Excel says the file format or extension is not valid, or it found unreadable content
Last reviewed 2026-09-19
An .xlsx workbook is a ZIP of XML files, one per sheet plus styles and shared strings. Excel refuses it when the ZIP is damaged, when a sheet's XML is cut off, or when the file is not really an .xlsx at all (a CSV or an older .xls renamed).
Messages you might see
- “Excel cannot open the file 'book.xlsx' because the file format or file extension is not valid.”
- “We found a problem with some content in 'book.xlsx'. Do you want us to try to recover as much as we can?”
- “Excel found unreadable content in 'book.xlsx'.”
Why it happens
- The workbook was cut off during a save, copy or download.
- The extension does not match the content, for example a .csv or an HTML export saved as .xlsx.
- A library or script that generated the file wrote invalid XML.
- The file was corrupted by a failing disk or an unfinished sync.
Try this first (free, no tool needed)
- Check the file size: a workbook that is only a few bytes or kilobytes was almost certainly cut off, so re-download or re-copy it.
- Open it in a text editor. If you see readable text lines or HTML tags instead of unreadable binary, it is not a real .xlsx; rename it .csv or .html and open it that way.
- In Excel use File > Open > Browse, select the file, click the arrow next to Open and choose Open and Repair, then try Repair first and Extract Data second.
- Try LibreOffice Calc or Google Sheets, which sometimes open a workbook Excel rejects.
- Check File > Info > Manage Workbook and the OneDrive or SharePoint version history for an earlier copy.
If that didn't work
These tools run in your browser. Your file is read and repaired on your own device and is never uploaded.
Corrupted Excel Rebuilder
Reads the workbook even with a broken ZIP directory, repairs damaged XML, removes sheets whose data is gone, rebuilds a missing workbook file from the sheets that remain and writes a clean .xlsx. Cell values are never edited.
Open Corrupted Excel RebuilderExcel-to-CSV Extractor
If you only need the data, this pulls the cells straight from the workbook's XML into one CSV per sheet. Formulas come out as their last saved result, not recalculated.
Open Excel-to-CSV ExtractorCommon questions
Will my formulas still work?
The rebuild keeps formulas as they are stored in the file. The CSV extractor keeps only the last calculated values.
Is my file uploaded anywhere?
No. The tool runs as a program inside your own browser, so the file is read and repaired on your device and never sent to a server.