File extension

Updated: 04/02/2019 by Computer Hope

A file extension or file name extension is the ending of a file that helps identify the type of file in operating systems, such as Microsoft Windows. In Microsoft Windows, the file name extension is a period that is often followed by three characters, but may also be one, two, or four characters long.

Example of a file extension

As an example the file name "myfile.txt" has a file extension of ".txt," which is a file name extension associated with text files.

File list in explorer with name and file extension

In the above example, the first file shown in Explorer is "Regedit.exe." "Regedit" is the name of the file and ".exe" is the file extension that identifies this file as an executable file. The next file, "RtlExUpd.dll," is a DLL file and the last two files are log files. As can be seen in this example, by having file name extensions, you can quickly identify the type of file and have a better idea of how that file may be opened.


If you are trying to find a file with a specific file extension, use wildcards in your search.

What makes a valid file name extension?

A file name extension is often between one and three characters and will always be at the end of the file name, starting with a period. Some programs also support file extensions that are more than three characters. For example, all the latest versions of Microsoft Word support .docx document files and some web pages end with the .html file extension.


In all new versions of Microsoft Windows, by default, the file extensions are hidden and you must have show file extensions enabled to view and edit file extensions.

Can a file extension be longer than three or four characters?

Yes. As long as the program was designed to identify and open a program with a longer file extension, it can be longer than four characters. However, to keep the overall file name short, most programs do not exceed four characters.

Is there a limit to how long a file extension can be?

There is no limit to the length of a file extension as long as the file path, name, and extension combined do not exceed the maximum file name character limit of the operating system. Below is a list of Microsoft Windows versions and their file name character limit.

Windows 10 - 260 character limit.
Windows 8 - 260 character limit.
Windows 7 - 260 character limit.
Windows Vista - 260 character limit.
Windows XP - 255 character limit.
Windows 2000 - 254 character limit.

How many types of file extensions are there?

There are thousands of file extensions that can each be associated with one or more applications. See the full list below for a listing of file extensions and their associated programs.

Some of the more common file extensions are listed below, separated by the types of files the file extensions are associated. See our most common file extension list for a more extensive list of the most used file extensions.

Picture files

  • .bmp
  • .gif
  • .jpg

Music and sound files

  • .mp3
  • .wav

Operating system files

  • .dll
  • .exe

Text and word processing documents

  • .doc
  • .docx
  • .rtf
  • .txt

Spreadsheet files

  • .xls
  • .xlsx
  • .xlr
  • .csv

Web Page files

  • .htm
  • .html

See our computer file extensions information and listing for a list of many more file extensions.

Does a directory have an extension?

No. A directory does not have an extension like a file.

Extension, File, File format, File name, Ftype, Operating system terms, Software terms