Fix "Excel cannot open the file because the file format or file extension is not valid"

Why Excel refuses the file

An .xlsx file is a zip package of XML files. When Excel opens something called .xlsx, it expects that package. If the file is really a text CSV, an old .xls, a web page or half a download, Excel can't read it and shows the full message: Excel cannot open the file 'Report.xlsx' because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file.

A close cousin appears for .xls files: The file format and extension of 'Report.xls' don't match. That one offers Yes to open anyway, and it's usually a report exported by another system as HTML or text with an .xls name. Only click Yes if you trust where the file came from.

1. See what the file really is

Windows hides extensions by default, so you may be looking at Report.csv.xlsx or Report.xlsm without knowing. In File Explorer choose View > Show > File name extensions (in Windows 10, tick File name extensions on the View tab).

Then right-click the file, choose Open with > Notepad, and look at the first few characters. The table above shows what each start means. Close Notepad without saving. Rename a copy of the file to the matching extension and open that copy in Excel.

The classic case: someone renamed export.csv to export.xlsx, thinking that converts it. It doesn't. To turn a CSV into a workbook, open the CSV and use File > Save As > Excel Workbook (*.xlsx).

2. A macro workbook saved with the wrong extension

Workbooks with macros must be .xlsm (or .xlsb). If an .xlsm file is renamed to .xlsx, its contents still say macro-enabled, and Excel won't open it under the .xlsx name. Rename it back to .xlsm and it opens. Excel may then show a security bar about macros; only enable them if you trust the file.

3. The file is incomplete: download, email or sync

Check the file size in File Explorer. A workbook that shows 0 KB, or far smaller than it should be, didn't finish copying. Download it again, or ask the sender to resend it. For email attachments, save the attachment to a folder first rather than opening it straight from the message.

For OneDrive or SharePoint files, look at the status icon next to the file. A cloud icon means it's online only; a red cross means a sync error. Right-click and choose Always keep on this device, or open the file from the OneDrive website instead.

Files from the internet or email can be marked as blocked. Right-click the file, choose Properties, and on the General tab tick Unblock if it's there, then click OK and try again.

4. A real workbook that's damaged: Open and Repair

If Notepad shows PK at the start and the size looks right, the workbook itself may be damaged. Excel has a built-in repair. If Repair can't fix it, run Open and Repair again and choose Extract Data, which recovers the values and, if you choose, the formulas.

Before you overwrite anything, check for an older good copy: File > Info > Version History for OneDrive and SharePoint files, or right-click > Properties > Previous Versions for files on a backed-up PC.

1. In Excel, go to File > Open > Browse.
2. Select the file, but don't double-click it.
3. Click the small arrow next to the Open button and choose Open and Repair.
4. Click Repair. If that fails, repeat and click Extract Data.
5. Save what you recover with File > Save As under a new name.

5. A CSV that keeps going wrong: import it instead

If the file is really text, open it through Data > Get Data > From File > From Text/CSV (Data > From Text/CSV on some versions). The preview lets you pick the delimiter and the file origin, so commas, semicolons and accented characters come through properly, and you can keep codes like 00123 as text instead of losing the zeros. Then save the result as .xlsx.