Calibre Font Size Wrong After Conversion: How to Fix It

When the body text, headings, footnotes, or captions look too small, too large, or inconsistent after an e-book conversion, the problem usually comes from one of four places: calibre rescaled the source sizes, the selected output profile changed the size mapping, the source file contains difficult CSS, or the reader app is applying its own display settings. The safest approach is to test one short conversion, change one relevant option at a time, and stop as soon as the output becomes consistent. This guide focuses specifically on calibre font size rescaling and conversion settings for Windows, macOS, and Linux.

1. Confirm the Symptom With a Small Safe Test

Before changing global preferences or editing the book, confirm whether the font problem is inside the converted file or limited to the program or device displaying it. This distinction prevents unnecessary conversions and helps you choose the correct fix.

1.1 Create a controlled test conversion

Make a copy of the original source file outside your calibre library. Add the copy as a temporary book, then convert it to the format you actually use, such as EPUB or AZW3. Do not test with a book that has already been converted several times because repeated conversions can preserve or compound earlier styling changes.

If the source is editable, choose a book or sample containing several representative elements:

• Normal body paragraphs
• At least two heading levels
• A footnote or endnote
• A caption, block quote, or other smaller text
• Italic and bold text

You do not need to read the entire result. Open a few pages that contain these elements and compare them with the source. A focused test is faster and makes subtle changes easier to identify.

1.2 Compare the result in two readers

Open the converted file in calibre's E-book viewer and, if possible, a second reader app or the intended device. Reset each reader's font controls to a neutral or default position before comparing them.

If the book looks correct in calibre but wrong on the device, the conversion may already be successful. The device or reader app may be overriding fonts, applying a minimum text size, enlarging accessibility text, or treating embedded sizes differently. If the same elements are wrong in both readers, investigate the conversion settings and source CSS.

Success looks like this: you know whether the error is reproducible across readers and can name the affected element. Stop testing display settings if the same size problem appears everywhere.

2. Check Calibre's Font Rescaling Settings

Most cases of calibre font size wrong after conversion can be traced to the Fonts section under the conversion dialog's Look & feel settings. These controls determine whether calibre preserves, normalizes, or proportionally changes font sizes.

2.1 Understand the base font size

The base font size represents the book's main body-text size. When you specify a base font size, calibre identifies the most common text size and rescales the other sizes proportionally around it. A larger base value generally produces larger body text, while a smaller value generally produces smaller body text.

This can fix an entire book whose text is uniformly too small or too large. It is less suitable when the body text is correct but only captions, headings, or footnotes are wrong.

1. Select the original source format in the calibre library.
2. Choose Convert books.
3. Open Look & feel, then the font-related settings.
4. Check whether a custom base font size is present.
5. If you did not intentionally choose it, restore the automatic or default behavior and reconvert.

Success looks like this: ordinary paragraphs display at a comfortable size while headings remain larger and notes remain smaller. Stop increasing the base size once the body text is correct because further increases can distort the hierarchy.

2.2 Check the font size key

The font size key controls the set of size steps calibre uses when mapping small text, body text, subheadings, headings, and unusually large text. It is normally a comma-separated sequence of sizes. A poorly chosen key can compress several source sizes into nearly identical output sizes or create jumps that make headings enormous and notes unreadably small.

If you changed the key previously, return it to the default associated with the output profile before trying to design a custom mapping. Base font size and font size key work together, so changing both at once makes the result difficult to diagnose.

When only one class of text is wrong, compare the converted heading, caption, or footnote with nearby body text. If the hierarchy is too compressed or exaggerated, the font size key is a stronger suspect than the base font size.

Success looks like this: the converted book has a predictable progression from notes and captions through body text to headings. Stop modifying the key when the visual hierarchy is restored, even if the result is not pixel-for-pixel identical to the source.

2.3 Disable font size rescaling when preservation is better

Some EPUB and HTML sources already contain a carefully designed system of relative sizes. In that situation, calibre's normalization can make a good source worse. Enable the option that disables font size rescaling, then convert the untouched source again.

This is particularly useful when the original EPUB looks correct before conversion, but its headings or footnotes change immediately afterward. Disabling rescaling tells calibre to preserve the source size relationships instead of fitting them to the selected size mapping.

Do not assume this option is always better. A source containing chaotic inline sizes, word-processor artifacts, or fixed point values may benefit from rescaling. Compare one test with rescaling enabled and one with it disabled.

Success looks like this: the output retains the source's sensible size relationships without introducing tiny notes or oversized headings. Stop if preservation solves the problem across your test pages.

