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Minimizing Your eBook File Size

Posted on 2014-Oct-07

by Paul Salvette

The Trouble with Large eBooks

Many of our clients are rightfully concerned that the file size of their eBooks be minimized to the fullest extent possible. Avid readers with a large library can quickly fill up the device space on smartphones and tablets, leaving little room for other apps and, of course, more eBooks. Additionally, eBooks that use large, uncompressed images can crash out a device—the iBooks app being a notorious offender. However, the biggest concern with large eBooks from a publisher or author perspective is the dreaded Amazon delivery fee.

When you take the 70% royalty option at Amazon (highly recommended if you are between $2.99 and $9.99 inclusive and used by most authors), Amazon slaps the publisher or author with a so-called “delivery fee.” The rate is $0.15/MB on every eBook delivered. So if your MOBI eBook is 2MB, then Amazon will take $0.30 from every sale. This can eat into your income very quickly, so it is important to keep the eBook file size as small as possible. Amazon has, arguably, the most powerful web-based infrastructure and servers on the planet, so it’s not clear why they charge so much for content delivery. However, it’s Amazon, so they can do whatever they want. I’m not anti-Amazon per se…I’m just sayin’.

Strategy to Keep the eBook File Size Small

The biggest culprit that takes up space in eBooks is images. Unlike print editions, eBooks do not need to have the highest-res images possible. This will cause major device lag and also eat into your well-deserved royalties at Amazon. With the exception of fixed-layout eBooks, we have never encountered a situation where the image had to be greater 127KB. If you are a client of BB eBooks, we’re happy to handle all this for you. For DIYers, we strongly recommend getting a decent image manipulation program like Photoshop or Gimp (which is free and wonderful!). You can degrade the quality of JPEG type images, usually to 70-80%, and this will cut down tremendously on the eBook’s file size. If you are converting your eBooks from Word, Pages, or another word processing program, embed the image in the document after reducing the file size with a program like Gimp.

The cover in the MOBI file is an exception to the rule. The cover embedded inside the eBook file and uploaded as the marketing image does not count toward the Amazon delivery fee. Therefore, please feel free to upload large covers for the marketing image and embed high-res covers inside the eBooks (please note: if you’re a client of BB eBooks we will handle the embedding of the cover in the eBooks).

Fonts are another culprit that can bloat an eBook. Many traditional publishers and InDesign users embed fonts for the actual body content rather than just headings and special content like text messages. BB eBooks typically advises against this practice because 1) many readers select their own font to use, 2) every eReading device comes equipped with its own selection of fonts that the user can choose, and 3) if a font like DeJaVu Sans or Charis SIL is embedded that has a massive set of glyphs to cover multiple languages, it can create close to 1MB of overhead. That’s money down the drain! It is recommended to only use fonts that have a limited glyph set and to only use them for headings, special content, and dingbats/fleurons (again, if you’re a client, we take care of this for you). If you do intend to use a font face with a huge number of glyphs, try to subset them to have only the glyphs you need for the eBook itself. Font Squirrel has a great app to reduce the size of font files. BB eBooks uses its own proprietary methods for our clients to reduce the size of font files.

The last culprit for big eBook file size is sloppy formatting or coding practices that “bloat” the eBook file size. This is an in-depth discussion beyond the scope of this blog post. However, BB eBooks’ clients can rest assured that our standards are very, very high to ensure clean, professional formatting that minimizes the eBook’s file size.

The MOBI Conundrum

Amazon’s command-line tool for developers to create MOBI files is called KindleGen, and it is also integrated into many eBook-making programs like Calibre. Honestly, the thing is a black box of proprietary Amazon code, and I don’t know how the hell it works. But, for some reason the file size of the outputted MOBI is 2-3 times the EPUB size. Rob Siders of 52 Novels explains why in this helpful comment:

…the packaged Mobi has as many three versions of the book: a KF8-ready version (if those features were used) for Kindle devices and apps that can play them, a version for legacy Kindle devices and apps, and also the source epub. This results in a final file size that’s as much as three times the size of the source epub.

Therefore, the physical file size of the MOBI eBook is not what Amazon will use to calculate the dreaded delivery fee. To find out what file size Amazon will use to calculate the delivery fee, upload your MOBI eBook through the KDP publishing process in Step 1 (Your Book). After you are done uploading the MOBI and get the “Upload and Conversion Successful” message, proceed to Step 2 (Rights & Pricing). If you scroll down below the royalties information you’ll see the file size in fine print. Below is an example screenshot:


In this case the actual file size of the eBooks was 1.1MB, but the delivery fee will be calculated for 0.4MB, so about 6 cents per eBook sold. Please note that you can do these steps without actually publishing the book itself if you just want to have a look.

Box Sets and File Size

2014 is definitely the year of the box set. Many of our clients have banded together to create 8-, 10-, and even 12-book box sets. This is a great way to make a run for the coveted NY Times and USA Today bestseller lists. Our hats are off to you! Many authors will price their box set at $0.99 for a limited time to maximize sales and get on the important lists. Unfortunately, there is a limit at Amazon to pricing your eBook at $0.99. If your book size, as calculated by Amazon for the delivery fee, is >3MB, you must publish at $1.99 or higher. If your book is >10MB, you must publish at $2.99 or higher. More on this policy from Amazon here.

Even if all the images, fonts, and code are squared away, some box sets are just too darn big in content to be <3MB for the MOBI. From experience, we advise the following if you would like to keep your box set under 3MB at Amazon so you can price at $0.99:

  • <500,000 words total if there are covers at the beginning of each story in the box set
  • <650,000 words total if there are no covers at the beginning of each story in the box set

As long as the total word count is less than these, you should be okay if you work with us. Please feel free to contact us for help with you multi-author box set.

Label: Technical and Design

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