An Excel file extension is the three or four letters after the dot in a filename that tells your computer what kind of spreadsheet it is

When you save a spreadsheet in Excel, the filename ends with a period and letters like .xlsx or .xls. That ending is the file extension. It is a label that tells your computer which program should open the file and what features are saved inside it. Different extensions mean different things — some files work in older versions of Excel, some preserve formulas and formatting, and some are designed to be opened by other programs entirely.

The extension matters because it controls what you can do with the file. If you send a .xlsx file to someone using Excel 2003, they may not be able to open it without converting it first. If you save a file as .csv (comma-separated values), you lose all your formatting and formulas. Understanding which extension you need prevents frustration and lost work.

Key Takeaways

  • .xlsx is the standard Excel format since 2007 and works on Windows, Mac, and most other programs that read spreadsheets.
  • .xls is the older Excel format from before 2007 and is rarely used now, but some older computers and programs still need it.
  • .csv removes all formatting and formulas and saves only the raw data, making it useful for sharing data between different programs.
  • The extension you choose affects who can open your file and what features survive the save, so pick based on who needs to use it and what it contains.

.xlsx: The current standard for Excel files

.xlsx is what Excel creates by default when you click File > Save on a modern computer. It has been the standard since Excel 2007. The file is actually a compressed folder containing multiple smaller files — XML code that stores your data, formatting, formulas, and charts all together. Your computer unpacks it automatically when you open it.

.xlsx files work on Windows, Mac, and Linux. They open in Excel, Google Sheets, LibreOffice Calc, and most other spreadsheet programs. If someone asks you to send them a spreadsheet and does not specify a format, .xlsx is the safest choice. It preserves everything: colors, fonts, formulas, named ranges, and pivot tables.

The only reason not to use .xlsx is if you know the person receiving it uses Excel 2003 or older, or if you are working with a very old computer that does not support it. In those rare cases, you would save as .xls instead.

.xls: The older Excel format you rarely need now

.xls is the format Excel used from 1997 through 2003. It is a single binary file — not a compressed folder like .xlsx — and it stores data differently at the technical level. Most modern computers and programs can still open .xls files, but Excel itself stopped using this format as its default in 2007.

You might encounter .xls files if you are working with spreadsheets created on very old computers or if you are sharing with someone who specifically requires it. If you receive a .xls file, you can open it in any modern version of Excel without any problem. If you need to send a file to someone using Excel 2003 or earlier, you would save as .xls to may provide compatibility.

There is no reason to choose .xls for new work. It has fewer features than .xlsx, is less widely supported, and takes up more disk space. If you find yourself saving as .xls, it is usually because you are working with an older system or following a specific requirement from your workplace.

.csv: Plain data with no formatting or formulas

.csv stands for comma-separated values. It is a text file that contains only the raw data from your spreadsheet — numbers, text, and dates — with commas between columns and line breaks between rows. No colors, no fonts, no formulas, no charts. When you open a .csv file in Excel, it looks plain because there is nothing else to display.

The advantage of .csv is that almost every program on earth can read it. If you need to move data from Excel to a database, a website, accounting software, or a mail merge in Word, .csv is often the format those programs expect. .csv files are also very small because they contain only text.

The cost is that you lose everything except the data itself. If your spreadsheet has a formula like =SUM(A1:A10), the .csv file will contain only the result of that formula, not the formula itself. If you save a file as .csv and then open it again, you cannot undo the loss — you would need to go back to the original .xlsx file to get the formulas back.

.xlsm: Excel files that contain macros

.xlsm is identical to .xlsx except that it can contain macros — small programs written in Excel's programming language that automate repetitive tasks. If your spreadsheet has a button that runs code when you click it, or if it has automated calculations that run on a schedule, it is saved as .xlsm.

Excel requires the .xlsm extension for security reasons. When you open a .xlsm file, Excel warns you that it contains code and asks whether you trust it. This prevents malicious spreadsheets from running hidden programs on your computer. If you try to save a file with macros as .xlsx, Excel will strip out the macros and ask if you want to proceed.

Most people never need .xlsm. You use it only if you or someone else has written code inside the spreadsheet. If you receive a .xlsm file and you do not know what the macros do, it is safe to open it — just click "Disable Macros" when Excel asks, and you can still see and edit the data.

.ods: The open-source spreadsheet format

.ods stands for OpenDocument Spreadsheet. It is the native format for LibreOffice Calc and other open-source spreadsheet programs. It is similar to .xlsx in that it is a compressed folder containing XML files, and it preserves formatting, formulas, and charts.

You might encounter .ods files if you are working with someone who uses LibreOffice instead of Excel, or if you are in an organization that uses open-source software. Excel can open and save .ods files, though some advanced features may not transfer perfectly. If you need to share a spreadsheet with someone using LibreOffice, .ods is a good choice because it preserves more features than .csv but is native to their program.

How to change a file extension

To save a spreadsheet in a different format, open the file in Excel and click File > Save As. A dialog box appears asking where you want to save the file and what name you want to give it. Below the filename field, there is a dropdown menu labeled "Save as type" or "Format". Click that dropdown and choose the extension you need — .xlsx, .xls, .csv, .xlsm, or .ods.

Type your filename (without the extension — Excel adds it automatically) and click Save. Excel will convert the file to the new format. If you are converting to .csv, Excel will warn you that you are about to lose formatting and formulas. If you are converting to .xlsx from a file that contains macros, Excel will ask if you want to keep the macros (which requires saving as .xlsm instead).

You can also change the extension by renaming the file directly in Windows File Explorer or Mac Finder, but this is risky because it does not actually convert the file — it just changes the label. Always use File > Save As inside Excel to may support the file is properly converted.

Frequently Asked Questions

Can I open a .xlsx file in Google Sheets?

Yes. Google Sheets opens .xlsx files directly. You can upload a .xlsx file to Google Drive, and it will convert automatically. You can also read a Google Sheet as .xlsx if you need to send it to someone using Excel. Some advanced features like certain chart types or conditional formatting may not transfer perfectly, but the data and most formatting will survive.

What happens if I send a .xlsx file to someone with Excel 2003?

They will not be able to open it without downloading a compatibility pack from Microsoft, or converting the file first. If you know someone is using Excel 2003, save the file as .xls before sending it. If you already sent .xlsx and they cannot open it, ask them to read the free compatibility pack, or convert the file yourself and resend it.

Is it safe to open a .xlsm file?

It is safe to open. Excel will warn you that the file contains macros and ask if you want to enable them. Click "Disable Macros" if you do not recognize the file or do not know what the code does. You can still view and edit all the data with macros disabled. Enable macros only if you trust the source and know what the code is supposed to do.

Why does my .csv file look ugly when I open it in Excel?

Because .csv contains only raw data with no formatting. All the colors, fonts, and column widths are gone. If you need to format the data, do it after opening the .csv file in Excel, then save as .xlsx to preserve the formatting. Never save your formatted work as .csv unless you are sending it to another program — you will lose all the formatting.

Can I use .csv to share a spreadsheet with formulas?

No. .csv saves only the results of formulas, not the formulas themselves. If your spreadsheet contains calculations that other people need to edit or update, use .xlsx instead. .csv is only for sharing raw data that does not need formulas.