2.4 Verify the output profile

The output profile influences screen assumptions and default font rescaling behavior. Choose a profile intended for your target device when one is appropriate. Otherwise, use a suitable generic profile rather than selecting an unrelated device merely because its name is familiar.

A mismatched output profile can produce unexpected results even when you did not manually set a base size or key. Convert the same source with the intended profile while leaving the font controls at their defaults. If you are targeting several modern devices and apps, a generic profile is usually a more useful baseline than a highly device-specific profile.

Success looks like this: the book is proportioned correctly on the target screen without requiring extreme reader-app adjustments. Once the correct profile works, do not compensate by adding an unnecessary custom base size.

3. Inspect Source CSS and Look & Feel Overrides

If only particular elements are wrong, the source file probably contains a rule that calibre is preserving or transforming. Common examples include fixed point sizes, pixel values, inline styles, percentage-based footnotes, and selectors that apply to more elements than intended.

3.1 Look for fixed and conflicting font sizes

For EPUB or AZW3 files, right-click the book and choose Edit book. Inspect the relevant paragraph, heading, footnote, or caption. The Live CSS and inspection tools can show which rules apply to the selected element and which rule ultimately wins.

Watch for declarations such as font-size: 8px, unusually large point values, inline style attributes, or repeated rules containing !important. Relative values such as em, rem, and percentages generally adapt more naturally, but they can still produce bad results if they inherit from an unexpectedly small or large parent.

Use the editor only when the conversion controls cannot correct a localized problem. Editing CSS requires more care than adjusting a conversion option, so keep an untouched copy of the source.

3.2 Review Look & feel styling options

In the conversion dialog, review every setting under Look & feel that can alter or override CSS. Pay particular attention to Extra CSS, style transformation rules, and any filtering of CSS properties.

Extra CSS has high priority and can override source rules. It is useful for a targeted repair, but a broad rule such as body { font-size: 120%; } may enlarge nested elements in unexpected ways. Likewise, a rule targeting every paragraph may also affect captions or footnotes if they use paragraph tags.

If you previously added custom CSS, remove it temporarily and reconvert. If the sizes become normal, restore only the specific rules you still need. Avoid forcing absolute sizes across every element unless the target format or device requires them.

Success looks like this: the problematic element is fixed without changing unrelated text. Stop adding overrides once the smallest targeted rule solves the issue.

3.3 Separate metadata from content styling

Ordinary metadata fields such as title, author, tags, series, and identifiers do not normally determine the font size of body text. Re-downloading metadata is therefore not a useful first response to this symptom. However, replacing a book format while updating metadata can change which source file is being converted.

Confirm the input format shown in the upper-left area of the conversion dialog. If the library contains EPUB, PDF, DOCX, and other versions of the same title, make sure calibre is converting the intended source. A clean EPUB may behave very differently from a fixed-layout PDF or a DOCX containing extensive direct formatting.

4. Rule Out Reader Controls and Source Limitations

4.1 Reset reader-app font controls

Reader apps commonly provide their own controls for font size, minimum font size, accessibility scaling, themes, and publisher formatting. Calibre's viewer also lets the reader enlarge or reduce text independently of conversion.

Reset the viewing size before judging a new conversion. In calibre's viewer, restore the default font size rather than assuming the current zoom represents the file's embedded styling. On the target device, compare the result with publisher formatting enabled and disabled if that option exists.

If moving the reader's size control fixes all body text while preserving a sensible hierarchy, the book may not need another conversion. If body text responds but captions or footnotes remain fixed and unreadable, the source probably contains hard-coded sizes.

4.2 Account for format and device limitations

Reflowable formats such as EPUB and AZW3 generally support adjustable text better than page-oriented or fixed-layout sources. A PDF conversion may contain inferred paragraphs, headings, or spans with awkward sizes because the source describes a printed page rather than a flexible reading layout.

When the source quality is poor, try a cleaner source format that you are authorized to use. Do not repeatedly convert the already converted output. Each generation can carry forward flattened or normalized CSS, making it harder to recover the original hierarchy.

USB mode, firewall settings, antivirus tools, email delivery, and the Content server do not normally change font sizes inside a successfully transferred file. Investigate them only if the wrong file is reaching the device, the new conversion is not replacing the old copy, or the server is serving a different stored format.

After sending a corrected book, remove or rename the old device copy if necessary and verify the file's modification time. Reader libraries sometimes keep cached versions or thumbnails, which can make a successful conversion appear unchanged.

5. Use Conversion Logs and Debug Output

Logs are useful when the visible settings look correct but the output remains wrong. They can confirm the input format, selected options, output plugin, warnings, and whether custom processing occurred.

