What MOBI to EPUB can read, and what it cannot write
Read an older Kindle file in an app that wants EPUB.
Text chapters are extracted and packed as an EPUB. Some MOBI files are really EPUBs in disguise and convert more cleanly.
Kindle files and DRM
Only convert books you own or may legally convert. Complex layouts and images may be simplified.
When MOBI to EPUB is the right page
MOBI to EPUB sits on this site because MOBI and EPUB are not interchangeable, even when they look similar in a file list. Read an older Kindle file in an app that wants EPUB. People want MOBI for an old Send-to-Kindle path, or EPUB so other apps can open an old Kindle file. EPUB is the copy to keep if you want the widest choice of reading apps. Kindle prefers its own formats.
Text chapters are extracted and packed as an EPUB. Some MOBI files are really EPUBs in disguise and convert more cleanly.
What you give up by leaving MOBI
MOBI is an older Kindle format. Amazon has moved on, but old files still appear. A browser cannot encode a new MOBI. Use Calibre on a computer when you must create one.
Keep the original ebook.
What you gain, and do not gain, with EPUB
The download has to be EPUB, because that is the format the next step named. EPUB is the copy to keep if you want the widest choice of reading apps. Kindle prefers its own formats. Chapter text you own can be repacked into EPUB or out of EPUB.
Pictures and precise page design will not match a print book.
A test that matches the real file
Start from the question the other software asked. If it asked for EPUB, MOBI to EPUB is the matching page. If it asked for something else, switch tools instead of hoping a similar extension will pass. Read an older Kindle file in an app that wants EPUB. Text chapters are extracted and packed as an EPUB. Some MOBI files are really EPUBs in disguise and convert more cleanly. The limit is the part people skip, and it is the part that decides whether the download is usable. Only convert books you own or may legally convert. Complex layouts and images may be simplified. 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. Check three things in the download: it opens, the MOBI material you cared about is still there, and the file size or the layout is what the form described. A new extension alone is not those three things. If a portal, a teacher, or a client rejects the file, send them the original MOBI as well when you are allowed to. MOBI to EPUB is a convenience copy. It is not a new master. Convert only a book you own and are allowed to convert. MOBI to EPUB 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 MOBI to EPUB. You start with MOBI and you stop when a real program accepts EPUB. A MOBI you own can often be read and turned into EPUB text. A fixed, illustrated layout is simplified. DRM-locked books fail. 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.
Only convert books you own or may legally convert. Complex layouts and images may be simplified.
A related step, if you still need one, is EPUB to MOBI.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.