calibre Sends the Wrong Format to Device: How to Fix It

You select a book, click Send to device, and calibre transfers EPUB, MOBI, AZW3, KEPUB, or PDF when you expected a different format. In most cases, calibre is not choosing randomly. It is following the connected device driver's supported-format list, the formats already attached to the book record, and any conversion needed to produce a compatible file. The fastest fix is to identify whether calibre selected an existing format, converted the book automatically, or used a different transfer route than you thought.

This guide focuses specifically on automatic format selection during a send operation. It covers direct USB connections, devices connected as folders, and the settings that can affect selection. Work through the tests in order. Once calibre sends the expected format reliably, stop changing settings. Additional changes can hide the original cause or introduce a second problem.

1. Confirm the Symptom With a Small Safe Test

Start with one book rather than changing global settings or testing an entire series. The best test book is an unrestricted file that you are allowed to convert and that has at least two formats attached to the same calibre record, such as EPUB and AZW3.

1.1 Verify which formats the book actually contains

Select the book in the calibre library and look at the formats listed in the Book details panel. You can also open Edit metadata and inspect the available formats. A single calibre book record can contain several files, and the normal Send command may choose one compatible with the detected device.

Do not rely on the cover, title, filename, or the format that opens when you click View. The viewer can open one format while Send to device selects another. Right-clicking View also lets you choose a particular format, but that choice does not necessarily become the automatic sending preference.

For a controlled test:

1. Select one book containing two or more known formats.
2. Write down the displayed formats before sending.
3. Remove older copies of that test book from the device, if safe to do so.
4. Send the book once using the normal Send to device command.
5. Open the Device view in calibre and inspect the transferred filename or format.

Success looks like this: you can state exactly which source formats existed and which extension reached the device. If you cannot yet distinguish an EPUB from an AZW3 or PDF on the device, do not change conversion settings. First establish what was actually transferred.

1.2 Force the expected format once

Right-click Send to device, choose Send specific format to, select the destination, and choose the format you expected. This bypasses automatic selection for that send and is the quickest way to determine whether the desired file is usable.

If the requested format transfers and opens correctly, the file and connection are probably sound. The issue is likely the automatic priority used for normal sends. If the requested format is unavailable, calibre may offer to convert it. If the transfer succeeds but the reader cannot display the book, the device or reading application may not support that format despite accepting the file.

Stop here if you only occasionally need a different edition or format. Send specific format is a safe, precise solution and avoids changing global behavior.

2. Check the Settings and Library State That Control Format Selection

Automatic selection is primarily shaped by the detected device, the ordered formats supported by its driver, and the formats available in the selected book record. Several settings sound relevant but affect different parts of the process.

2.1 Check the device's supported formats and their order

Connect the reader and wait for calibre to display the Device button. Open the device configuration options, often available by right-clicking the Device button or through the relevant device plugin under Preferences. Look for the list of formats associated with that device.

A device plugin can provide an ordered list of supported formats. That order matters because calibre needs a consistent way to choose among several compatible files. If EPUB appears ahead of PDF and the record contains both, EPUB may be selected even though you expected PDF. Similarly, a Kindle-oriented driver may prefer AZW3 or another Kindle-compatible format over EPUB, depending on the device and connection method.

Enable only formats that the actual device or reading application can handle, then place the format you normally want ahead of less desirable alternatives when the interface allows reordering. Do not mark a format as supported merely to force a transfer. A successful copy does not guarantee that the device can open the file.

Success looks like this: after changing the format priority, the normal Send button transfers the expected existing format from the test record. Once that happens twice with the same test book, stop changing device settings.

2.2 Understand the preferred output format

Go to Preferences > Interface > Behavior and inspect Preferred output format. This preference is important for conversion defaults and some format-oriented actions, but it should not be treated as an unconditional command to send that extension every time.

For example, setting EPUB as the preferred output does not make EPUB appropriate for a device whose driver does not report EPUB as a supported transfer format. It also does not necessarily override the device's priority when the book already contains several supported formats.

