Sec-CH-UA-Bitness · HTTP Client Hints

Sec-CH-UA-Bitness reports whether the browser is a 32-bit or 64-bit build, the second half of picking the right binary download, behind an Accept-CH opt-in.

Sec-CH-UA-Bitness is the companion to Sec-CH-UA-Arch: where Arch says which instruction set, Bitness says how wide: "64" or "32", as a quoted string.

Accept-CH: Sec-CH-UA-Arch, Sec-CH-UA-Bitness

Sec-CH-UA-Arch: "x86"
Sec-CH-UA-Bitness: "64"

When you actually need it

In 2026, "64" is the overwhelming answer on every desktop platform, so the interesting signal is the exception: a "32" from a Windows machine tells your download page to serve the legacy x86 installer rather than the x64 one. That single decision, still relevant for long-tail enterprise fleets, is essentially the whole reason this header exists.

Note the subtlety that the value describes the browser build, not the OS: 32-bit Chrome on 64-bit Windows reports "32". If you need to untangle that specific case, that is exactly what Sec-CH-UA-WoW64 encodes.

In JavaScript

const { bitness } = await navigator.userAgentData
  .getHighEntropyValues(['bitness']);
// "64" | "32"

Server-side use

Combine platform, architecture and bitness for a complete download matrix:

Platform	Arch	Bitness	Serve
`"Windows"`	`"x86"`	`"64"`	`app-win-x64.exe`
`"Windows"`	`"x86"`	`"32"`	`app-win-x86.exe`
`"Windows"`	`"arm"`	`"64"`	`app-win-arm64.exe`
`"macOS"`	`"arm"`	`"64"`	`app-mac-arm64.dmg`
`"macOS"`	`"x86"`	`"64"`	`app-mac-intel.dmg`

As with every opt-in hint: Chromium only, no Firefox or Safari, so keep the architecture dropdown visible as a fallback.