Calibre vs Sigil: Free eBook Formatting Tools Compared

Last reviewed by Robert Prime — March 2026

If you’re a UK author diving into self-publishing, two free tools will inevitably come up: Calibre and Sigil. Both promise to get your eBook formatted correctly for distribution on platforms like Amazon KDP, Apple Books, and Kobo. The truth? Neither tool is a silver bullet. They come with steep learning curves, technical quirks, and UK-specific challenges that many guides overlook.

This article isn’t just a technical manual. It’s a no-nonsense, UK-focused, business-oriented comparison of Calibre vs Sigil. I’ll walk you through their strengths and weaknesses, share practical UK market insights, and offer tips to help you make an informed decision based on real-world experience—not fluff.

Understanding the Role of eBook Formatting Software

Before you even open Calibre or Sigil, it’s crucial to understand what these tools do—and what they don’t. Both are primarily ePub editors/converters: they help you prepare your manuscript for digital distribution, converting from formats like DOCX or HTML into ePub or MOBI files.

• Calibre is best known as a comprehensive eBook management system with powerful conversion features.
• Sigil is a dedicated ePub editor focused on manual editing and precise control over ePub content.

Neither replaces professional formatting expertise, especially for complex layouts or commercial-quality books. They’re tools for authors who want to get hands-on but come with significant technical overhead.

Basic Terminology

• ePub: The standard open format for eBooks, supported by most retailers except Amazon Kindle, which prefers MOBI or Amazon Kindle Format 8 (KF8).
• MOBI: Amazon’s older, proprietary eBook format, gradually replaced by AZW3/KF8.
• HTML/CSS: The coding languages behind ePub files, controlling structure and styling.
• Metadata: Book information like title, author, publisher, ISBN.
• Cover Image: The front cover file embedded or linked in the eBook. While free tools like Calibre and Sigil may seem appealing, the hidden costs—time, technical errors, and lost sales from poor formatting—can outweigh the initial savings.

Note also that UK authors face specific considerations:

• ISBNs cost £93 each or £174 for 10 through Nielsen, the UK ISBN agency, unlike free Amazon-issued ISBNs that list Amazon as the publisher.
• VAT rules mean print books are zero-rated, but eBooks have varying tax implications depending on the platform.
• Professional cover design typically costs £300–£600, and poor formatting can undermine even the best cover.

Knowing this helps frame the value of investing in professional formatting or using tools like publishing.co.uk's automated services to avoid costly mistakes.

Here’s a practical walkthrough of how to use both tools for a typical UK author preparing a book for Amazon KDP and other retailers. I’ll highlight critical menu paths and tips for each.

Using Calibre: Conversion and Management Powerhouse

Calibre is more than an editor—it's a full eBook library and converter.

1. Import Your Manuscript

• Open Calibre and click Add books in the top-left.
• Select your manuscript file, ideally a clean DOCX or HTML.
• Calibre supports many formats but prefers well-structured DOCX or HTML files for conversion.

Pro Tip: Before importing, clean your Word file of excessive styles and use consistent heading levels. UK authors should ensure UK English spelling is used to avoid odd autocorrects during conversion.

2. Edit Metadata

• Select the book, then click Edit metadata > Edit metadata individually.
• Fill in Title, Author, Publisher (use your imprint or your name, not Amazon if using your own ISBN).
• Add your ISBN here if you have one from Nielsen.
• Add Language (set to en-GB) to ensure UK English spelling and hyphenation.

Note: Failure to input your Nielsen ISBN correctly can cause your book to be misrepresented in retailer databases, impacting discoverability and sales.

3. Convert to ePub or MOBI

• Select the book, then click Convert books.
• In the top-right Output format dropdown, choose EPUB for wide distribution or MOBI for Kindle.
• Navigate to Look & Feel for font embedding and style tweaks.
• Under Page Setup, select the appropriate output profile (e.g., Kindle Fire, Generic eInk).
• In the Structure Detection tab, ensure chapter breaks are correctly identified by headings to avoid merged chapters.
• Click OK to start conversion.

UK-Specific Tip: Amazon UK will accept MOBI or ePub (KDP now accepts ePub uploads), but ePub is preferred for wider distribution, including Apple Books and Kobo UK.

4. Check the Result

• Calibre includes a built-in eBook viewer; double-click the converted file to preview.
• Look for common issues: missing images, broken links, weird paragraph spacing.
• Export the converted file and test on multiple devices or with Amazon’s Kindle Previewer.

5. Edit ePub Internally (Optional)

• Calibre allows basic editing via Edit book but it’s limited compared to Sigil.
• Use this for quick fixes like tweaking metadata or cover image replacement.

Using Sigil: Precision ePub Editing

Sigil is ideal for hands-on control over your ePub files but requires familiarity with HTML and CSS.

1. Create or Open an ePub File

• Open Sigil and select File > New to start fresh or File > Open to edit an existing ePub.
• To import, convert your manuscript first to ePub with Calibre or other tools, then open in Sigil for fine-tuning.

2. Edit Book Structure and Content

• Use the Book Browser panel to view chapters and files.
• Edit HTML files directly in the Code View or WYSIWYG Book View.
• Insert images via Insert > Add Existing Files.
• Manage Table of Contents with Tools > Table of Contents > Generate ToC.

Pro Insight: UK authors should ensure their TOC uses consistent chapter naming and that the toc.ncx file is properly generated to meet retailer requirements.

3. Edit Styles

• Access CSS stylesheets in the Book Browser.
• Modify fonts, paragraph indents spacing via CSS rules.
• Avoid overly decorative fonts. Stick with web-safe options such as Georgia, Times New Roman, or Arial to ensure compatibility with all UK e-readers.

