Calibre vs Sigil: Free eBook Formatting Tools Compared

Checked 6 September 2026: calibre 9.14 (released 28 August 2026) and Sigil 2.8.1 (released 18 July 2026), both free and open-source under GPL-3.0; KDP’s accepted formats and delivery cost re-read on KDP’s own pages. Reviewed by Robert Prime, 6 September 2026.

Table of Contents

• Introduction
• Calibre and Sigil: what to know first
• Step-by-Step Guide to Using Calibre and Sigil
• Calibre and Sigil in the UK
• Calibre and Sigil: mistakes to avoid
• Calibre and Sigil: tools and resources
• Calibre and Sigil: what it costs
• Frequently Asked Questions

Introduction

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.

When I was formatting Google. Panic. Repeat. about my battle with health anxiety, I thought writing was the hard part. I was wrong. The real nightmare was getting it KDP-ready. After hiring a formatter for £130 who delivered a messy file that cost me time and money, I tried Calibre and Sigil to regain control. What I discovered was a world of complicated menus, cryptic error messages, and formatting headaches that almost made me give up.

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.

Calibre and Sigil: what to know first

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, and since 2022 the format KDP asks for as well (it no longer accepts MOBI for new reflowable titles). Apple Books, Kobo and Google Play all take the same file.
• MOBI: Amazon’s old proprietary format, retired for new uploads. If a guide tells you to make a MOBI, it is out of date; KDP converts your EPUB itself.
• 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.

UK Market Context

The UK self-publishing market has grown substantially in recent years. 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 KDP ISBNs, which only work on KDP and show “Independently published” as the imprint.
• VAT rules mean both print books and eBooks are zero-rated (eBooks since May 2020), though audiobooks still carry 20% VAT.
• Professional cover design typically costs several hundred pounds, 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.

Step-by-Step Guide to Using Calibre and Sigil

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

• Select the book, then click Convert books.
• In the top-right Output format dropdown, choose EPUB. That one file goes to KDP, Apple Books, Kobo and Google Play alike.
• 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 accepts EPUB or DOCX (KDP no longer accepts MOBI uploads for new titles), 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, margins, and line 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 ePubCheck to check for errors.
• Fix errors manually in the HTML/CSS code.
• Save your clean ePub file.

UK-Specific Note: Validation is crucial because retailers can reject uploads with metadata or formatting errors.

When to Use Each Tool

Calibre and Sigil in the UK

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.

The UK basics, in one place: ISBNs, VAT on books, tax on book income and legal deposit. Those guides are kept up to date, so this page doesn’t repeat them.

Calibre and Sigil: mistakes to avoid

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 ePubCheck 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

When I self-published Google. Panic. Repeat., I lost dozens of hours just wrestling with Calibre’s quirks and Sigil’s HTML editing. It’s not unusual for new authors to lose many hours to formatting, which is time better spent on writing or marketing.

Calibre and Sigil: tools and resources

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/

Calibre and Sigil: what it costs

Publishing on KDP is free; what costs money is the help you buy. Every part, with dated sources and a calculator for your own book, is in how much it costs to self-publish a book in the UK.

Frequently Asked Questions

What is the most common mistake UK authors make with Calibre vs Sigil?

The biggest mistake is assuming these free tools will automatically produce a professional-quality eBook without manual tweaking. Many authors run default conversions and upload files riddled with formatting errors, poor metadata, and missing covers. This leads to rejection or poor customer reviews.

How much does Calibre vs Sigil cost in the UK?

Both Calibre and Sigil are completely free to download and use. The real cost is your time and technical learning curve. Buying a Nielsen ISBN costs £93 for a single or £174 for ten, which is necessary for professional publishing. Additional costs for covers or professional formatting are separate.

What tools do UK authors recommend for Calibre vs Sigil?

UK authors often start with Calibre for conversion and basic management, then use Sigil for detailed ePub editing if they have HTML/CSS skills. Many eventually move to professional services like publishing.co.uk or paid software like Vellum (Mac only) for ease and quality.

How long does the Calibre vs Sigil process typically take?

For a novice, expect 15–30 hours to learn and produce a clean ePub file. This includes multiple conversion attempts, validation, and device testing. Experienced users can reduce this to under 5 hours.

Can I handle Calibre vs Sigil myself or should I hire a professional?

If you enjoy tech and have time, you can learn to use both tools effectively. However, for commercial-quality results and to avoid costly delays, many UK authors find professional formatting or automated services a better investment. Publishing.co.uk offers UK-focused, affordable automated formatting that saves you from the steep learning curve and common pitfalls.

What are the UK-specific requirements for Calibre vs Sigil?

UK authors must embed proper Nielsen ISBN metadata, use UK English spelling, and comply with VAT implications in pricing. Also, physical print books require EAN-13 barcodes linked to Nielsen ISBNs. These requirements are not always highlighted in US-centric guides and can cause issues if overlooked.