Characters to Avoid in Filenames and Directories

There are a number of good tips to keep in mind when you name your folders, files, and web pages. These tips will help to keep your URLs short and specific to what they involve. Good file naming will also help with search engine optimization.

  • Don’t include spaces or other punctuation in your folder or file name. If necessary, use dashes in this manner: “my-new-file.pdf”.
  • Try to keep your folder or file name short while still making sense.
  • Use key words.
  • Use lowercase.
  • Pick a filename and stick to it. Not renaming your file will keep the file indexed by any search engines.
  • Try to avoid using years, version numbers, or other naming patterns that date a file. Maintenance is reduced and user experience is sustained when you can just overwrite the old file with the new one each year and keep the filename the same. However, for archiving purposes, different filenames may be important (e.g., FY09). To ensure that your new filename updates properly, please request a redirect when you change a filename.

What Characters are not Allowed in Filenames?

You may only work in one operating system, however your web files will be viewed by numerous systems. Therefore, it becomes essential that you play it safe and avoid common illegal directory and filename characters. Some are illegal because a specific operating system doesn’t support them, others because they are used as variables or reserved characters. Naming conventions are important in web folders as well as for downloadable files such as HTML files, images, PDFs, Word documents, and Excel spreadsheets. Folder and file names build out the path used for URLs.

Illegal Filename Characters

Do not use any of these common illegal characters or symbols in your filenames or folders:

  • # pound
  • % percent
  • & ampersand
  • { left curly bracket
  • } right curly bracket
  • \ back slash
  • < left angle bracket
  • > right angle bracket
  • * asterisk
  • ? question mark
  • / forward slash
  •   blank spaces
  • $ dollar sign
  • ! exclamation point
  • ' single quotes
  • " double quotes
  • : colon
  • @ at sign
  • + plus sign
  • ` backtick
  • | pipe
  • = equal sign
  • emojis
  • alt codes

In addition to forbidden characters, keep these rules in mind when choosing a folder name or filename:

  • Don’t start or end your filename with a space, period, hyphen, or underline.
  • Keep your filenames to a reasonable length and be sure they are under 31 characters.
  • Most operating systems are case sensitive; always use lowercase.
  • Avoid using spaces and underscores; use a hyphen instead. This will also improve your search engine rankings.
  • Non-ASCII characters need to be escaped.
Bad filename Web browsers see Good filename
F&A Costs.html F&amp;A%20Costs.html index.html
my PDF file#name.pdf my%20PDF%20file%23name.pdf my-pdf-file-name.pdf

Note: This list is not exhaustive. It is meant to help you avoid common errors in filenames. If you have any questions or need any assistance applying this information at Michigan Tech, do not hesitate to contact