
Three colour schemes
Classic (red / green), Colorblind (orange / blue, deuteranopia & protanopia friendly) or Contrast (cyan / magenta). Recolours every layer instantly.

Quick answers

Are my STL files uploaded to a server?
No. The whole comparison runs in your browser inside a Web Worker. Your STL files never leave your machine — nothing is uploaded, stored, or seen by us.
Is the STL diff tool really free with no signup?
Yes. STL Diff Online is 100% free, with no signup, no registration, no account, and no email required to compare two STL files online.
What is the maximum STL file size I can compare?
The browser-based comparison handles STL files up to 80 MB per file. Performance depends on your device — modern laptops handle 10–40 MB STLs comfortably.
What does the STL diff actually show?
A true volumetric 3D diff. STL Diff Online voxelizes both meshes, aligns them, and computes added (B minus A) and removed (A minus B) volume. Added geometry is rendered in green, removed in red, so you can spot revisions instantly — even when triangulation differs between the two files.
How do I compare two STL files online?
Open STL Diff Online in your browser, drop your original .stl file in the left zone and the modified one in the right zone. The tool voxelizes both models, auto-aligns them, and renders a 3D diff in seconds — green for added geometry, red for removed. No signup or upload required.
Does STL Diff Online work on Mac, Windows and Linux?
Yes. STL Diff Online runs in any modern browser (Chrome, Edge, Firefox, Safari) on Mac, Windows, Linux and Chromebooks. There is nothing to install.