Set the preferred output format to the format you most commonly create. Then configure the connected device separately. This keeps conversion defaults and device compatibility aligned without assuming they are the same setting.

Success looks like this: manual conversions default to the intended output, while normal sends still follow the compatible device configuration.

2.3 Determine whether conversion on send is occurring

If none of the files attached to a book are suitable for the detected device, calibre can offer to convert the book to a format the device understands. This can make it appear that calibre ignored the original format when it actually created a compatible replacement.

Watch for a conversion prompt after clicking Send, and check the Jobs area in the lower portion of the calibre window. A conversion job followed by a device transfer indicates conversion on send. Open the completed job details to confirm the input and output formats.

If the conversion output is wrong, cancel the send and convert the book manually. In the conversion dialog, verify the input format in the upper-left area and the output format in the upper-right area. After conversion, confirm that the newly created file appears in the book's formats list. Then use Send specific format for the first test.

Success looks like this: the Jobs details name the expected input and output, and the device receives the resulting file. If that file opens normally, further conversion changes are unnecessary.

2.4 Inspect multiple formats attached to one record

A common cause of calibre device formats appearing wrong is an old or unwanted format stored alongside the current one. You may have edited an EPUB but left an older AZW3 attached, or replaced a PDF while an earlier EPUB remained in the record. Automatic sending can select any compatible format according to device priority, not according to which file you edited most recently.

Open Edit metadata and review every available format. If one is obsolete, back it up if necessary and remove only that format from the record. Do not delete the entire book entry. Alternatively, reconvert the corrected source into the other formats so every attached version contains the intended content.

Also check that separately imported files have not become duplicate book records with nearly identical titles. Metadata matching and duplicate handling can place editions where you did not expect them. Confirm the author, title, identifiers, and available formats on the exact row you are sending.

Success looks like this: the selected record contains only current files, or all attached formats represent the same edition. A normal send then chooses a current compatible file.

2.5 Use the device output profile for layout, not format priority

The device output profile appears in the conversion dialog under Page setup. It helps calibre tailor measurements, screen dimensions, margins, and related layout behavior for the target. It does not by itself tell the Send button to prefer EPUB over AZW3, MOBI, KEPUB, or PDF.

Choose a profile matching the target device when converting, or use an appropriate generic profile if no exact option exists. Fix format selection through the device's supported-format configuration, Send specific format, or deliberate manual conversion.

Success looks like this: the chosen extension is correct first, and the output profile then improves how that converted file appears on the screen.

2.6 Separate USB sending from email and Content server downloads

Email delivery and the Content server are separate workflows. An email recipient configuration can require or favor particular formats, while the Content server presents the available book formats for reading or downloading in a browser. Neither should be assumed to behave exactly like the USB Send to device button.

If the unexpected format arrived by email, inspect the email-sharing configuration and the recipient's accepted formats. If it came from the Content server, return to the book's server page and deliberately select the required download format. If a reading application automatically chose a server download, check that application's settings as well.

Success looks like this: you can reproduce the problem through one named route only, such as USB, email, or browser download. Troubleshoot that route instead of changing unrelated device settings.

2.7 Check device plugins and format-specific plugins

Device plugins tell calibre how to recognize and communicate with readers. Third-party plugins can also add format handling or device-specific behavior, including workflows involving KEPUB. If automatic selection changed after installing, updating, enabling, or configuring a plugin, test that plugin directly.

Open Preferences > Plugins, identify the active device interface plugin, and inspect its customization options. Record the current settings before changing anything. If safe, temporarily disable only the suspected third-party plugin, restart calibre, reconnect the device, and repeat the one-book test.

Do not disable every plugin simultaneously. That makes it difficult to identify the cause and can prevent calibre from recognizing the device at all.

Success looks like this: disabling or correcting one plugin makes automatic selection predictable. At that point, stop and decide whether to keep the plugin disabled or adjust its documented options.

2.8 Do not confuse viewer or editor behavior with sending behavior

The calibre viewer can open a selected format, and the editor works with supported editable formats. Editing an EPUB does not automatically update an AZW3, MOBI, or PDF attached to the same record. If the device receives one of those other files, it may show the old text or formatting.

