How do I embed a custom font in an EPUB?

Place the OTF or TTF file in the EPUB's Fonts/ folder, add it to the OPF manifest, then reference it in CSS: @font-face { font-family: 'MyFont'; src: url('../Fonts/MyFont-Regular.otf') format('opentype'); } body { font-family: 'MyFont', Georgia, serif; }. Calibre's Edit Book and Sigil both let you add font files via the file manager and edit the CSS directly.

Can I legally embed any font in an EPUB?

No — font licensing determines what's allowed. Fonts under SIL Open Font License (OFL) or Apache 2.0 can be freely embedded and distributed. Commercial fonts (Adobe, Monotype) often require a separate 'ebook embedding' license. System fonts (Arial, Times New Roman) must not be embedded — they're licensed for device use only. Check the font's fsType value: 0 means freely embeddable.

How do I reduce font file size in an EPUB?

Use pyftsubset (part of fonttools): pip install fonttools, then pyftsubset MyFont.otf --unicodes='U+0020-007E,U+00A0-00FF' --output-file=MyFont-subset.otf. This removes all glyphs outside the specified Unicode ranges. For an English-only book, subsetting Latin characters reduces a 400 KB font file to 30–80 KB — an 80–95% reduction.

What is EPUB font obfuscation?

EPUB font obfuscation (IDPF method) scrambles the first 1040 bytes of embedded font files so they can't be extracted and reused elsewhere, while EPUB readers that implement the spec can still render them. It's not DRM — it only protects against casual font extraction. Apply it in Sigil via Tools → Obfuscate Fonts; the OPF encryption.xml is updated automatically.

EPUB Fonts — Embedding, Subsetting, and Licensing for E-Reader Distribution

Custom fonts in EPUBs improve readability and brand consistency, but embedded fonts add file size and raise licensing questions. Here's everything publishers and developers need to know.

How EPUB Fonts Work

EPUB uses standard web font technology. Fonts are embedded as OTF, TTF, or WOFF2 files in the EPUB's Fonts/ folder and referenced via CSS @font-face declarations:

@font-face {
  font-family: "MyFont";
  src: url("../Fonts/MyFont-Regular.otf") format("opentype");
  font-weight: normal;
  font-style: normal;
}

body {
  font-family: "MyFont", Georgia, serif;
}

If the embedded font fails to load (corrupted file, wrong path, or device restriction), the e-reader falls back to the next font in the stack — in this case Georgia, then the system serif.

EPUB 3 Font Support

EPUB 3 supports:

OTF/TTF — OpenType and TrueType. Universally supported.
WOFF — Web Open Font Format. Supported in EPUB 3 spec, but not all e-readers handle it.
WOFF2 — EPUB 3.3 added WOFF2 support. Best compression for web fonts.

For maximum compatibility, embed OTF or TTF. Convert to WOFF2 for size savings on modern readers that support it.

Font Licensing: What You Can Embed

Not all fonts can be legally embedded in EPUBs. The font's license determines what's allowed:

SIL Open Font License (OFL): Free to embed, subset, and distribute in EPUBs. No restrictions. Most Google Fonts use OFL.
Apache 2.0 / MIT: Free to embed and distribute. Check the specific license.
Commercial fonts (Adobe, Monotype, etc.): Embedding rights depend on the license tier. Many commercial fonts require an "ebook embedding" license add-on. Check the fsType field in the font file or the license documentation.
System fonts (Arial, Times New Roman): Do not embed — licensed for device use only, not for redistribution inside EPUBs.

To check the fsType embedding permission in a font file:

python3 -c "
from fontTools.ttLib import TTFont
f = TTFont('MyFont.otf')
print(f['OS/2'].fsType)
# 0 = installable (freely embeddable)
# 4 = print/preview only
# 8 = editable embedding
"

fsType 0 means freely embeddable. Any other value requires checking the license before distributing in EPUBs.

Font Subsetting to Reduce File Size

Full font files include characters for many languages. Subsetting removes unused glyphs, keeping only the characters actually used in the book. This reduces font file size by 80–95% for Latin-only text.

# Install fonttools
pip install fonttools brotli

# Subset to characters used in an English book
pyftsubset MyFont-Regular.otf   --unicodes="U+0020-007E,U+00A0-00FF,U+2018,U+2019,U+201C,U+201D,U+2026"   --layout-features="kern,liga,calt"   --output-file="MyFont-Regular-subset.otf"

The Unicode ranges cover: ASCII printable (U+0020–007E), Latin-1 Supplement (U+00A0–00FF), curly quotes (U+2018–201D), and ellipsis (U+2026). Add ranges for any special characters in your book.

Font Obfuscation (IDPF Method)

EPUB 3 defines a font obfuscation method to prevent font extraction from EPUBs. This is separate from DRM — it only scrambles the first 1040 bytes of the font file so it can't be extracted and used elsewhere, while e-readers that understand EPUB obfuscation can still render it correctly.

Calibre and Sigil can apply IDPF font obfuscation. In Sigil: Tools → Obfuscate Fonts. The OPF encryption.xml file is updated automatically.

When to Skip Embedded Fonts

For trade fiction and most nonfiction, embedded fonts are optional. Every major e-reader platform (Kindle, Kobo, Apple Books) has excellent system fonts and user-adjustable typography. Consider skipping embedded fonts when:

File size is a concern (each font weight adds 50–400 KB even subsetted)
The target audience reads on devices where they prefer their own font settings
The font adds style but not readability benefit

Embedded fonts are worth including for: poetry (line spacing matters), illustrated books with matching display fonts, textbooks with monospace code blocks, and any EPUB where brand typography is central to the design.

Converting a PDF to EPUB? toolkit.bot generates clean EPUB output ready for font customization.

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