OTF File (.otf): What It Is, How It Works + Sample Files

The basics

What is an OTF file?

An OTF file holds one font: the shapes of its letters, digits and symbols, their spacing, the font's names and licence, and often a set of typographic features such as ligatures, small caps and alternative figures. You install it to use the font in your apps, the same way as a TTF.

OpenType was announced in 1996 by Microsoft and Adobe as a way to end the split between TrueType fonts and Adobe's PostScript Type 1 fonts. It kept TrueType's file structure, a directory of tables, and allowed the outlines to be either TrueType curves or PostScript curves in a compact form called CFF. It also added the GSUB and GPOS tables for advanced typography and for scripts such as Arabic and the Indic scripts.

By convention, .otf means an OpenType font with CFF (PostScript) outlines and .ttf one with TrueType outlines. Both are OpenType fonts, and the extension isn't binding: our sample-truetype-outlines.otf is a valid .otf file with TrueType outlines inside.

Anatomy

What's inside an OTF file

An OTF file has the same layout as a TTF: a header, a directory of tables, then the tables. The difference is the outline table. Below is our sample-regular.otf. Numbers are big-endian: 00 00 0B 20 is 0xB20, which is 2,848.

sample-regular.otf (hex)

offset  bytes                                            meaning
000000  4F 54 54 4F                                      version "OTTO": CFF outlines
000004  00 09                                            9 tables
000006  00 80 00 03 00 10                                search helpers
        table directory, 16 bytes per table:
        tag         checksum    offset      length
00000C  43 46 46 20 1F FA AE 2B 00 00 0B 20 00 00 22 E0  CFF
00001C  4F 53 2F 32 …                                    OS/2
00002C  63 6D 61 70 …                                    cmap
00003C  68 65 61 64 …                                    head
00004C  68 68 65 61 …                                    hhea
00005C  68 6D 74 78 …                                    hmtx
00006C  6D 61 78 70 00 63 50 00 00 00 00 F8 00 00 00 06  maxp
00007C  6E 61 6D 65 …                                    name
00008C  70 6F 73 74 …                                    post
        the CFF table:
000B20  01 00 04 02                                      CFF header, version 1.0
000B24  00 01 01 01 18                                   one font name, 23 bytes
000B29  39 39 68 65 6C 70 65 72 73 53 61 6D 70 6C 65 …   "99helpersSample-Regular"
        … Top DICT, strings, charset, charstrings
        the glyph "o" as a charstring, decoded:
        593 474 66 rmoveto                               width, first point
        49 51 25 61 72 vvcurveto                         a cubic curve
        72 -25 61 -49 51 vhcurveto                       another one
        … 8 curves per contour, 2 contours, endchar
002F8C                                                   end of file: 12,172 bytes

14F 54 54 4F
The version tag
The ASCII letters OTTO, meaning the outlines are in a CFF or CFF2 table. A TrueType-flavoured font has 00 01 00 00 here, whatever its extension.
200 09
Nine tables
One fewer than the TTF version of the same font: the CFF table replaces both glyf and loca, because it has its own index of where each glyph starts.
343 46 46 20 1F FA AE 2B 00 00 0B 20 00 00 22 E0
The CFF entry
The tag is CFF plus a space (tags are always four bytes), then a checksum, the offset 0xB20 and the length, 8,928 bytes.
46D 61 78 70 … 00 00 00 06
A shorter maxp
In a CFF font, maxp is version 0.5: just 6 bytes with the glyph count. The extra fields a TrueType font needs for its hinting engine are left out.
501 00 04 02
The CFF header
Compact Font Format version 1.0. CFF is Adobe's compact repackaging of PostScript Type 1 font data, and is a complete font format of its own, nested inside the OpenType wrapper.
600 01 01 01 18 / 39 39 68 65 …
The Name INDEX
A list with one entry (00 01), one-byte offsets, and a name 23 bytes long: the PostScript name 99helpersSample-Regular, which printers and PDF files use to refer to the font.
7593 474 66 rmoveto
A charstring
Each glyph is a tiny program in the Type 2 charstring language: numbers followed by operators, stored compactly as bytes. The first number is the glyph's advance width, then it moves to the first point of the o.
849 51 25 61 72 vvcurveto
Cubic curves
Curve operators take the control points of cubic Bézier curves, as relative moves. vvcurveto is a shortcut for a curve that starts and ends vertically, which saves bytes.

Under the hood

How an OTF font works: CFF outlines

Most tables in an OTF are the same as in a TTF: cmap maps characters to glyphs, hmtx gives widths, name the names and licence, OS/2 weight, line spacing and embedding permissions. What differs is how the outlines are stored:

Cubic curves. CFF glyphs are drawn with cubic Bézier curves, each with two control points: the same kind of curve as in PostScript, PDF, SVG paths and design apps such as Illustrator. Type designers often draw in cubics, so CFF fonts keep their drawings as they were made.
Charstrings. Each glyph is a short program of numbers and drawing operators (Type 2 charstrings). Pieces shared by many glyphs can be stored once as subroutines and called from each glyph, which keeps large fonts compact.
Hints, not instructions. CFF fonts carry hints: where the stems are, and alignment zones such as the baseline and x-height. The rasteriser decides how to fit them to the pixel grid. TrueType instead stores the fitting as programs inside the font. CFF hinting is far less work for the font maker; TrueType hinting gives more control on low-resolution screens.
CFF2. Added in OpenType 1.8 (2016), CFF2 is a leaner version of CFF that also supports variable fonts. It drops some PostScript leftovers, such as the font's own copy of its names.

