Compare two STL files.
See every geometric change.

A true volumetric diff for 3D-printable models — what's added and what's removed, side by side. Runs entirely in your browser. Your files never leave your device.

No signup

No account, no email, no login. Drop two STLs and you're comparing.

Completely free

Every feature is free. The local-only diff is and always will be.

Security first

The comparison runs entirely in your browser via Web Workers — your STL files and geometry are never uploaded. No tracking cookies, no third-party analytics; just anonymous, aggregate usage stats (file sizes, timings — never your files).

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

Added geometry highlighted in one colour, removed in another, with three palettes to pick from. Toggle layers, switch modes, and read live volume stats.

Advanced - modes, alignment, sensitivity
Four display modes
Voxels (volumetric blocks), Git (red/green deltas over a grey kept baseline), Heatmap (signed-distance field on the real mesh) or Overlay (two transparent meshes side-by-side). Switch instantly, no recompute.
Voxel resolution 96³ → 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.
the result
faq
Are my STL files uploaded to a server?
No. The whole comparison runs in your browser inside a Web Worker - your STL files and their geometry never leave your machine. We record only anonymous, aggregate usage metrics (such as file sizes and how long a compare took), never your files, filenames, or their contents.
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 one colour and removed in another, 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. 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.