Build tag

A build tag or buildtag is a common name for the version string format used by recent versions of Microsoft Windows. In its fully expanded form, the build tag is a unique identifier of a Windows compile that includes the version number, target architecture and build type, branch or lab name, account username for private builds, and compilation timestamp.

Build tags started to be introduced towards the end of Windows 2000 development, although they were not inserted into Windows builds themselves until very early in the development of Windows XP.

Format[edit | edit source]

Modern build tags most commonly use the following format:

version.arch.lab.timestamp

However, depending on the Windows version, the following formats may also be encountered:

version (lab.timestamp)
version built by: lab at: timestamp
version built by: lab
  • version is the full version number, i.e. 6.3.9600.16384. The major and minor version or the delta number is often omitted, in which case there is no default implied value.
  • arch is the target platform identifier suffixed by the build type (fre for free/retail builds, chk for checked/debug builds). This part is often left out when it is not considered necessary to identify the exact compile. Known target platform identifiers are as follows:
    • x86 for 32-bit x86
    • x64, amd64 for 64-bit x86
    • ia64 for Itanium
    • woa, arm for ARM32
    • arm64 for ARM64
  • lab is the name of the branch or build lab that the build was compiled from. For private builds, this is followed (or replaced for early build tags) by the username of the account that initiated the build in parentheses; this can sometimes be an official build service account, like wincbld (Windows Central Build Account) or wmbla (Windows Main Build Lab Account).
  • timestamp is the compilation date and time in the YYMMDD-hhmm format.

The build tag may be followed by a hash of the product key and build lab, although it is not considered to be a part of the build tag itself.

Applying the above logic, the build tag 10.0.10240.16384.x86fre.th1.150709-1700 is the free/retail 32-bit x86 build of version 10.0.10240.16384, which was compiled from the th1 branch on 9 July 2015 17:00 (PT).

Location[edit | edit source]

The build tag of the currently running version of Windows is located in the registry under the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion. Depending on the exact version being used, there are two string values named BuildLab and BuildLabEx. On most pre-release versions of Windows, the build tag is also shown as a part of the watermark.

In older versions of Windows, the build tag is inserted into the file version information for system files. However, in recent Windows 10 updates, most files only contain a dummy build tag with the full version number and a placeholder lab and timestamp component, which says (WinBuild.160101.0800).