Converting between outline types

Quadratic TrueType curves can always be turned into cubic CFF curves exactly. The other way round is an approximation: each cubic curve becomes one or more quadratic curves within a small tolerance, so a font converted from OTF to TTF usually has more points and loses its CFF hints.

Typography

OpenType features: ligatures, small caps and figures

OpenType features are rules inside the font that swap or reposition glyphs. GSUB substitutes glyphs (fi becomes the ﬁ ligature, a letter becomes its small capital), and GPOS positions them (kerning, mark placement for accents). Each feature has a four-letter tag. Some are on by default; others an app or a stylesheet has to switch on.

Tag	Feature	Usually
`kern`	Kerning: adjusts space between pairs such as AV	On
`liga`	Standard ligatures such as fi and fl	On
`smcp`	Small capitals	Off
`onum` / `lnum`	Old-style or lining figures	Off; the font's default applies
`tnum` / `pnum`	Tabular (equal-width) or proportional figures	Off; the font's default applies
`ss01`…`ss20`	Stylistic sets: alternative letter shapes	Off

style.css

/* Turn OpenType features on with CSS */
.figures  { font-variant-numeric: tabular-nums; }      /* tnum */
.smallcap { font-variant-caps: small-caps; }           /* smcp */
.oldstyle { font-feature-settings: "onum" 1; }         /* low-level switch */
.nolig    { font-variant-ligatures: none; }            /* liga off */

Features work the same in TTF and OTF files: they live in the GSUB and GPOS tables, not in the outlines. Our samples keep things simple and have none; their digits are tabular by design.

Related formats

OTF vs TTF vs WOFF2

	OTF (CFF)	TTF	WOFF2
Outlines	Cubic curves (`CFF` or `CFF2`)	Quadratic curves (`glyf`)	Either: it wraps a TTF or OTF
First four bytes	`OTTO`	`00 01 00 00`	`wOF2`
Hinting	Stem and zone hints	Instructions in the font	Same as the font inside
OpenType features	Yes	Yes	Yes, whatever the font has
Variable fonts	Yes, with `CFF2`; less widely supported	Yes, the common choice	Yes
Our regular sample	12,172 bytes	11,588 bytes	4,428 bytes

For installing on a computer, use whichever the font maker supplies: in current apps, neither is better. Some foundries recommend OTF for print and design work and TTF for Windows screen use, because of the hinting difference. For a website, convert either to WOFF2.

Variants

Variable OTF fonts and CFF2

A variable OTF stores its outlines in a CFF2 table, with blend operators that say how each coordinate changes along a design axis. Our sample-variable-cff2.otf covers weights 300 to 700 with five named styles, using the same fvar, HVAR and STAT tables as a variable TTF.

Browsers and current macOS and Windows versions render CFF2 fonts, but support in apps, older systems and some PDF tools is patchier than for variable TTFs. That's why most variable fonts, including those on Google Fonts, are distributed with TrueType outlines.

How to

How to install, open and convert an OTF file

Windows: double-click the file to preview it, then click Install (or right-click, Install for all users).
macOS: double-click to open it in Font Book and click Install.
Linux: copy it to ~/.local/share/fonts and run fc-cache -f.
On a website: convert it to WOFF2 (fontTools, or FontForge) and load it with @font-face. Browsers accept raw OTF files too, but they're larger.
Convert to TTF: FontForge or fontTools (otf2ttf in its snippets, which uses cu2qu) convert CFF outlines to quadratic ones. Expect more points and the loss of CFF hints.
Inspect it: ttx -l font.otf lists the tables, and ttx -t CFF font.otf dumps the outlines as readable charstrings.

Troubleshooting

Common problems with OTF fonts

Features don't appear. The app has to support OpenType features and expose them. Design apps usually do; office apps support only some. In CSS, use font-variant-* or font-feature-settings.
The font isn't in the menu after installing. Restart the app, which reads the font list at start-up.
Thin or soft text on Windows at small sizes. CFF hints leave more to the rasteriser, and some older Windows apps rendered CFF fonts less crisply than well-hinted TrueType. At larger sizes and on high-resolution screens the difference disappears.
Can't embed in a PDF or document. The font's fsType embedding flags, or the app's settings, may forbid it. Check the licence and the font's properties.
A converter gives a broken TTF. Cubic-to-quadratic conversion is approximate and can produce overlapping or reversed contours. Use fontTools or FontForge and check the result in a font validator.