After editing, save the edited file and reconvert it into any alternate format you intend to send. Then verify the available formats and use Send specific format once. This confirms that the updated edition, not a stale companion file, reaches the device.

3. Check the Connection, Operating System, and Device Limitations

Connection problems usually prevent sending altogether, but they can also cause calibre to use a generic or unexpected device interface. Check the connection layer if the device configuration is missing, changes between sessions, or reports implausible supported formats.

3.1 Confirm the correct device and USB mode

Connect only one supported reader, phone, or tablet during testing. Ensure the operating system can see it. Android-based devices commonly require an unlocked screen and a USB mode such as File Transfer or MTP rather than charging only. Some devices also display a permission prompt that must be accepted.

On macOS and Linux, another application may hold the device connection. Close file managers, phone-transfer utilities, or other programs accessing the reader, then restart calibre and reconnect. On Windows, confirm that the device appears normally in File Explorer or the appropriate portable-device interface.

Success looks like this: calibre consistently identifies the same device and exposes the same device configuration after reconnection.

3.2 Verify permissions and writable storage

If the device or destination folder is read-only, calibre may fail before completing the intended send. Check free space and confirm that you can create a harmless test folder or copy an ordinary file through the operating system when the device permits direct file access.

On macOS, review privacy permissions for files, folders, and removable volumes if calibre reports access errors. On Linux, confirm that the mounted storage is writable by your user. On Windows, check whether security software or controlled-folder protection is blocking calibre or its temporary conversion files.

A permissions problem usually produces an error rather than silently selecting another format. Treat it as the primary cause only when the Jobs details or operating system reports access, write, lock, or temporary-file failures.

3.3 Pause cloud synchronization around the test library

If the calibre library is stored in a folder actively synchronized by a cloud service, another process can lock, delay, or replace files while calibre is converting or sending. Pause synchronization briefly and repeat the one-book test. Avoid manually modifying files inside the calibre library folder.

If the test succeeds only while synchronization is paused, relocate the active library to a normal local folder and use a separate backup strategy. The objective is to let calibre remain the sole manager of its library structure during database and format operations.

3.4 Respect reader and application limitations

A device may accept a file through USB but still be unable to index or open it. Support can also differ between the device's built-in reader and an installed reading application. PDF support does not mean PDF will reflow like EPUB, and a device that accepts EPUB may not support a vendor-specific EPUB variant in the same way.

Verify supported formats in the device manufacturer's documentation or the reading application's documentation. If the expected format is unsupported, choose a compatible output instead of forcing the extension. Renaming a file extension does not convert its internal format.

4. Use Logs and Debug Output When the Choice Still Makes No Sense

Logs are useful after you have reproduced the issue with one book and one connection. They are less useful after many simultaneous settings changes.

4.1 Read the Jobs details first

Click the Jobs indicator after the send finishes or fails. Open the relevant conversion and transfer jobs and note:

• The input format selected for conversion
• The output format created
• The path or filename sent to the device
• The detected device or destination
• Warnings, access errors, or plugin messages

If the log clearly shows the expected format, inspect the device for an older duplicate copy. Readers sometimes retain two editions under similar titles or continue showing cached library information until they rescan or restart.

4.2 Run Debug device detection

If calibre identifies the wrong device, exposes the wrong format list, or behaves differently on each connection, open Preferences > Miscellaneous and run Debug device detection with the device attached. Save the output without editing it.

This report is particularly useful for an unrecognized, unusual, or newly changed USB device. It can show which interface calibre sees and help determine whether the operating system, built-in driver, or user-defined device plugin is involved.

4.3 Use conversion debug output only for conversion defects

If calibre creates the requested extension but its layout or contents are wrong, enable debug output in the conversion dialog and choose an empty destination folder. calibre can save intermediate stages of the conversion pipeline, allowing advanced troubleshooting of structure, CSS processing, and output generation.

