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Invalid Filenames

This post discusses the valid filenames in Windows.

Characters

You cannot use the following in a filename:

• Backslash \
• Forward Slash /
• Colon :
• Asterisk *
• Question Mark ?
• Speech Mark "
• Less than sign <
• Greater than sign >
• Pipe/Bar |

Windows warning of invalid characters for a filename.

Not recommended

While these are allowed, they can cause problems in some system software:

• Ampersand &
• Apostrophe '
• Percentage %
• Caret ^
• Semicolon ;
• Full stop (if not being used to separate the extension from the filename) .

Also, note that some non-standard software can have issues with spaces in a filename, which is why you will often see people use underscores instead of spaces in filenames. However, most Windows software will not be affected by spaces in filenames.

Names

The following names are not allowed to be used in Windows:

• CON
• NUL
• AUX
• PRN
• COM1, COM2, COM3, ... COM9
• LPT1, LPT2, LPT3, ... LPT9

This means, for instance, you cannot create a Microsoft Word document called "aux.docx".

Length

The default maximum length of a file-path on Windows is 254 characters. This means that a file in "C:\" can have a filename of a maximum length of 251 characters, but a file on "C:\Users\Client\Documents\" can have a filename of maximum length of only 228 characters. Therefore, keep filenames as short as possible to avoid running into this limit.

The limit can be changed in Windows settings these days but may then cause issues with some software and since it’s set at 254 characters by default and most people don’t change it, long filenames will cause issues when sending files to other people.