Free downloads

Sample OTF files to download

Free sample OTF fonts for testing font installers, renderers, converters and @font-face set-ups. They use 99helpers Sample, an original geometric typeface we drew for these pages, with Latin letters, digits, common punctuation and a few accented letters.

sample-regular.otf · 11.9 KBDownload

Aa Gg 99

ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz
0123456789 .,:;!?&@#()

14pxThe quick brown fox jumps over the lazy dog

20pxThe quick brown fox jumps over the lazy dog

32pxThe quick brown fox jumps over the lazy dog

48pxThe quick brown fox jumps over the lazy dog

What's inside sample-regular.otf

The regular weight with CFF outlines, built with fontTools. The specimen above is drawn with this exact file.

Nine tables
CFF , OS/2, cmap, head, hhea, hmtx, maxp, name and post: no glyf or loca, since the CFF table holds the outlines.
Cubic outlines
Every glyph as a Type 2 charstring with cubic curves. The design is the same monoline geometric sans as the TTF; only the curve type differs.
99 glyphs
A to Z, a to z, 0 to 9, space, .,:;!?&@#()-'"/, á à â ä é è ê ë í ï ó ô ö ú ü ñ É Ü Ñ, plus .notdef and a dotless i.
Names
Family 99helpers Sample, style Regular, PostScript name 99helpersSample-Regular, weight class 400.
Licence
In the name table and in the CFF Notice string, with a link back to this page. fsType is 0: installable embedding.

All sample OTF files

Regular weight, CFF outlines
sample-regular.otf · 11.9 KB (12,172 bytes)
99helpers Sample Regular (weight 400) as an OpenType font with a CFF table: 99 glyphs, cubic curves, no hints.
- Testing font installation and CFF rendering
- A source file for OTF to WOFF2 or TTF conversion
Download
Bold weight, CFF outlines
sample-bold.otf · 12.0 KB (12,256 bytes)
The same family at weight 700, with the bold style bits set so apps pair it with the regular file.
- Testing style linking in apps
- Comparing real and synthetic bold
Download
Variable font with CFF2 outlines
sample-variable-cff2.otf · 26.3 KB (26,952 bytes)
A variable OpenType font with a CFF2 table and one wght axis from 300 to 700, with five named styles (Light to Bold).
- Testing CFF2 and variable-font support
- Checking which apps and PDF tools handle CFF2
Download
OTF file with TrueType outlines
sample-truetype-outlines.otf · 11.3 KB (11,588 bytes)
A valid .otf file that starts with 00 01 00 00 and stores its outlines in glyf, like a TTF. Tests code that guesses the outline type from the extension.
- Testing format detection by signature, not by name
- Checking upload filters and converters
Download

Ways to test with them

Rendering: install sample-regular.otf and compare it with sample-regular.ttf from the TTF page at small sizes; any difference comes from the outline type and rasteriser.
Format detection: a tool should report sample-truetype-outlines.otf as TrueType-flavoured, because its first four bytes are 00 01 00 00, not OTTO.
Variable fonts: open sample-variable-cff2.otf in an app with variable-font support and move the weight slider; if nothing happens, the app doesn't support CFF2.
Converters: convert sample-regular.otf to TTF and compare point counts with ttx; quadratic outlines need more points.

Free to use for testing, teaching and demos, commercially or not, with no attribution needed. A link back to this page is always appreciated.

Put it online

Have an OTF file? Give it a link.

Drop an OTF file on 99helpers and it gets its own address in seconds. Opening the link downloads the font, since browsers don't display font files on their own, and a page or site you host on 99helpers can load it with @font-face.

Put an OTF file online

Questions

OTF files, frequently asked

What is an OTF file?

An OpenType font, usually one with PostScript-style (CFF) outlines. You install it like any font to use it in your apps.

How do I install an OTF font?

On Windows, double-click it and click Install. On macOS, double-click it and click Install in Font Book. On Linux, copy it to ~/.local/share/fonts and run fc-cache -f.

Is OTF better than TTF?

Neither is better in current apps. OTF usually has cubic CFF outlines and simpler hints; TTF has quadratic outlines and can carry detailed hinting. Both support the same OpenType features.

Can I use an OTF font on a website?

Yes, browsers accept OTF in @font-face, but converting it to WOFF2 makes it much smaller. Check that the licence allows web use.

How do I convert OTF to TTF?

With FontForge or fontTools. The curves are converted from cubic to quadratic, which is a close approximation, and CFF hints are lost.

What is the MIME type of an OTF file?

font/otf, registered in RFC 8081 in 2017. It covers OpenType fonts with either kind of outlines.

Why don't ligatures or small caps show up?

Your app has to support OpenType features and you may have to switch them on. Some features, like small caps, are off by default; check the font includes them too.

Related formats

Other file types explained

TTF file WOFF2 file WOFF file EOT file PDF file SVG file All file types

Keep going

Host an OTF file

Give a font file its own link in seconds.

Share an OTF file

Send a font as a link instead of an attachment.

Host a website

Put a site online with its fonts, styles and images.

Sources

Last reviewed 27 October 2026.