Conversion debug output is not the first tool for a simple EPUB-versus-AZW3 selection problem. Use it only after the logs confirm that conversion occurred and that the selected output extension was correct.

4.4 Start calibre in debug mode when necessary

If the graphical logs omit a recurring plugin or device error, close calibre and launch its GUI from a terminal or command prompt with:

calibre-debug -g

Reproduce one send, then save the terminal output. Avoid posting logs publicly without checking them for usernames, local paths, IP addresses, email addresses, or other private information.

5. Run a Clean Temporary Test Before Reinstalling

Reinstalling calibre rarely fixes a format priority stored in device, library, or plugin settings. Before reinstalling or deleting anything, isolate the problem with a temporary library and a harmless test file.

1. Record your current device and plugin settings.
2. Create or switch to a temporary calibre library in a local, writable folder.
3. Add one unrestricted EPUB or another suitable test source.
4. Convert it manually into a second format supported by the device.
5. Confirm that both formats appear on one book record.
6. Connect only the target device.
7. Use Send specific format for each format and verify whether both open.
8. Remove the device copies, then test the normal Send command.

If the normal send works correctly in the temporary library, the installation and basic device connection are probably functional. Return to the main library and inspect the affected record, saved conversion settings, or plugin configuration. If the temporary test also chooses the unwanted format, concentrate on the device driver and supported-format priority.

Do not delete the main library, its database, or all configuration files to test this symptom. Make backups before library maintenance, and change one variable at a time.

6. Quick Fix Checklist

• Test one unrestricted book containing two known formats on the same record.
• Confirm the actual extension in calibre's Device view after sending.
• Use Send specific format to to prove the expected file can transfer and open.
• Review the connected device's supported formats and their priority.
• Set Preferred output format for conversion defaults, not as a universal send override.
• Check Jobs to see whether calibre converted the book during the send.
• Remove or update stale formats attached to the book record.
• Use the output profile to improve layout, not to choose the extension.
• Troubleshoot USB, email, and Content server transfers as separate workflows.
• Test one suspected third-party plugin at a time.
• Confirm the device is in the correct USB data-transfer mode.
• Run Debug device detection if calibre identifies the device incorrectly.
• Use a temporary local library before reinstalling or deleting settings.

7. Frequently Asked Questions

7.1 Why does calibre send EPUB when I want PDF?

The connected device may report both formats as supported while ranking EPUB ahead of PDF. If the book record contains both, automatic Send can choose EPUB. Use Send specific format for occasional PDF transfers, or adjust the device's format priority if PDF should normally come first. Remember that EPUB generally adapts to screen size, while PDF preserves a fixed page layout.

7.2 Why does calibre convert my EPUB to AZW3 or another format?

Conversion on send can occur when the detected device does not accept the available source format through its configured driver. Check the conversion prompt and Jobs details. If the device supports your desired format through a different transfer method or reading application, send that format explicitly or revise the correctly detected device configuration.

7.3 Does Preferred output format control Send to device?

It influences conversion defaults and several format-oriented actions, but it is not an unconditional override for device compatibility. Automatic sends also depend on the device driver's supported formats, their ordering, and the files already attached to the book.

7.4 Why does the device receive an older version of my edited book?

You may have edited one format while calibre sent another attached to the same record. For example, editing EPUB does not automatically update an existing AZW3. Reconvert the edited source, remove obsolete companion formats if appropriate, and use Send specific format to verify the updated file.

7.5 Is KEPUB selection controlled by the normal calibre settings?

KEPUB workflows commonly involve device-specific behavior or an installed plugin. Inspect the active Kobo-related device and format plugin settings, then test with one book. Temporarily disable only the suspected third-party component if necessary. Keep a copy of the original EPUB and avoid changing several plugin settings at once.

7.6 Should I reinstall calibre if it keeps sending the wrong format?

Not initially. Reinstallation usually does not correct device format priority, multiple files attached to a record, saved conversion choices, or plugin configuration. First run the temporary-library test. Reinstall only when debug output points to damaged program files or calibre itself cannot start or operate normally.