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.
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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. The same idea underpins a lot of everyday Excel work, so the few minutes spent getting it right here pay back across every sheet you build afterwards. Treat it as a pattern, not a one-off, and it stops being something you look up and starts being something you reach for. The difference between a quick fix and a sheet you can trust is the extra minute you spend validating “excel file format or extension is not valid”. Start on a copy or a tiny sample, keep the affected cells visible, and compare the result with the tool above before you touch the real workbook. 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. The point is a data step that keeps your analysis trustworthy, but the practical win is that someone else can open the file and understand what happened without asking you.
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. If this is something you do often, add the command to the Quick Access Toolbar (right-click it on the ribbon and choose Add to Quick Access Toolbar) or learn its shortcut, so next time it is one click instead of a hunt through the menus.
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. Keep this page bookmarked for the next time the same question comes up. Better still, repeat the steps once in your own workbook — doing it yourself is what turns a copied answer into something you remember. If you take one thing from this page on “excel file format or extension is not valid”, make it the order of checks rather than the individual clicks: confirm what is selected, apply the step, and look at the result before moving on. That small routine is what keeps Excel work predictable when the same task comes back in a slightly different workbook.
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.