5.1 Read the completed job details

Open calibre's Jobs list after the conversion completes and view the detailed log for that job. Confirm that the expected input and output formats were used. Search the log for references to fonts, CSS, profiles, warnings, or plugins.

A completed conversion does not guarantee that the source structure was good. It only means calibre produced an output file. Warnings about malformed markup or unsupported source features can explain why a localized size rule was lost or interpreted unexpectedly.

5.2 Generate conversion debug output

Calibre can save intermediate files from different stages of the conversion pipeline. Specify a new empty folder for conversion debug output, run the test conversion, and compare the generated stages.

The most useful comparison for this problem is between the stage before font size conversion and the processed stage near the end of the pipeline. If the sizes are sensible before rescaling but wrong afterward, concentrate on the base font size, font size key, output profile, and rescaling switch. If they are already wrong near the beginning, investigate the source file or input-format interpretation.

Advanced users can also run ebook-convert with its debug-pipeline option. This is not required for ordinary troubleshooting, but it can isolate the exact stage at which CSS changed.

5.3 Use calibre-debug only when plugins or the interface are suspect

If the issue appears only when a plugin is enabled, or calibre behaves differently outside your normal setup, start the graphical interface in debug mode with calibre-debug -g. This can expose plugin errors or unusual application behavior.

Do not start with application-wide debugging when one book has bad CSS. Conversion debug output is more directly relevant to font rescaling. General debug mode becomes useful when several unrelated books fail in the same way or a plugin modifies conversion behavior.

6. Run a Clean Temporary Test Before Reinstalling

Reinstalling calibre rarely repairs a font mapping problem because conversion preferences, source styling, and library files may remain unchanged. A clean test gives better evidence without risking the main library.

1. Create a temporary empty library.
2. Add an untouched copy of one affected source file.
3. Disable optional conversion-related plugins for the test if applicable.
4. Convert with the appropriate output profile and otherwise default settings.
5. Inspect a few representative pages in two readers.
6. Change only one font option and repeat if necessary.

If the temporary conversion works, compare its settings with the original library's conversion setup. Stored per-book conversion settings may be responsible. Reset the affected book's saved conversion options before trying again in the main library.

If the clean test still fails, the source file or target reader is the stronger suspect. Open the source in the editor, test another authorized source, or compare the output on another device.

Cloud synchronization and operating system permissions matter only if they prevent calibre from reading the intended source or writing the new output. Perform the clean test in a normal local folder where your user account has write access. Avoid placing debug output or temporary working files in a folder actively synchronized while the conversion runs.

7. Quick Fix Checklist

• Convert an untouched source rather than a previous conversion.
• Test only a few pages containing body text, headings, notes, and captions.
• Reset the calibre viewer and device font controls before comparing results.
• Remove an unintended custom base font size.
• Restore the default font size key before creating a custom one.
• Disable font size rescaling when a well-designed source should be preserved.
• Select an output profile appropriate for the target device or use a suitable generic profile.
• Remove old Extra CSS or transformation rules during testing.
• Inspect fixed, inline, or conflicting font sizes in the source CSS.
• Confirm that the corrected file, not a cached older copy, reached the device.
• Read the conversion job details for warnings and unexpected options.
• Use conversion debug output to identify when the font sizes changed.
• Run a temporary-library test before reinstalling or deleting anything.

8. Frequently Asked Questions

8.1 Why is body text tiny after conversion?

The likely causes are an unsuitable base font size, an output profile with an unexpected mapping, or hard-coded source CSS. Reset the font options, choose the intended output profile, and compare conversions with rescaling enabled and disabled.

8.2 Why are headings huge while normal text looks correct?

The font size key may be exaggerating the difference between body and heading sizes, or the source may define headings with very large fixed values. Restore the default key first. If the problem remains localized, inspect the heading CSS.

8.3 Should I disable font size rescaling?

Disable it when the source already looks correct and conversion damages its size hierarchy. Leave it enabled when the source has inconsistent sizes that need normalization. A two-conversion comparison is the safest way to decide.

8.4 Why does the book look different in calibre and on my device?

The two readers may apply different font controls, minimum sizes, publisher-style settings, or CSS support. Reset both readers, then test the same file. If only one reader is wrong, adjust that reader before converting again.

8.5 Can metadata downloads repair incorrect font sizes?

No. Standard metadata does not control the styling of the book's body text. Confirm the selected source format instead, especially when the library contains several formats for the same title.

8.6 When should I stop changing settings?

Stop when body text is comfortable, headings have a clear hierarchy, notes and captions remain readable, and the result works on the intended reader. Do not keep adjusting conversion values to compensate for a display-only preference that the reader app can handle safely.