What Is an SID Number?

Every Windows user has a unique security identifier

A SID, short for security identifier, is a number used to identify user, group, and computer accounts in Windows.

SIDs are created when the account is first created in Windows and no two SIDs on a computer are ever the same.

The term security ID is sometimes used in place of SID or security identifier.

Why Does Windows Use SIDs?

Users (you and me) refer to accounts by the account's name, like "Tim" or "Dad", but Windows uses the SID when dealing with accounts internally.

If Windows referred to a common name like we do, instead of a SID, then everything associated with that name would become void or inaccessible if the name were changed in any way.

So instead of making it impossible to change the name of your account, the user account is instead tied to an unchangeable string (the SID), which allows the username to change without affecting any of the user's settings.

While a username can be changed as many times you like, you're unable to change the SID that's associated with an account without having to manually update all the security settings that were associated with that user to rebuild its identity.

Decoding SID Numbers in Windows

Example SID in the Windows 10 registry editor

All SIDs start with S-1-5-21 but will otherwise be unique. If you need to, you can find a user's security identifier (SID) in Windows for matching users with their SIDs.

A few SIDs can be decoded without the instructions we linked to above. For example, the SID for the Administrator account in Windows always ends in 500. The SID for the Guest account always ends in 501.

You'll also find SIDs on every installation of Windows which correspond to certain built-in accounts.

For example, the S-1-5-18 SID can be found in any copy of Windows you come across and corresponds to the LocalSystem account, the system account that's loaded in Windows before a user logs on.

Here's an example of a user SID: S-1-5-21-1180699209-877415012-3182924384-1004. Yours will be different, but the purpose of each part of the SID remains the same:

S 1 5 21-1180699209-877415012-3182924384 1004
Indicates that this is a SID SID specification version number Identifier authority Domain or local computer identifier Relative ID

Any group or user that was manually created (i.e., not included in Windows by default) will have a Relative ID of 1000 or greater.

The following are a few examples of the string values for groups and special users that are universal across all Windows installs:

  • S-1-0-0 (Null SID): Assigned when the SID value is unknown, or for a group without any members.
  • S-1-1-0 (World): This is a group of every user.
  • S-1-2-0 (Local): This SID is assigned to users who log on to a local terminal.

More on SID Numbers

While most discussions about SIDs occurs in the context of advanced security, most mentions on our site revolve around the Windows Registry and how user configuration data is stored in certain registry keys that are named the same as a user's SID. So in that respect, the above summary is probably all you need to know about SIDs.

However, if you're more than casually interested in security identifiers, Microsoft has a full explanation of SIDs. You'll find information about what the various sections of the SID actually mean and you can see a list of well-known security identifiers like the S-1-5-18 SID we mentioned above.

SID Used in Other Terms

SID is also an acronym for other technology terms but none of them are related to an SID as it's explained below. Some examples include session identifier, serial interface device, standard integrated desktop, Safer Internet Day, and subscriber identification.

The .SID letter group is also used in some file extensions. For example, ScanIt Documents, SID Audio, MrSID Image, and Steam Game Data Backup files all use the SID suffix to indicate their relative file formats.