In Excel: .xlsx is the default (no macros), .xlsm is the same file with macros allowed, .xls is the pre-2007 binary format, .xlsb is a binary format for very large files, and .csv is plain text with one sheet and no formatting.
On this page7
File ▸ Save As, then choose the format in the "Save as type" list beneath the file name.
Use .xlsx for normal work, .xlsm the moment the file contains VBA.
Use .xlsb when a large workbook is slow to open or save and no other tool needs to read it.
Use .csv only as an interchange format, and reimport it with Data ▸ From Text/CSV so text columns keep their leading zeros.
To see extensions in Windows Explorer, turn on View ▸ Show ▸ File name extensions.
If Excel reports "file format or file extension is not valid", the file was renamed rather than converted — reopen the original and use Save As to convert properly.
What this does
The extension decides what the file can hold. .xlsx is the zipped XML default since Excel 2007 and cannot store VBA at all — save a macro workbook as .xlsx and the code is silently removed. .xlsm is byte-for-byte the same format with macros permitted, which is why security policies treat the two differently. .xls is the old binary format, capped at 65,536 rows and 256 columns. .xlsb stores the same content in binary, which opens and saves noticeably faster on very large workbooks at the cost of tool compatibility. .csv is not a spreadsheet: one sheet, values only, no formulas, no formatting, no multiple tabs. And .xltx/.xltm are the template versions, which open as a new copy rather than the file itself. Before you run it on a workbook other people depend on, try it on a copy or a few rows first. Undo only reaches back through the current session, so a quick trial run is the cheapest way to see exactly what will change before the file is saved and shared. Treat “extensions for excel files” as a small repeatable workflow rather than a one-off click you hope to remember next time. Use a small test block before the live file, so any surprise in the affected cells shows up while it is still harmless. When this is a ribbon command, the selection matters more than the button: confirm the range, apply the command, then spot-check the output before saving. That turns a data step that keeps your analysis trustworthy into a method you can reuse, explain, and defend when the workbook leaves your screen.
A worked example
A workbook with a macro button, saved as report.xlsx, loses the macro with only a warning dialog most people click through — save it as report.xlsm instead. A 180 MB .xlsx that takes 40 seconds to open often drops to a third of the size and opens in a few seconds as .xlsb. And exporting to .csv keeps one sheet of values: the formulas become their results, the other tabs vanish, and leading zeros in product codes are gone. Picking the wrong extension is how macros disappear, how a 200 MB workbook stays slow, and how a CSV export quietly drops the leading zeros from every product code. When the result is not what you expected, undo straight away rather than repairing it by hand — undo restores the sheet exactly, while manual fixes tend to leave small inconsistencies behind that surface later.
In Google Sheets
This is a command rather than a formula, so there is no syntax to carry over. Google Sheets arranges its commands in menus across the top instead of a ribbon, names some of them differently and does not have every Excel command — look for the equivalent in the Sheets menus or its help before assuming the steps match. The aim was to get you unstuck fast and leave you a little more capable than a copy-paste would. The answer is at the top and the detail above shows why it holds — so the next time a colleague asks, you can answer without reaching for search. Treat “extensions for excel files” as a routine rather than a one-off. Once you know which setting or command controls it, the same few steps handle every workbook where it comes up in Excel, and you can explain them to a colleague in a sentence.
Common mistakes
- Renaming a file from .xls to .xlsx in the file manager, which changes the name but not the contents — Excel then refuses to open it.
- Saving a macro workbook as .xlsx and losing the code.
- Treating .csv as a backup, when it keeps one sheet of values and discards everything else.
- Emailing .xlsm to recipients whose policy blocks macro-enabled attachments.
- Choosing .xlsb for a file other tools have to read; the binary format has much weaker third-party support.
Frequently asked questions
What file extension does Excel use?
.xlsx by default. .xlsm is for workbooks containing macros, .xls is the pre-2007 binary format, .xlsb is a faster binary format for large files, and .csv is plain text.
What is the difference between .xlsx and .xlsm?
The format is identical; .xlsm is the one permitted to contain VBA macros. Saving a macro workbook as .xlsx removes the code.
When should I use .xlsb?
When a large workbook is slow to open or save. It stores the same content in binary, is usually smaller and faster, but is less compatible with non-Microsoft tools.
Why does Excel say the file format or extension is not valid?
The file was renamed rather than converted, is corrupt, or was downloaded and blocked. Open the original and use File ▸ Save As to convert it, or unblock it in the file's Properties.
Other ways people ask this
People reach this page typing “excel file extension”, “excel file extensions”, “extension excel file” and “extension for excel files”, among other phrasings; whichever wording you used, the fix above is the one you want.
Why do people search for this in so many different ways?
Because the same task has many names. “excel file extension”, “excel file extensions”, “extension excel file” all point at the one operation explained on this page, which is why they all lead here.