Excel files use different extensions depending on what program created them and what they contain

An Excel file extension is the three or four letters after the dot at the end of a filename — like .xlsx or .xls. The extension tells your computer what program should open the file and what format the data is stored in. When you're organizing spreadsheets, the extension matters because it affects whether the file will open in Excel, Google Sheets, or other programs, and whether formulas, formatting, and macros will work the way you saved them.

The most common extension you'll see is .xlsx, which is the standard format for Excel 2007 and newer versions. If you have an older file, it might end in .xls. You may also encounter .csv files, which are spreadsheets stripped down to just data with no formatting. Each extension has a specific purpose, and knowing the difference helps you keep files organized and prevents problems when you share them or open them later.

Key Takeaways

  • .xlsx is the current standard Excel format and preserves all formatting, formulas, and multiple sheets in one file.
  • .xls is the older Excel format from before 2007 and may lose some formatting or features if you open it in newer versions.
  • .csv files contain only raw data with no formatting or formulas, making them useful for sharing data between different programs.
  • The extension determines which program opens the file by default and whether special features like macros will function.
  • Renaming a file extension without converting the actual file will cause it to fail to open or display incorrectly.

.xlsx — the modern Excel standard

.xlsx is the default format when you save a spreadsheet in Excel 2007 or any newer version, including Excel 2010, 2013, 2016, 2019, and Microsoft 365. The "x" in the name stands for XML, which is the underlying code structure Microsoft switched to. This format can hold multiple sheets in a single file, preserves all your formatting (colors, fonts, cell borders), keeps formulas intact, and supports charts and images.

When you save as .xlsx, the file is actually a compressed folder containing multiple XML files bundled together. This is why .xlsx files are usually smaller than .xls files with the same data. If you open an .xlsx file in Google Sheets or LibreOffice Calc, most of the formatting and formulas will transfer over, though some advanced Excel features may not work perfectly in those programs.

.xls — the older Excel format

.xls is the format Excel used from 1997 through 2003. If you have a file from that era or someone sent you one, it will have this extension. The format is less efficient at storing data, so .xls files are often larger than .xlsx files. More importantly, .xls has limits on what it can hold — fewer rows and columns, no support for newer Excel features, and some formatting may not survive if you open it in a modern version of Excel.

If you open an .xls file in Excel 2007 or newer, Excel will usually convert it to .xlsx automatically when you save. However, if the original file used features that only existed in older Excel versions, those features may be lost or changed. For this reason, if you need to preserve an old .xls file exactly as it was, you should keep a backup before opening it in newer software.

.csv — data only, no formatting

.csv stands for "comma-separated values." A .csv file is a plain text spreadsheet where each cell is separated by a comma and each row is on a new line. When you open a .csv file, it looks like a spreadsheet, but there are no formulas, no colors, no fonts, no multiple sheets — just the raw data. This makes .csv files useful for moving data between Excel, Google Sheets, databases, and other programs that might not understand Excel's native formats.

If you save an Excel file as .csv, all the formatting and formulas disappear and only the values remain. For example, if a cell contains a formula that calculates a total, the .csv file will only store the number the formula produced, not the formula itself. This is intentional — .csv is designed to be straightforward and universal so any program can read it. When you open a .csv file in Excel, Excel will ask you which character separates the columns (usually a comma, but sometimes a semicolon or tab).

.xlsm — Excel files with macros

If a spreadsheet contains macros (automated tasks written in Excel's programming language), the file extension is .xlsm instead of .xlsx. The "m" stands for "macro." When you open an .xlsm file, Excel will ask whether you want to enable macros, because macros can potentially run unwanted code. If you save a file with macros as .xlsx, the macros will be deleted and you'll lose that functionality.

Most people don't create macros themselves, but you may receive .xlsm files from coworkers or read them from websites. If you're organizing files and you see .xlsm, treat it the same way you would .xlsx — it's a full Excel file with all formatting and formulas intact, just with the added capability to run automated tasks.

Other extensions you might encounter

.xltx and .xltm are Excel template files. A template is a pre-made spreadsheet you can use as a starting point for new files. When you open a template, Excel creates a copy so you don't accidentally overwrite the original. .ods is the format used by LibreOffice Calc and Google Sheets sometimes saves files in this format. .numbers is Apple's spreadsheet program format. If you receive any of these, you can usually open them in Excel, but some formatting may not transfer perfectly.

You may also see .txt files that contain spreadsheet data. These are plain text files with no structure — they're harder to work with than .csv files because the data isn't organized into columns. If someone sends you a .txt file with data, you may need to import it into Excel manually or ask them to send it as .csv instead.

How to change a file extension safely

straightforward renaming a file from .xlsx to .csv by changing the letters at the end will not actually convert the file — it will just confuse your computer about what program should open it. To truly convert a file from one format to another, you need to use Excel's "Save As" feature. Open the file, click File, then Save As, and choose the format you want from the dropdown menu labeled "Save as type."

When you use Save As to convert to .csv, Excel will warn you that you're about to lose formatting and formulas. This is normal and expected. If you want to keep both versions — one with all the formatting and one as plain data — save the file twice with different names. For example, save "Budget.xlsx" for the formatted version and "Budget.csv" for the data-only version.

Frequently Asked Questions

Can I open an .xlsx file in an older version of Excel?

Not directly — Excel 2003 and earlier don't recognize .xlsx. However, Microsoft released a free compatibility pack that allows older versions to open .xlsx files. If you don't have the pack, you can ask someone with newer Excel to save the file as .xls instead, or convert it online using a free converter tool.

What happens if I change .xlsx to .xls in the filename?

The file won't actually convert — it will just have the wrong extension. When you try to open it, your computer may be confused or the file may fail to open. Always use Excel's Save As feature to convert between formats, not just renaming the file.

Is .csv safe to use for important data?

.csv is safe, but it's not ideal for long-term storage because it strips away all formatting and formulas. Use .csv when you need to share data with someone using a different program, but keep your main file in .xlsx so you don't lose your work.

Why does my .xlsx file look different when I open it in Google Sheets?

Google Sheets supports most Excel features, but not all. Some advanced formatting, certain functions, and macros may not work the same way. If you need exact formatting, keep the file in Excel. If you just need to view or edit the data, Google Sheets usually works fine.

Can I tell what's inside a file just by looking at the extension?

The extension tells you the format, but not the content. A .xlsx file could contain a budget, a contact list, or anything else. The extension only tells your computer which program should open it and what features are available — it doesn't describe what data is actually in the file.