Side by side
ACL2 vs Lean
Both stand on formal verification tools. Which one you can start free with, which one lets you read the code, and what each costs once you pay.
Free plan LinuxMacSelf-hosted+1
ACL2 combines a theorem prover with libraries and documentation for formal verification across several operating systems. Check the Lisp prerequisite and version note before installing.
Visit ACL2
Free plan LinuxMacWeb+1
Lean combines general programming with formal proof work and a substantial community mathematical library. It is a focused choice for people willing to work with formalization and proof tooling.
Visit LeanThe facts
| ACL2 | Lean | |
|---|---|---|
| Free plan | Yes | Yes |
| Open source | Not published | Not published |
| Runs on | Linux, Mac, Self-hosted, Windows | Linux, Mac, Web, Windows |
| Lowest paid plan | Not published | Free or on request |
| Plans published | None | 1 |
| Maker | Not published | Not published |
| Website | acl2.org | lean-lang.org |
| On formal verification tools | #7 of 34 | #11 of 34 |
ACL2: what it does well
- Free and self-hosted.
- Runs on Linux, macOS, FreeBSD, and Windows.
- Community Books include proof and hardware-verification libraries.
- Documentation is available in several formats.
- Supports counterexamples and proof artifacts.
What to know first
- Requires a Common Lisp implementation.
- Some books are guaranteed only with CCL or SBCL.
- Version 8.7 lacks fixes and improvements since March 2026.
- Building all Community Books can take hours.
Lean: what it does well
- Free and open source under Apache 2.0.
- Mathlib contains over a million lines.
- Lake manages Lean projects.
- Lean4Web runs Lean code in a browser.
- Learning resources include tutorials and interactive games.
What to know first
- VSCodium and Cursor support is unofficial.
- x86-64 macOS and WebAssembly are Tier 2 platforms.