What fonts can you assume Windows users will have? Mac users? And how about (gulp!) Linux & other UNIX users? Oh, & don't forget mobile users: iOS (iPhone, iPad, iPod Touch), Android, & Windows Phone 7!

One answer can be found at Code Style Font Sampler, which asks visitors to fill out a survey asking them about the fonts they have on their computers. You should take the results with a grain of salt, as the results are biased in favor of users who:

1. visit the site,
2. agree to take the survey, &
3. are able to correctly determine the fonts on their computers

Still, it provides some useful results. Go take a look & it'll give you some numbers you can work with.

Various resources also exist that give web developers some hard numbers & lists they can use when determining the fonts available on various operating systems.

Windows

The following fonts have served as the default system fonts on Windows:

The number of typefaces & fonts Microsoft includes with Windows has been steadily increasing. First a table:

And now a detailed breakdown:

• Windows XP: 31 typefaces⁴ & 136 fonts⁵
• Windows Vista: 90 typefaces⁶ & 191 fonts
• Windows 7: 111 typefaces & 257 fonts⁷; 15 are new typefaces & 49 are new fonts⁸
• Windows 8: 118 typefaces & 279 fonts; 6 are new typefaces & 17 are new fonts⁹
• Windows 8.1: 125 typefaces, 322 fonts; 10 are new typefaces & 45 are new fonts¹⁰
• Windows 10: 177 typefaces, 440 fonts
  • In the base install, there are 93¹¹ typefaces & 171¹² fonts.
  • In that base install are 10 new typefaces & 45 new fonts.¹²
  • Windows 10 also makes other typefaces & fonts available in Feature on Demand packages. According to Microsoft, FoD packages “are installed automatically by Windows Update when the associated languages are enabled in language settings (for example, by enabling a keyboard)” but can also be installed manually by using Settings > Apps > Apps & Features > Manage Optional Features. The FoD packages include 84 typefaces & 269 fonts.¹³
  • In those FoD packages are 3 new typefaces & 9 new fonts.¹³

For more info:

• Windows XP
  • Fonts Supplied with Windows XP
  • Fonts supplied with Windows XP
• Windows Vista
  • Fonts Supplied with Windows Vista
  • Fonts supplied with Windows Vista
• Windows 7
  • New Fonts in Windows 7
  • Fonts supplied with Windows 7
• Windows 8/8.1
  • Windows 8 font list
  • Windows 8.1 font list
• Windows 10
  • Windows 10 font list
  • New fonts in Windows 10 (including Georgia Pro & Verdana Pro)
  • List of Installed Typefaces in Windows 10 (N. America)

Wikipedia’s “List of typefaces included with Microsoft Windows” lists fonts included with Windows 3.1–10.

In addition to these, you should also check out Microsoft's “Fonts and Products”, which lists fonts supplied not only with Microsoft’s products, but also included in UNIX, Mac OS X, & Adobe products.

Mac OS X/OS X/macOS

Apple has a page detailing System Fonts for macOS & iOS.

The following fonts have served as the default system fonts on macOS:

The number of typefaces & fonts Apple includes with Mac OS X/OS X/macOS has varied. First a table:

And now a detailed breakdown:

• Mac OS X 10.3 Panther
  • 74 fonts installed¹⁴
• Mac OS X 10.4 Tiger
  • 117 fonts installed¹⁵
• Mac OS X 10.5 Leopard
  • 160 fonts installed¹⁶
  • 155 fonts installed¹⁷
• Mac OS X 10.6 Snow Leopard
  • 249 fonts installed¹⁸
• Mac OS X 10.7 Lion
  • 225 fonts installed¹⁹
• OS X 10.8 Mountain Lion
  • 242 fonts installed²⁰
• OS X 10.9 Mavericks
  • 507 fonts installed²¹
• OS X 10.10 Yosemite
  • Full list not available
  • 11 new typefaces & 29 new fonts²²
• OS X 10.11 El Capitan
  • Full list not available
  • 6 new typefaces²³
• macOS 10.12 Sierra²⁴
  • 473 fonts installed
  • 106 fonts available for download
  • 158 fonts available in documents or apps that use or request the font(s)
  • 8 new typefaces & 12 new fonts²⁵
• macOS 10.13 High Sierra²⁶
  • 193 (?) typefaces & 488 fonts installed
  • 58 typefaces (?) & 107 fonts available for download
  • 134 typefaces (?) & 197 fonts available in documents or apps that use or request the font(s)
• macOS 10.14 Mojave²⁷
  • 180 typefaces & 488 fonts installed
  • 61 typefaces (?) & 187 fonts available for download
  • 134 typefaces (?) & 198 fonts available in documents or apps that use or request the font(s)

Some interesting info is at Unicode fonts for Macintosh OS X computers.

UNIX & Linux

There are hundreds of different UNIX variants out there (Linux itself is one, and there are hundreds of variants of Linux!), so saying with certainty what fonts are safe to assume are on any particular user's UNIX box is difficult. Fortunately, even though there are hundreds of UNIX flavors, there are only a few that make up the majority of desktop use.

Fedora 29: 59 typefaces & 300 fonts installed²⁸

Ubuntu Server²⁸

• 12.04: 15 typefaces & 56 fonts installed
• 14.04: 13 typefaces & 47 fonts installed
• 18.04: 15 typefaces & 43 fonts installed
• 18.10: 4 typefaces & 16 fonts installed

For more, see Fonts supplied with UNIX / XFree and GhostScript.

iOS

The following fonts have served as the default system fonts on iOS:

When the iPad came out, it had more fonts on it than did the iPhone. Apple has since rectified that, & it's my guess that it will continue to make sure both devices have the same number of fonts on them.

Android

Android is a bit more complicated, because while it doesn't include very many built in fonts, apps can bundle whatever fonts they want (note that this is now true for Apple's iOS as of version 4; however, since iOS also comes with over 50 built in fonts, most apps don't need to include their own).

Android versions 1 through 3 (code names: Cupcake, Donut, Eclair, Froyo, Gingerbread, & Honeycomb) included only 3 fonts:

• Droid Sans
• Droid Serif
• Droid Sans Mono

You can read more about Droid:

• Droid (font) at Wikipedia
• The Droid Fonts Story

Starting with version 4 of Android (codenamed Ice Cream Sandwich), Google introduced a new standard font: Roboto. Here are some webpages about Roboto:

• Android Design: Typography
• Roboto is a Four-headed Frankenfont
• Roboto vs. Helvetica

Windows Phone 7

Windows Phone 7 out of the box supports 16 fonts, including those in Core Fonts for the Web plus a few others. If you do not specify font-family, Segoe WP is used by default. However, like Android & iOS, developers can embed their own fonts in their apps if they desire.

• Supported Fonts on IE for Windows Phone 7

If you’re looking for my comments on Microsoft’s Core Fonts for the Web (Andalé Mono, Arial, Comic Sans, Courier New, Georgia, Impact, Times New Roman, Trebuchet MS, Verdana, & Webdings) & Microsoft’s ClearType Font Collection (Calibri, Cambria, Candara, Consolas, Constantia, Corbel), see My Thoughts on Fonts.