File Extension

Example: "You need to decompress a .zip file to view the archive's contents."

A file extension is the last part of a filename, after the dot. For example, if a file is named document.pdf, "pdf" is the file extension. Most extensions are three characters long, but they can be shorter or longer. Many popular file formats now use four-letter extensions, such as Microsoft Word documents (.docx), Excel spreadsheets (.xlsx), and PowerPoint presentations (.pptx).

A file's extension defines the corresponding file format. For example, if a filename ends in ".jpg" or ".jpeg", you can tell it's a JPEG image before you open it. Operating systems use extensions to associate files with a "default program" — the application that will open the file when you double-click it. They also display the appropriate icon for associated file format.

Visit the PC.net File Extensions Reference to view an extensive list of file extensions and their associated file formats.

First Published: September 17, 2007
Last Updated: March 23, 2024