This EPUB to MOBI conversion does not run here
Older Kindle workflows asked for MOBI.
This browser cannot encode MOBI.
What to do instead
Send the EPUB, convert with Calibre on a computer, or make a PDF with EPUB to PDF. MOBI to EPUB does work here if you already have a MOBI file.
The page that does work is EPUB to PDF.
When EPUB to MOBI is the right page
EPUB to MOBI sits on this site because EPUB and MOBI are not interchangeable, even when they look similar in a file list. Older Kindle workflows asked for MOBI. EPUB is the copy to keep if you want the widest choice of reading apps. Kindle prefers its own formats. People want MOBI for an old Send-to-Kindle path, or EPUB so other apps can open an old Kindle file.
This browser cannot encode MOBI.
What you give up by leaving EPUB
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.
What you gain, and do not gain, with MOBI
The download has to be MOBI, because that is the format the next step named. People want MOBI for an old Send-to-Kindle path, or EPUB so other apps can open an old Kindle file. A MOBI you own can often be read and turned into EPUB text.
DRM fails. Chapter text moves. Illustrated layouts are simplified.
A test that matches the real file
Start from the question the other software asked. If it asked for MOBI, EPUB to MOBI is the matching page. If it asked for something else, switch tools instead of hoping a similar extension will pass. Older Kindle workflows asked for MOBI. This browser cannot encode MOBI. The limit is the part people skip, and it is the part that decides whether the download is usable. Send the EPUB, convert with Calibre on a computer, or make a PDF with EPUB to PDF. MOBI to EPUB does work here if you already have a MOBI file. 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 EPUB 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 EPUB as well when you are allowed to. EPUB to MOBI is a convenience copy. It is not a new master. Convert only a book you own and are allowed to convert. EPUB to MOBI 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 MOBI. You start with EPUB and you stop when a real program accepts MOBI. Chapter text you own can be repacked into EPUB or out of EPUB. A browser cannot encode a new MOBI. Use Calibre on a computer when you must create one. 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.
Send the EPUB, convert with Calibre on a computer, or make a PDF with EPUB to PDF. MOBI to EPUB does work here if you already have a MOBI file.
A related step, if you still need one, is MOBI to EPUB.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.