Understanding License Compatibility
Open source software thrives on code reuse, but each license carries conditions governing how that code may be combined and redistributed. Compatibility refers to whether a project under one license can legally incorporate code under another license and distribute the resulting work. Incompatible licenses can create legal conflicts, forcing developers to remove components or change licensing strategies. This calculator offers a simplified matrix for several common licenses to provide quick guidance. It is not a substitute for professional legal advice, but it helps developers flag potential issues early.
How the Matrix Works
The compatibility database behind this tool classifies pairs of licenses into three outcomes:
- Compatible: You can combine the dependency with your project without changing your license, provided you comply with attribution requirements.
- Conditionally compatible: Combination is possible, but only if you satisfy additional obligations, such as re-licensing your project under a copyleft license or offering source code to users.
- Incompatible: Legal terms conflict, so using the dependency may violate one or both licenses unless you obtain a separate commercial license or alternative component.
The matrix focuses on situations where you distribute binaries that include the dependency’s code. Using a library merely as a separate service (e.g., via network APIs) may trigger different rules like the AGPL. Always read the full license text.
Compatibility Table
| Project \ Dependency | MIT | Apache-2.0 | GPLv3 | GPLv2 | BSD-3 | LGPLv3 |
|---|---|---|---|---|---|---|
| MIT | ✔ | ✔ | Conditional | Conditional | ✔ | ✔ |
| Apache-2.0 | ✔ | ✔ | Incompatible | Incompatible | ✔ | ✔ |
| GPLv3 | Requires GPLv3 | Requires GPLv3 | ✔ | ✔ (if v2+ | Requires GPLv3 | ✔ |
| GPLv2 | Requires GPLv2 | Incompatible | Incompatible | ✔ | Requires GPLv2 | Conditional |
| BSD-3 | ✔ | ✔ | Conditional | Conditional | ✔ | ✔ |
| LGPLv3 | ✔ | ✔ | Requires GPLv3 if linked statically | Incompatible | ✔ | ✔ |
Legend: ✔ = compatible; “Requires GPL” indicates you must relicense your project under GPL to incorporate that dependency.
Step-by-Step Use
- Select your project’s license. If you haven’t chosen one yet, pick the license you intend to apply to your distribution.
- Select the dependency’s license. This is typically found in the library’s repository or package metadata.
- Click check compatibility. The result highlights whether the combination is safe, requires re-licensing, or is incompatible.
- Copy the result. Use the copy button to save the outcome in your project documentation.
Example Scenarios
Imagine your project is licensed under MIT, and you want to include an LGPLv3 library. The matrix shows compatibility, but you must allow users to replace the LGPL component, typically by providing dynamic linking. If you instead wish to include a GPLv3 library, your project must also adopt GPLv3 to comply with copyleft terms.
Another example: an Apache-2.0 project cannot include a GPLv2-only library because Apache’s patent termination clause conflicts with GPLv2, which lacks explicit patent grants. However, GPLv2+ (or LGPLv2.1+) permits relicensing under later versions, enabling compatibility if you adopt GPLv3.
Caveats and Nuances
License compatibility can depend on factors beyond the license text:
- Linking method: Static linking often creates a derivative work, triggering copyleft requirements, while dynamic linking or inter-process communication may avoid this in some jurisdictions.
- Copyright vs. patent grants: Apache-2.0 includes an explicit patent license. Combining Apache-licensed code with GPLv2 software introduces a patent grant mismatch, hence incompatibility.
- Additional permissions: Some projects add exceptions allowing broader use than the base license. Always check for NOTICE files or custom clauses.
Frequently Asked Questions
Does compatibility guarantee compliance? No. You must still follow attribution, notice, and source availability requirements described in each license.
What about proprietary software? Many permissive licenses like MIT or BSD are compatible with closed-source distribution, but copyleft licenses may require releasing source. Consult legal counsel for commercial use.
Is LGPLv3 compatible with MIT? Yes, but if you modify the LGPL library or statically link it, you must distribute source and allow relinking.
Why is GPLv3 incompatible with Apache-2.0 in this matrix? Actually, the Free Software Foundation considers Apache-2.0 compatible with GPLv3. However, Apache-licensed code cannot be included in GPLv2-only projects. Our matrix reflects the direction from project license to dependency; an Apache project cannot include GPLv3 code without changing its own license to GPLv3.
Using the Result
If the tool flags incompatibility, consider alternative libraries, seeking a dual-licensed version, or contacting the author for a commercial license. For conditional cases, weigh the implications of switching licenses or providing source code. Document the decision in your project’s README or CONTRIBUTING file to guide future contributors.
Disclaimer
This calculator provides informational guidance only and does not constitute legal advice. Software licensing can be complex, and interpretations vary by jurisdiction. When in doubt, consult a lawyer experienced in intellectual property law.