CONVERT
OPML → EPUB
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Fast, secure OPML to EPUB conversion. No registration required.
Setup: OPML is a document format oriented around a particular office suite or publishing pipeline. Goal: an interchangeable EPUB. A OPML to EPUB conversion is one of those tiny jobs that blocks real work until it is done. KaijuConverter turns the OPML into a usable EPUB in the background so you can move on — uploads are encrypted in transit and both files disappear automatically within a couple of hours. Worth knowing: OPML is a document format oriented around a particular office suite or publishing pipeline. Meanwhile EPUB is the open IDPF ebook standard, a ZIP of HTML and CSS supported by every modern e-reader.
OPML Outline
Source formatOPML (Outline Processor Markup Language) is an XML format for structured outlines and lists. It is most widely used for exchanging RSS feed subscription lists between podcast apps and feed readers, and for hierarchical note-taking.
EPUB eBook
Target formatEPUB is the open standard for reflowable digital books. It adapts text to any screen size and is supported by most e-readers except Kindle. EPUB 3 adds support for multimedia and interactivity.
Why convert OPML to EPUB
Sending OPML to someone who expects EPUB regularly leads to errors or quality loss as the receiving software performs its own silent re-encode. Converting upstream lets you control the quality trade-offs rather than leaving them to an external tool.
HOW TO CONVERT
OPML → EPUB
Provide the file
Drag and drop or select a OPML file up to 25 MB. No account required on the free tier.
Run the conversion
We pick the right backend for this pair automatically and produce a EPUB that matches the source data exactly.
Retrieve the output
A download link appears as soon as the EPUB is ready. For batch jobs, you get a single ZIP download.
Common Use Cases
Share across platforms
Send EPUB files to anyone without worrying about whether they have the right software for OPML.
Embed in documents
Drop EPUB output into Word, Google Docs, PowerPoint, Notion or a website without conversion warnings.
Optimize size
EPUB often produces smaller files than OPML for web, email and storage.
Archive & future-proof
Store in a widely-supported format that will still open on future operating systems without legacy plugins.
OPML vs EPUB — Strengths and limitations
What each format does best, and where it falls short.
OPML Strengths
- Standard RSS subscription interchange format.
- Simple XML — easy to parse and generate.
- Highly extensible via arbitrary attributes.
- Supported by every major outline and RSS tool.
Limitations
- XML verbosity — larger than a JSON-based equivalent.
- Specification is loose — different tools disagree on edge cases.
- Primary use (RSS reading) has shrunk dramatically since Google Reader.
EPUB Strengths
- Open standard — no vendor lock-in, no DRM required.
- Reflowable text — adapts to any screen size, font size, or orientation.
- Rich typography via CSS, embedded fonts, and SVG.
- Accessibility-first: native support for screen readers, adjustable text, and alt-text.
- Universal across every non-Kindle ebook reader and library app.
Limitations
- Kindle does not support EPUB natively (Amazon wants you to convert to AZW3).
- Fixed-layout EPUBs (for children's books, comics) are awkward to author.
- Rendering quality varies between apps — some CSS works everywhere, some does not.
OPML vs EPUB — Technical specifications
Side-by-side comparison of the technical details.
OPML
- MIME type
- text/x-opml
- Extension
- .opml
- Format
- XML with nested <outline> elements
- Standard
- OPML 2.0 (2006)
- Primary use
- RSS subscription interchange
EPUB
- MIME type
- application/epub+zip
- Extension
- .epub
- Container
- ZIP archive
- Markup
- XHTML 1.1 (EPUB 2); HTML5 (EPUB 3)
- Standards
- IDPF/W3C EPUB 2.0.1, 3.0, 3.2, 3.3
| Specification | OPML | EPUB |
|---|---|---|
| MIME type | text/x-opml | application/epub+zip |
| Extension | .opml | .epub |
| Format | XML with nested <outline> elements | — |
| Standard | OPML 2.0 (2006) | — |
| Primary use | RSS subscription interchange | — |
| Container | — | ZIP archive |
| Markup | — | XHTML 1.1 (EPUB 2); HTML5 (EPUB 3) |
| Standards | — | IDPF/W3C EPUB 2.0.1, 3.0, 3.2, 3.3 |
OPML vs EPUB — Typical file sizes
Approximate file sizes for common scenarios.
OPML
- Typical RSS reader export (50 feeds) 5-30 KB
- Deep outline (Scrivener novel plan) 20-200 KB
EPUB
- Novel (300 pages, text only) 200-800 KB
- Illustrated reference book 5-30 MB
- Fixed-layout children's book 30-100 MB
Quality & Compatibility
Fidelity depends on how close the two formats sit conceptually. Same-family conversions (document↔document, audio↔audio) keep the essentials intact. Cross-family jobs (image→video, text→PDF) reinterpret the source into a genuinely different artefact, so expect the EPUB to emphasise different attributes than the OPML.
Tips for Best Results
- If the OPML contains sensitive data, strip metadata in the EPUB export (toggle under Advanced) before sharing publicly.
- Conversion settings that look obscure (bitrate, colour profile, compression level) matter mostly for archival and professional workflows — defaults are fine for everyday use.
- Batch conversions share settings across every file in the job; set them once, apply to many.
Frequently Asked Questions
Yes. The free tier accepts files up to 25 MB without registration, email capture or watermarks; paid plans go up to 2 GB. Paid plans raise the size cap, enable batch conversions and provide a REST API for automation, but nothing on the free tier is quality-limited — the output is exactly the same as on any paid plan.
Uploads run over HTTPS, files are processed in isolated containers, and both the source OPML and the EPUB output are auto-deleted within two hours. No account is required, file contents are never logged, and KaijuConverter does not use uploads for AI training. The paid plan adds a signable data-processing agreement for regulated workflows.
Most files finish in well under a minute. Small images and documents are typically ready in a few seconds; large video or audio files scale roughly with duration. Upload speed from your network is usually the dominant factor, not server time.
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Related comparisons
See these formats side by side to understand which fits your use case best.
Related Guides
EPUB E-Book Format: The Complete Technical Guide
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Read guideEPUB Format: Electronic Publication Ebook Standard — Complete Guide
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Read guideEPUB 3: The Open Standard for Ebooks and Digital Publications
What is EPUB 3? Learn the ZIP-based structure of .epub files, OPF package documents, navigation, XHTML5 content, how to create EPUB files in Python with ebooklib, and how to validate with EPUBCheck.
Read guideSecure & Private Conversion
Your files are encrypted during transfer, processed in isolated containers, and automatically deleted within 60 minutes. We never read, share, or store your data.