4. Validate and Save

• Use Tools > Validate EPUB with FlightCrew to check for errors.
• Fix errors manually in the HTML/CSS code.
• Save your clean ePub file.

UK-Specific Note: Validation is crucial because UK retailers like Waterstones’ Kobo platform reject uploads with metadata or formatting errors more strictly than Amazon.

When to Use Each Tool

ISBN and Metadata: Why It Matters

A massive oversight UK authors make is relying on free Amazon ISBNs, which list Amazon as the publisher. If you want to build a lasting brand and maintain control over your catalogue, buying your own ISBNs from Nielsen is essential.

• Cost: £93 per single ISBN, or £174 for a block of 10.
• Metadata: You must input your ISBN correctly in Calibre or Sigil metadata fields.
• Barcode: For print books, you’ll need an EAN-13 barcode matching the ISBN.

This investment is non-negotiable if you want to be taken seriously in the UK market. For example, my own imprint was ignored by UK libraries and some retailers when I used Amazon’s free ISBNs, but once I switched to Nielsen ISBNs, discoverability improved significantly.

VAT and Tax Implications

• Print books are zero-rated for VAT in the UK, but eBooks carry 20% VAT when sold through Amazon, Apple, or other UK platforms.
• If you price your eBook at £4.99, your actual revenue after VAT and retailer cuts is closer to £2.50–£3.00.
• This affects pricing strategy and royalties, so you must factor this into your business plan.

UK Market Expectations on Formatting Quality

UK readers expect professional formatting—odd indents, inconsistent paragraph spacing, or poor font choices can hurt reviews and sales. While Calibre and Sigil can produce professional results, the learning curve is steep, and mistakes common.

1. Relying on Default Settings

Many authors run conversions in Calibre with default settings, resulting in inconsistent fonts, broken TOCs, and missing images. Always spend time adjusting Look & Feel and Page Setup options.

2. Ignoring Validation Errors in Sigil

I’ve seen authors ignore FlightCrew validation warnings, leading to rejected uploads on Amazon KDP. Always review and fix validation errors before publishing.

3. Using Overused or Unprofessional Fonts

Fonts like Papyrus or Bleeding Cowboys are a red flag for readers and reviewers. Stick to standard, web-safe fonts like Georgia, Palatino, or Helvetica.

4. Skipping UK-Specific Metadata Fields

Leaving out your Nielsen ISBN or UK imprint information in the metadata can cause listing issues on UK platforms.

5. Underestimating Time Investment

While Calibre and Sigil are powerful, here are additional tools and services UK authors should consider:

• publishing.co.uk: Automated, UK-focused formatting services that remove the headache and deliver KDP-compliant files quickly. This is a solid alternative to wrestling with fiddly free tools or overpriced freelancers.
• Kindle Previewer: Amazon’s free tool to preview how your eBook will look on Kindle devices.
• Vellum (Mac only): Paid software with user-friendly interface, not UK-specific but popular among authors.
• Adobe InDesign: Professional layout software, excellent for print and complex eBooks but costly and complex.
• ISBNs from Nielsen: Purchase your UK ISBNs here: https://www.nielsenisbnstore.com/

When you factor in the time lost fixing mistakes or learning HTML/CSS, paying for a professional or reliable automated tool often pays for itself.

Use Calibre vs Sigil — quick guide

Use Calibre when:

• Converting between formats (DOCX→EPUB, EPUB→MOBI)
• Managing a library of multiple ebooks
• Bulk metadata edits across many files
• Quick conversions where output quality isn't critical

Use Sigil when:

• You need precise control over EPUB HTML/CSS
• Fixing a specific layout bug in an existing EPUB
• You're comfortable hand-editing HTML
• You need to validate to strict EPUB 3 spec

Use neither (use Atticus, Vellum, or a service) when:

• You want polished, professional output without learning HTML
• You're publishing commercially and presentation matters
• You don't enjoy fiddly software

Quick verdict

Use this if:

• You're publishing your first 1-3 books and want a fast workflow
• You don't want to learn complex software
• Cross-platform compatibility matters

Skip it if:

• You're publishing 10+ books and need granular control
• Your books have complex layouts (cookbooks, picture books)
• You already have a workflow that works

Best alternative: depends on your priority — speed (paid tool), price (free Calibre), or polish (paid service).

Frequently asked questions

What's the cheapest viable option for serious self-publishing?

Calibre (free) for conversion, Atticus (£147 one-off, cross-platform) for layout, Canva Pro (£11.99/month) for cover. Total: under £200 for full workflow.

Should I learn the tool myself or pay someone to format for me?

If you'll publish 3+ books and enjoy software, learn the tool — it'll save thousands long-term. If this is a one-off launch, paying a formatter (£100-£300) is faster and avoids the 20-30 hour learning curve.

Does the tool affect KDP acceptance?

Indirectly. Tools that produce cleaner output (Vellum, Atticus) reduce KDP rejection rates. Calibre conversions and DIY Word formatting cause about 40% of formatting rejections in our queue.

Can I switch tools mid-project?

Most tools let you export to DOCX. You lose tool-specific features (snapshots, dynamic captions) but the manuscript transfers. Re-formatting in the new tool typically takes 2-4 hours.

About this guide

Written by Robert Prime for publishing.co.uk. Last reviewed May 2026. Specs and pricing change — verify current figures with the linked sources before relying on them.

Related guides

• White Vs Cream Paper
• Vellum Vs Atticus
• Self Publishing Vs Traditional Uk
• Reedsy Vs Scrivener
• Plotting Vs Pantsing

External references

• For current UK indie publishing trends, see the ALCS Author's Earnings report.