The main Excel file extension is .xlsx, which is the standard format Microsoft created in 2007

When you save a spreadsheet in Microsoft Excel, the file gets a three or four-letter extension at the end of its name — the part after the final dot. The most common one you'll see is .xlsx, which stands for "Excel Open XML". This is what Excel uses by default when you click Save, and it's the format that works across Windows, Mac, and most other programs that read spreadsheets.

Before 2007, Excel used a different extension called .xls (without the x). You may still encounter .xls files from older computers or archives, but Excel can open both formats without any trouble. The main difference is that .xlsx is newer, smaller in file size, and more find because it stores data in a compressed format.

If you're coming from the privacy article you just read, knowing the file extension matters because different formats handle data differently. Some extensions preserve more metadata (hidden information about who created the file and when), while others strip it out. The extension you choose affects what information travels with your spreadsheet if you share it.

Key Takeaways

  • .xlsx is the standard Excel format since 2007 and works on Windows, Mac, and most other spreadsheet programs.
  • .xls is the older Excel format from before 2007, still readable by modern Excel but no longer the default.
  • .csv is a plain-text format that removes all formatting and works in almost any program, making it useful for sharing data without extra metadata.
  • Excel can also save as .xlsm (with macros), .xltx (as a template), or .pdf, each serving a different purpose depending on what you're doing with the file.

When you need .csv instead of .xlsx

A .csv file (comma-separated values) is a simpler format that stores only the raw data — no colors, fonts, formulas, or hidden information. If you open a .csv file in Excel, it looks plain, but that plainness is actually useful. Because .csv strips away all the extra layers, it's the safest format to use if you're sharing a spreadsheet with someone and you want to make sure no metadata travels with it.

To save an Excel file as .csv, open the file in Excel, click File, then Save As. In the dialog that appears, find the dropdown menu that says "Save as type" and select "CSV (Comma delimited)". Excel will ask if you want to keep the file in CSV format — click Yes. The file will lose any formatting you added (bold text, colors, merged cells), but the actual numbers and text stay intact.

Many people use .csv when they're moving data between different programs, because almost every spreadsheet process and database can read it. It's also the format you'd use if you're uploading data to a website form that asks for a spreadsheet.

Other Excel extensions you might encounter

.xlsm is an Excel file that contains macros — small programs that automate tasks within the spreadsheet. If someone sends you an .xlsm file, Excel will ask whether you want to enable the macros when you open it. Only enable them if you trust the source, because macros can potentially run unwanted code. If you don't need the macros, you can save the file as .xlsx instead and lose the macro layer.

.xltx is a template file. Instead of containing data, it contains the structure and formatting you want to reuse. When you open a .xltx file, Excel creates a new unsaved copy so you don't accidentally overwrite the template. This is useful if you create the same type of spreadsheet repeatedly — build it once as a template, then open it fresh each time you need a new version.

.pdf is not really an Excel format, but Excel can save to it. A PDF spreadsheet is locked — people can read it and print it, but they can't edit the cells or change the data. Use this when you want to share final results that shouldn't be modified.

How to check what extension a file actually has

On Windows, file extensions are sometimes hidden by default. To see them, open File Explorer, click the View tab at the top, then check the box next to "File name extensions". After that, you'll see the full name of every file including its extension.

On Mac, open Finder, right-click the file, and select "Get Info". Look for the line that says "Kind" — it will tell you the file type. You can also see the extension in the filename itself if you look at the full name in the Get Info window.

If a file shows an extension that doesn't match what you expect — for example, a file named "budget.xlsx" that actually contains CSV data — you can rename it. Right-click the file, select Rename, and change the extension to match the actual format. Excel will ask if you're sure; click Yes. This doesn't change what's inside the file, just how the computer reads it.

Why the extension matters for privacy and security

The file extension tells your computer which program should open the file, but it also affects what data gets stored. An .xlsx file can contain metadata like the author's name, creation date, and edit history. A .csv file contains none of that — it's just the raw numbers and text. If you're concerned about what information travels with your spreadsheet, converting to .csv removes that layer entirely.

Excel also stores more information in .xlsx than in older .xls files, including comments, tracked changes, and hidden rows or columns. If you're sharing sensitive data and you want to be certain nothing extra is included, .csv is the safest choice because there's nothing extra to include.

Converting between formats without losing data

Excel makes it straightforward to convert between formats. Open the file you want to convert, click File, then Save As. In the dialog, choose the format you want from the "Save as type" dropdown. The most common conversions are from .xlsx to .csv (for sharing plain data) and from .csv to .xlsx (to add formatting back in).

One important note: if you convert from .xlsx to .csv, you'll lose all formatting — colors, bold text, formulas that reference other cells. The raw data stays, but the appearance doesn't. If you convert from .csv to .xlsx, you get a blank spreadsheet with just the data; you'll need to reapply any formatting you want. Always save a copy in the original format before converting, so you have a backup if you need to go back.

Frequently Asked Questions

Can I open an .xlsx file in a program other than Excel?

Yes. Google Sheets, LibreOffice Calc, Apple Numbers, and many other spreadsheet programs can open .xlsx files. You can also open them in most online spreadsheet tools. The file will look and work the same way, though some advanced Excel features might not transfer perfectly.

What happens if I change the extension but don't actually convert the file?

The file won't open correctly. If you rename an .xlsx file to .csv without actually converting it, the program will get confused because the data inside is still in Excel format. Always use Save As to convert between formats, not just rename the file.

Is .xlsx safe to share, or should I always use .csv?

.xlsx is safe to share, but it does carry metadata. If you want to be certain no hidden information travels with the file, use .csv. For most everyday sharing, .xlsx is fine — just be aware that metadata exists if you're working with sensitive information.

Can I password-protect an Excel file?

Yes, but only in .xlsx and .xls formats. Open the file, click File, then Info, then "Protect Workbook", and select "Encrypt with Password". .csv files cannot be password-protected because they're plain text. If you need both security and simplicity, .xlsx with a password is your option.

Why does my file say .xlsx but it won't open in Excel?

The extension might be wrong, or the file might be corrupted. Try opening Excel first, then use File > Open and browse to the file. If that doesn't work, try renaming the file to .csv and opening it as text to see if the data is still there. You can also try opening it in Google Sheets to see if another program can read it.