Why won't my EPUB open in Apple Books / Kobo / Calibre?

Most common cause: the download was incomplete or the file extension is wrong (often .zip renamed to .epub without actually being an EPUB). Second most common: the OPF manifest references files that aren't in the ZIP, or the mimetype file is missing or compressed. Run the EPUB validator first — it pinpoints the exact error in 90% of cases.

My EPUB opens in one app but not another — why?

Different readers apply different strictness. Calibre and Adobe Digital Editions are lenient. Apple Books is middle. EPUBCheck / Kobo are strict and will reject files the others accept. An EPUB that works in Calibre but fails in Apple Books almost always has a structural issue that the validator will flag.

What does "invalid EPUB" actually mean?

Usually one of: (1) the first file in the ZIP isn't the mimetype file stored uncompressed, which is required by spec; (2) container.xml is missing or malformed; (3) the OPF package file has broken XML; (4) manifest lists files that aren't in the ZIP. The validator tells you which.

Can I fix a corrupted EPUB myself?

If the ZIP is corrupted (couldn't be opened at all) — probably not without a re-download. If the structure is broken but the ZIP opens — yes. The Epublys validator auto-fixes three common structural problems: incorrect mimetype content, a missing container.xml, and a missing language declaration. For the rest (duplicate IDs, missing manifest entries, bad references), open the EPUB in Sigil and edit the OPF/manifest directly.

My EPUB has DRM — now what?

DRM-locked EPUBs (from Kobo store, Adobe Digital Editions ecosystem, libraries via OverDrive) only open in the ecosystem that issued the license. Removing DRM is a legal gray area in most countries and out of scope for any mainstream tool. If you own the book and the DRM is expiring, contact the store for a refresh.

EPUB Won't Open — How to Fix It

EPUB Validator & Checker →

The fastest fix: run the file through the free EPUB repair tool — it pinpoints the exact fault (corrupt ZIP, missing mimetype, broken OPF, duplicate IDs) and auto-fixes the three it safely can: mimetype, container.xml, and language. Below are the manual diagnoses for everything else.

An EPUB that won't open almost always fails for a specific, diagnosable reason. Work through this list top-to-bottom and you'll find the problem in 90% of cases.

Step 1: Is the file actually an EPUB?

Check the extension — should end in .epub, not .epub.zip, .epub.part, .epub.crdownload, or similar.
Check the size. A legitimate EPUB is typically 200 KB to 20 MB. If it's under 10 KB, the download was truncated. If it's suspiciously round (like exactly 1024 KB), likely a placeholder.
Rename a copy to .zip and try to open it. An EPUB is a ZIP — if the ZIP doesn't open, the file itself is corrupt. Re-download from the source.

Step 2: Run the validator — this finds the error 90% of the time

Upload the file to the EPUB validator. It runs a structural scan — the same error classes that most often cause store rejections — and returns the specific problem. Typical errors and what they mean:

"Mimetype file missing or invalid"

The EPUB spec requires the first file in the ZIP to be named exactly mimetype, containing only the text application/epub+zip, stored uncompressed. If someone recreated the EPUB with a generic ZIP tool, this gets broken. The validator's auto-fix repackages it correctly.

"Container.xml missing or malformed"

META-INF/container.xml tells the reader where the OPF package file is. Missing or malformed → reader can't find anything. Auto-fix regenerates it pointing to the OPF location.

"Package file (.opf) has invalid XML"

The OPF is the manifest of everything in the EPUB. Invalid XML (unclosed tags, bad attributes, encoding mismatch) means the reader can't parse it. Auto-fix can't repair invalid OPF XML — this one needs a manual fix. Open in Sigil, whose OPF editor catches errors on save.

"Manifest references missing files"

The OPF lists files (chapters, CSS, images) that aren't actually in the ZIP. Usually from a broken export. Either re-export from the source, or edit the OPF to remove the missing entries.

"Duplicate IDs" / "Invalid XHTML"

Common after merging or programmatic export. Two chapters both have id="chapter1", or the XHTML has unclosed <p> tags. The validator flags both, but neither is auto-fixed — rename duplicate IDs and close tags in Sigil.

Step 3: Test in multiple readers to isolate strictness

Different readers apply different strictness levels. If the file opens in some but not others, the file has a real issue that strict readers reject.

Calibre — most lenient. If it fails here, the ZIP itself is probably corrupt.
Adobe Digital Editions — lenient. Passes most non-strict files.
Apple Books — middle. Rejects many structural issues.
Thorium / Kobo / EPUBCheck — strict. Rejects anything the validator flags.

Step 4: Is it DRM?

If the EPUB came from a store (Kobo, B&N, library via OverDrive) and opens only in that store's app or a DRM-capable reader like Adobe Digital Editions, it's DRM-locked. Generic readers (Calibre without plugins, Apple Books, Thorium) will reject it. Open in the ecosystem that issued the license, or use the store's dedicated app.

If you bought from a store and the DRM is broken (license expired, device de-authorized), contact the store to re-issue the license. DRM removal is legally gray in most countries and out of scope for this guide.

Step 5: Still broken? Last resorts

Re-download from the source. Especially if the download was interrupted. Many downloads look fine but are subtly truncated.
Rebuild with Calibre. Import the broken EPUB, convert to EPUB (yes, EPUB-to-EPUB), and export. Calibre rebuilds the package from scratch and often repairs structural damage in the process.
Open in Sigil. Sigil's editor tolerates more damage than end-user readers and can often show you what's wrong — missing files, malformed tags, wrong encoding.
Accept it might be dead. Some EPUBs are genuinely unrecoverable (ZIP corruption, critical files missing from the original). If the source is replaceable, replace it.

EPUB Validator — the first stop for any broken EPUB
How to fix EPUB errors — deeper dive on common errors
What is an EPUB file? — the structure explained
Accessible EPUB — write files that pass validation from the start

Try it now — free

Free online EPUB checker and validator — no signup. Catch the structural, well-formedness, and reference errors that get books rejected from KDP and Apple Books.