What EPUB to FB2 can read, and what it cannot write
Some readers, especially Russian-language apps, want FB2 instead of EPUB.
Chapter text from an EPUB you own is repacked as FB2 in the browser. A fixed, illustrated layout is simplified to words.
Kindle files and DRM
DRM-locked books fail. Do not convert a book you are not allowed to convert. Pictures and precise page design will not match the EPUB.
EPUB to FB2 without the extra promises
EPUB to FB2 sits on this site because EPUB and FB2 are not interchangeable, even when they look similar in a file list. Some readers, especially Russian-language apps, want FB2 instead of EPUB. EPUB is the copy to keep if you want the widest choice of reading apps. Kindle prefers its own formats. Some readers ask for FB2 instead of EPUB, and the other way around for phone apps.
Chapter text from an EPUB you own is repacked as FB2 in the browser. A fixed, illustrated layout is simplified to words.
Read this about EPUB before you upload
EPUB is the open ebook format. Apple Books, Google Play Books, and most reading apps open it. A fixed, illustrated layout is simplified. DRM-locked books fail.
Keep the file you bought. Convert only books you are allowed to convert.
Read this about FB2 before you send the download
The download has to be FB2, because that is the format the next step named. Some readers ask for FB2 instead of EPUB, and the other way around for phone apps. Chapter text can move between FB2 and EPUB.
The result is a book of words, not a replica of a designed page.
Keep, check, then delete
People lose time when they convert before they know which rule failed. Size, pixel dimensions, codec, font encoding, and layout are different rules. EPUB to FB2 only addresses the EPUB to FB2 step. Some readers, especially Russian-language apps, want FB2 instead of EPUB. Chapter text from an EPUB you own is repacked as FB2 in the browser. A fixed, illustrated layout is simplified to words. DRM-locked books fail. Do not convert a book you are not allowed to convert. Pictures and precise page design will not match the EPUB. The file stays in your browser and is not uploaded. The limit is 100 MB. Keep the original until the download has been opened in the program that will use it. Do the test with a short or simple file first when the real file is large. A one-page PDF, a single photo, or a one-minute clip will show you whether the output is the kind of file you needed. After that test looks right, run the real file once. Keep both the EPUB original and the FB2 download until the receiving person or the upload form accepts it. Convert only a book you own and are allowed to convert. EPUB to FB2 moves chapter text. A fixed illustrated layout is simplified, and DRM-locked files fail. Kindle formats can often be read here and not written. Keep the original ebook until the new file opens in the reader you actually use.
Here is the decision for EPUB to FB2. You start with EPUB and you stop when a real program accepts FB2. Chapter text you own can be repacked into EPUB or out of EPUB. Illustrated fixed layouts are simplified. DRM fails. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
DRM-locked books fail. Do not convert a book you are not allowed to convert. Pictures and precise page design will not match the EPUB.
A related step, if you still need one, is FB2 to EPUB.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.