FB2 to MOBI

How to do it

1. Download Asteria and open it. It's free and needs no account.
2. Drop your FB2 files onto its window, or press ⌘O (Ctrl O on Windows) to choose them. Drop as many as you like.
3. Type convert to mobiand press Enter. MOBI for older Kindles from an e-book.
4. The first time, Asteria asks to download Calibre, a free add-on from Kovid Goyal, under the GPL-3.0-only license. It shows the download size before you say yes, and needs no administrator password.
5. Press ⌘↵ (Ctrl ↵ on Windows) to save. The MOBI files appear next to the originals, which stay exactly as they were.

What it needs

FB2 to MOBI uses Calibre, a free program for ebooks. Asteria doesn't bundle it. The first time you need it, Asteria offers to download it, checks the download against a pinned checksum, and keeps it in its own folder, where Settings can remove it again. If Calibre is already on your computer, Asteria uses that copy.

How it was tested

A real FB2 file was converted on Linux, and the MOBI that came out was opened and checked with MOBI header to make sure it is what it says it is.