Free software, as defined by the fsf, preserves the users freedom to study, change and distribute the software. The right to study entails that the source code be made available: Wikipedia article. Calibre is licensed as GPL3, thus it is free software as defined by the fsf.
Open source is a subset of free software. Free software is always open source.
Hmmm no free software can be closed source. Neither is the subset of the other but they intersect
Free software, as defined by the fsf, preserves the users freedom to study, change and distribute the software. The right to study entails that the source code be made available: Wikipedia article. Calibre is licensed as GPL3, thus it is free software as defined by the fsf.