The correct spelling is E-X-T-E-N-S-I-O-N

Extension is spelled with ten letters: E, X, T, E, N, S, I, O, N. The word comes from the verb "extend," meaning to stretch out or make longer. In tech, you'll see it most often referring to file extensions (the letters after the dot in a filename, like .pdf or .docx) or browser extensions (add-ons that change how your browser works).

The most common misspelling is "extention" — dropping the second E and writing it as E-X-T-E-N-T-I-O-N. This happens because the word sounds like it could end in "-tion" rather than "-sion," but the correct form uses S, not T, before the I. You'll see this mistake in emails, documents, and forum posts often enough that it's worth knowing the right way.

Key Takeaways

  • Extension is spelled E-X-T-E-N-S-I-O-N, with an S before the I, not a T.
  • The word appears in tech contexts like file extensions (.txt, .jpg) and browser extensions (add-ons).
  • The most common error is spelling it "extention," which drops the second E and uses T instead of S.
  • Remembering that it comes from "extend" can help you spell it correctly every time.
  • File extensions tell your computer what program should open a file, while browser extensions are actual programs you install.

Where you'll encounter the word in tech

File extensions appear at the end of every document or image you save. When you save a Word document, it gets the extension .docx. A photo from your phone might be .jpg or .png. Your browser shows these extensions in the file name, and knowing what extension a file has tells you what program can open it. Without the extension, your computer doesn't know whether it's looking at a picture, a video, or a spreadsheet.

Browser extensions are small programs you add to Chrome, Firefox, Safari, or Edge to change what your browser can do. A password manager extension stores your login information. An ad blocker extension hides advertisements. A translation extension converts web pages to your language. You install them from your browser's extension store, and each one has a name — but the word "extension" itself is always spelled the same way, whether you're talking about files or browser add-ons.

Why "extention" is wrong

English spelling rules can be tricky, and this word trips up a lot of people. When you add "-tion" to a verb that ends in "t," you sometimes drop the T first — like "act" becomes "action," not "acttion." This pattern makes people expect "extend" to become "extention," following the same logic.

But "extend" doesn't follow that rule. The correct form is "extension," keeping both the E and the S. The same pattern shows up in related words: "extensive," "extent," "extended," and "extensible." All of them use S, not T, in that position. Once you notice this pattern across multiple words, the spelling becomes easier to remember and use correctly.

How to remember the correct spelling

One reliable trick is to think of the base word: "extend." If you can spell "extend," you can spell "extension" by adding "-sion" at the end. The S comes from the word itself, not from a spelling rule about adding suffixes. This method works because you're building on something you already know rather than trying to memorize an arbitrary rule.

Another way is to remember that words ending in "-sion" are more common in English than words ending in "-tion" when the root verb ends in a D or similar sound. "Extend" becomes "extension," just like "pretend" becomes "pretension," "suspend" becomes "suspension," and "defend" becomes "defense." Grouping these words together helps your brain recognize the pattern.

File extensions you'll see most often

Your operating system uses extensions to know which program should open a file. Here are the ones that appear most often in everyday use:

  • .txt — plain text files, readable in any text editor
  • .pdf — documents that look the same on any device
  • .docx — Microsoft Word documents
  • .xlsx — Microsoft Excel spreadsheets
  • .jpg and .png — image files
  • .mp4 — video files
  • .mp3 — audio files
  • .zip — compressed folders holding multiple files

Windows and Mac both hide file extensions by default, so you might not see them unless you turn on the setting to show them. If you need to change what program opens a file, knowing the extension helps you understand what you're working with. On Windows, you can show extensions by opening File Explorer, clicking View, and checking the box next to "File name extensions." On Mac, open Finder, go to Preferences, and check "Show all filename extensions."

Frequently Asked Questions

Is the extension part of the filename or separate?

The extension is technically part of the filename, but your operating system treats it separately. When you save a document called "Report," the full filename is "Report.docx" — the .docx is the extension. You can change the extension to change what program opens the file, but changing it incorrectly can make the file unreadable.

Can I remove a file extension?

You can delete the extension from the filename, but the file won't open properly afterward. Your computer uses the extension to know which program to use. If you remove .pdf from a PDF file, your computer won't know it's a PDF anymore. You can rename files and change extensions, but keep the extension at the end so your programs can find them.

What's the difference between a file extension and a browser extension?

A file extension is just letters after a dot in a filename — it tells your computer what type of file it is. A browser extension is an actual program you install that adds features to your web browser. They're completely different things that happen to share the same word.

Why do some files have double extensions like .tar.gz?

Some files are compressed or processed multiple times, so they end up with two extensions. A .tar.gz file is first compressed as a .tar file, then compressed again as a .gz file. Your computer reads both extensions to know it needs two different programs to fully extract the contents. This is common with files downloaded from Linux or Unix systems.