Ebook Conversion and File Optimization

Quick Answer: Ebook conversion means turning your manuscript (usually a Word document) into the EPUB format that ebook platforms require. The cleanest path is to use a dedicated formatting tool (Atticus, Vellum, or Reedsy Book Editor) rather than converting DOCX files directly. File optimization means keeping your ebook's file size small enough to minimize delivery costs on Amazon KDP while maintaining image quality.

Converting a manuscript from one format to another sounds simple. In practice, it's where most formatting problems originate. A well-structured Word document converts cleanly. A messy one, with manual formatting, inconsistent styles, and oversized images, produces an ebook full of spacing errors, broken tables of contents, and bloated file sizes.

Converting DOCX to EPUB

The most common conversion path for self-published authors is from Microsoft Word (DOCX) to EPUB. There are three ways to do this, each with trade-offs.

Method 1: Formatting tools (recommended). Import your DOCX into Atticus ($147), Vellum ($249.99, Mac only), Lacuna ($149), or the free Reedsy Book Editor. These tools handle the conversion, let you apply chapter styles and formatting, and export a clean, validated EPUB. This is the most reliable method because the tool manages the underlying code.

Method 2: Direct upload to KDP. Amazon KDP accepts DOCX files and converts them automatically. This works for simple manuscripts but often produces inconsistent results: unexpected spacing, broken tables of contents, and formatting that looks different from what you see in Word. If you go this route, always preview the converted file using Amazon's Kindle Previewer before publishing.

Method 3: Calibre (free, technical). Calibre is a free, open-source tool that converts between nearly any ebook format. It handles DOCX to EPUB conversion, but the output requires manual cleanup. Calibre is most useful for authors comfortable with editing EPUB code or for converting between formats after the initial EPUB is created.

Regardless of method, your DOCX file should use Word's built-in heading styles (Heading 1 for chapter titles, Heading 2 for sections) rather than manually sized text. Conversion tools rely on these style tags to generate the table of contents and apply formatting. A document that uses manual formatting instead of styles will produce a messy conversion every time.

Optimizing File Size

File size matters on Amazon KDP because Amazon charges a delivery fee of approximately $0.15 per megabyte for ebooks enrolled in the 70% royalty option. A text-only ebook might be 500 KB to 1 MB, costing $0.08 to $0.15 per sale in delivery fees. An image-heavy ebook could be 10 MB or more, costing $1.50 or more per sale, which significantly cuts into your royalty.

Image optimization is the biggest lever. Images account for the vast majority of ebook file size. Reduce image resolution to 150 DPI for screen viewing (300 DPI is for print). Use JPEG compression for photographs and complex images. Use PNG only when transparency is needed. Resize images to the largest dimension they'll actually display at, not the original camera resolution.

Remove embedded fonts you don't need. Each font weight (regular, bold, italic, bold italic) adds 50 to 200 KB. If you're embedding fonts and the total adds up, evaluate whether the embedded fonts are necessary. On most platforms, readers override the font anyway.

Strip unnecessary metadata. Word documents can carry revision history, comments, and embedded document properties that inflate file size. Clean these out before conversion.

Compress the final EPUB. Some formatting tools include compression options. For manual optimization, tools like EPUBCheck can validate and identify oversized elements in your file.

Validating Your EPUB

Before uploading to any platform, validate your EPUB file to catch errors that might cause rejection or display problems.

EPUBCheck is the standard validation tool. It's free, open-source, and available as a desktop application or web-based tool (through the DAISY Consortium's online validator). It checks your EPUB against the EPUB specification and reports errors (which will cause problems) and warnings (which might cause problems).

Kindle Previewer is Amazon's free tool for testing how your ebook will look on Kindle devices. Import your EPUB and it converts to Kindle format, showing you exactly how the book will display on different Kindle devices and apps. This catches Amazon-specific issues that EPUBCheck won't flag.

Apple Books preview is available through the Books app on Mac. Drop your EPUB in and see how it renders in Apple's ecosystem.

Testing on at least two of these before publishing catches most formatting problems before readers find them.

What Good Conversion Looks Like vs What Does Not

Works well: - The EPUB passes EPUBCheck validation with no errors. The table of contents is fully functional. Chapter headings display consistently. Images are sharp but appropriately sized. The file opens cleanly in Kindle Previewer and on a physical device. - The file size is under 2 MB for text-only ebooks and under 10 MB for image-heavy ones, keeping delivery costs manageable.

Red flags: - The author uploads a DOCX directly without testing the conversion. The result has random spacing, missing headings, and a non-functional table of contents. - Images are full-resolution camera files (3000 x 4000 pixels at 300 DPI) embedded in a book that displays them at one-third that size. The file is 25 MB and delivery costs eat the royalty. - The EPUB fails validation but the author publishes anyway. Some platforms reject invalid EPUBs; others accept them but display them incorrectly on certain devices.

The Takeaway

Use a dedicated formatting tool to convert your manuscript to EPUB rather than uploading a raw DOCX or relying on platform conversion. Optimize images before embedding them. Validate the final EPUB with EPUBCheck and test in Kindle Previewer before publishing. These steps take an hour or two and prevent the formatting problems that lead to negative reviews and lost sales. For the full process, see the Ebook Formatting and Conversion guide.

Q: How do you convert a Word document to ePub? A: Import your DOCX into a formatting tool like Atticus, Vellum, or the free Reedsy Book Editor. Apply chapter styles, adjust formatting, and export as EPUB. Alternatively, use the free Calibre tool for a more manual conversion. See How Do You Convert a Word Document to ePub? for a step-by-step walkthrough.

Q: How do you reduce ebook file size? A: Optimize images (reduce to 150 DPI, compress as JPEG, resize to display dimensions), remove embedded fonts you don't need, and strip unnecessary metadata from the source document. A well-optimized text-only ebook should be under 1 MB. Image-heavy ebooks should aim for under 10 MB.

Q: What is EPUBCheck? A: EPUBCheck is the free, standard validation tool for EPUB files. It checks your file against the EPUB specification and reports errors that could cause display problems or platform rejection. Run it before uploading to any ebook platform.