Sec-CH-UA-Bitness

The HTTP Sec-CH-UA-Bitness request header provides the CPU architecture bitness of the platform on which the browser is running.

Table of Contents

Baseline: Limited availability

Supported in Chromium-based browsers (Chrome, Edge, Opera). webstatus.dev

Usage

The Sec-CH-UA-Bitness header is part of the User-Agent Client-Hints infrastructure. Browsers send this hint to inform servers whether the operating system is 32-bit or 64-bit. The information helps servers select the appropriate binary format when offering software downloads, browser plugins, or native applications.

This header is a high-entropy hint. Browsers only send high-entropy hints after the server explicitly requests them via Accept-CH. The default behavior omits this header to preserve privacy. Servers needing the architecture bitness must opt in by listing Sec-CH-UA-Bitness in the Accept-CH response header.

Download sites commonly request Sec-CH-UA-Bitness alongside Sec-CH-UA-Arch and Sec-CH-UA-Platform to ensure the correct installer is offered. A server receiving "64" knows the user is running a 64-bit operating system and provides a 64-bit binary by default. Sites serving 32-bit binaries to 64-bit platforms risk confusion and support issues when software performs suboptimally.

The header is sent only over secure connections (HTTPS). Chromium-based browsers support this hint.

Values

"32"

The value "32" indicates a 32-bit platform architecture. The browser is running on a 32-bit operating system or a 32-bit browser build on a 64-bit system.

"64"

The value "64" indicates a 64-bit platform architecture. The vast majority of modern desktop and mobile operating systems use 64-bit architectures.

Example

A download page for a desktop application requests architecture details to present the correct installer. The server sends Accept-CH requesting bitness information. The browser responds with "64", and the server defaults to offering the 64-bit download link.

Sec-CH-UA-Bitness: "64"

A site serving WebAssembly modules uses the bitness hint to select optimized builds. A 32-bit platform receives a module compiled for 32-bit addressing, while a 64-bit platform gets a module tuned for larger memory spaces.

Sec-CH-UA-Bitness: "32"

Takeaway

The Sec-CH-UA-Bitness header tells the server whether the browser runs on a 32-bit or 64-bit platform, enabling servers to deliver architecture-appropriate binaries and optimized content.

See also

Last updated: March 6, 2026