Mobi file size

Can Mobi file size be estimated by adding up the file size of each illustration in a book and then adding an amount for text?


That would be a very rough estimate. I have not done any testing so I could be really off in what I’m saying here. This is all speculation. Text size is going to be amplified initially by the HTML markup, but then it is compressed in the end, so I don’t know for sure, but I’d guess 1:6 to 1:9 ratio of compression for the text. Images don’t usually compress any as they already are compacted as much as they can be, and since they’ll be the bulk, the text size will just be a small variable on their shoulders. Are you on a very slow machine, though? Compiling a .mobi doesn’t take very long for me, and that is of course going to be the most accurate.


If I add up the file sizes, I get about 2 Megs. This is with no credit for text compression. There are 77,000 words. The mobi file is coming in at 3.7 megs. Does this sound right?


So, I did a little research, and it turns out KindleGen archives a copy of the original source files in the container, as well as the actual book that the Kindle opens and reads. Thus, while the file is certainly more archival friendly in that you could extract the original .epub from it if you needed to, half of that thing is totally useless to the reader. There used to be an option to suppress this archival, but Amazon developers strangely removed it some time back. So that must be why you are seeing the results you are.

The text is negligible, even doubled. Common language text compresses really well. The pictures might compress a little if the originals are high quality. So 2 to 3.7 is probably where that is going. 4mb minus a little for skimming the fat off of the top.

You might try using Calibre to convert from .epub to .mobi, instead of KindleGen, for this book. The latter isn’t really optimised for illustration heavy works.

Hi – I have a concern about my compiled mobi file sizes as well.

When I compile my novel project into an epub file it comes out at 1.95 MBs.

When I compile the exact same project into mobi format, it comes out at 3.14 MBs.

I don’t get why this is.

I have a high quality cover image and a few small images within my docs that I have been compressed as low as I want them. Doing so shaved off a few KBs but I’m still getting this large mobi file.

At 3.14 MBs my Kindle delivery fee would be around 50 cents per download. Which is quite a lot when I’ll only be selling my novel for a few dollars. So I need to have my max mobi file size at around 2 MB. That will still enable me to make a decent profit from each sale. At 2MB, my delivery fees would be around 30 cents per download which is what I’ve been expecting. It might not seem like much but saving that extra 20 cents per download over time will really add up (Say I were to sell 10,000 copies, that’s nearly $2,000 more in profit!).

I’ve tried importing my epub file into Calibre and then exporting it as a mobi file but I’m not happy with the way my novel is rendering after going through Calibre so that is not an option for me.

If anyone has any suggestions on what I can do to bring my mobi file down to around the same size as my epub file, I would greatly appreciate if you could share. Perhaps there’s another program like Calibre that can covert an epub to mobi? Or some trick inside of Scrivener that I can perform?

Thank you!

After further thought and some more tinkering with different ebook programs, I’m hoping that this will be a decent solution:

  • I’m going to compile my Scrivener doc into an epub file.

  • Open the epub file in Sigil.

  • Customize it and hopefully reduce its size a bit more.

  • Then I’m going to upload it to Amazon KDP as an epub file and convert it to Amazon’s format.

Hopefully that will reduce bloat and make it a smaller file. However, I’m not sure that this will work since Scrivener already uses KindleGen. But it’s worth a try. I will report back.

Well I attempted converting from my Sigil epub to mobi on Kindle Direct Publishing. The file came out beautiful and shaved off 0.29 MBs. Now it’s down to 2.85 instead of 3.14 MBs. I wish it could be compressed more though. Perhaps I can alter the HTML further in Sigil it will. Otherwise I guess I’ll just be stuck with a large file.

Thanks for reading!

Part of the problem you are facing is Amazon being in the middle of switching formats from the old .mobi format to the new kf8 format. Right now KindleGen (not Scrivener, to be clear) packs both the old .mobi and new .kf8 formats into a single e-book file. If you are using an older Gen2 or so device to read books, then the old .mobi data gets used. If you are using a newer device then the kf8 device is used. So it means twice the book for a few years, but that’s a better alternative than making it so that older devices are suddenly obsolete.

So right now, if you are somehow making a .mobi file that isn’t roughly twice the size it should be, then you are most likely making an old style .mobi file, which probably isn’t the best thing to be doing in the middle of a format upgrade, and may result in quality drops with newer devices. If you have an older version of KindleGen hanging around, that would be one way of doing so—but I wouldn’t advise it.

That’s all understandable and logical. The pity is that Amazon hasn’t updated their delivery cost system to compensate for the bloat.

Compression is only going to grant you negligible returns. There is a little difference between optimised compression routines, but not much. The best thing you can do for it is shave down those graphics. If you are using PNG, switch to JPG and do your pixel examinations on Kindle devices if you can, rather than on your computer where it is much easier to see minor flaws that will be invisible on e-ink or a Fire display.