head to head · open source
calibre-web vs substance
calibre-web has 18,216 GitHub stars, 1,863 forks, 409 open issues and last shipped yesterday. substance has 2,733 stars, 121 forks, 41 open issues and last shipped 6 months ago. calibre-web leads on adoption by 567% (18,216 vs 2,733 stars). calibre-web is written in Fluent under GPL-3.0; substance is written in JavaScript under MIT. calibre-web has attracted 10% as many forks as stars, substance 4%. calibre-web was the more recently maintained of the two, and both are self-hostable with no licence fee.
Two open source projects, one decision. Both are free and self-hostable — the differences are community size, license terms, language stack and release pace.
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Side by side
| calibre-web | substance | |
|---|---|---|
| GitHub stars | ★ 18K | ★ 2.7K |
| License | GPL-3.0 | MIT |
| Written in | Fluent | JavaScript |
| Last push | 2026-09-19 | 2026-04-02 |
| Forks | ⑂ 1.9K | ⑂ 121 |
| Self-hosting | Yes | Yes |
| Data ownership | Your server | Your server |
pick calibre-web if
- You weight community size — 18K stars and counting
- You want the GPL-3.0 license terms
- Your stack matches Fluent
- You value the larger contributor base for long-term maintenance
pick substance if
- You want the substance feature set and don't need the biggest community
- You prefer the MIT license terms
- Your stack matches JavaScript
- You evaluated both and substance fits your workflow better
About calibre-web
Calibre Web is a web application that provides a clean and intuitive interface for browsing, reading, and downloading eBooks from a valid Calibre database. It is a fork of the library project (calibreserver) and is licensed under the GPL v3. The project lives in the Calibre ecosystem, and it is distributed through PyPI as the calibreweb package as well as through Docker images. It has been developed for over eleven years and carries roughly 18,183 stars and 1,856 forks on GitHub.
read the full calibre-web overview →
About substance
Substance is a JavaScript library for web based content editing. It lives in the JavaScript and Content & Publishing / Publishing ecosystem, and it is distributed under the MIT license. The README describes it as a library that provides building blocks for realizing custom text editors and web based publishing systems, and it was developed to power the online editing platform Substance.
read the full substance overview →
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Frequently asked questions
Is calibre-web or substance more popular?
calibre-web has 18,216 GitHub stars and substance has 2,733. calibre-web has the larger community by that measure.
Are calibre-web and substance free?
Both are open source. calibre-web is licensed under GPL-3.0 and substance under MIT. Neither carries a licence fee.
What is the difference between calibre-web and substance?
calibre-web is written in Fluent and substance in JavaScript. The practical differences are community size, licence terms, language stack and release cadence — all compared in the table above.
Which should I choose, calibre-web or substance?
Choose calibre-web if you want the larger community (18,216 stars) or its GPL-3.0 licence terms. Choose substance if its feature set, stack or MIT licence fits better. Both are self-hostable.