Conversion errors are rarely about the tool failing — they're usually about input quality, size limits, or format mismatches. This guide covers the five most common error scenarios we see, what each one means, and exactly how to fix it.
When Conversion Fails Before It Starts
The upload endpoint rejects your file immediately. This is usually one of three things:
File too large. Free tier: 10 MB. Pro: 50 MB. API: 100 MB. If your file exceeds the limit, the error names the cap and you won't waste time waiting for a job that was never going to run.
DRM-protected. Books purchased from Kindle, Kobo, or Google Play often carry DRM. We reject them on upload because we don't strip protection. Look for a DRM-free edition of the same title.
Corrupted or truncated. A half-downloaded EPUB or PDF that ends mid-file gets refused. Open the source file in your normal reader. If it fails there, no converter will repair it.
When Conversion Starts but Doesn't Finish
You uploaded successfully, the job kicked off, but then failed partway through. This is where most user errors surface.
Unsupported source format. Not every format converts to every other format. If you selected an impossible pair, the engine tells you immediately.
Source file has structural issues. EPUB files are ZIP archives with XML inside. If the archive is malformed, Calibre-based engines can't read the book, even if your reader happens to open it.
Network timeout. On shared Wi-Fi or slow connections, the upload itself may drop. Refresh the page and try again.
When the Output Looks Wrong
The conversion finished successfully, but the result isn't what you expected.
Missing table of contents. Some source formats don't include TOC metadata. If your output has no chapters, the source likely didn't either. This isn't a bug — it's source quality.
Lost images. Scanned PDFs or heavily illustrated EPUBs sometimes lose images during conversion if the output format doesn't support embedded graphics well.
Formatting breaks. Complex layouts (columns, footnotes, special fonts) rarely survive round-trip conversion cleanly. For best results, convert once to the target format and adjust manually if needed.
Rate Limits and Quotas
Free users get 5 conversions per hour. This is a soft limit designed to prevent abuse on shared infrastructure.
If you see a rate-limit error, wait 60 minutes for the window to reset, or upgrade to Pro for unlimited hourly conversions.
On campus or office Wi-Fi, someone else on the same connection may have used part of your quota. Switch networks or wait.
When Nothing Works: What to Send Us
If you've tried everything above and the conversion still fails, here's what helps us reproduce the issue fastest:
- Source format and target format
- File size (not the file itself — we don't want your book)
- Browser and operating system
- The exact error message or code
That combination lets us identify the problem without waiting for a back-and-forth. Email us at hello@bookconv.com or open a GitHub issue.
Key Takeaways
- Upload errors are fast — size, DRM, or corruption are rejected immediately, not after a long wait.
- Output quality depends on source quality — broken or complex source files produce broken or simplified outputs.
- Rate limits are hourly — if you hit them, wait or upgrade.
- Send us metadata, not files — we can reproduce the issue without seeing your book.