“Excel found unreadable content”

Excel says “Excel found unreadable content in 'Sales.xlsx'. Do you want to recover the contents of this workbook? If you trust the source of this workbook, click Yes.” Newer versions word it as “We found a problem with some content”. Either way, click Yes on a copy first, then read what Excel says it removed.

Reading Excel's repair log

After recovering, Excel shows “Excel was able to open the file by repairing or removing the unreadable content” and a list of what it changed (click the link to see the full log). The lines mean:

Repaired Records: Cell information from /xl/worksheets/sheet1.xml part. Excel fixed some cells on the first sheet. Usually little or nothing is lost. Check the totals.
Removed Records: Formula from /xl/worksheets/sheet2.xml part. Some formulas were removed. The cells keep their last values, but they won't update any more.
Removed Feature: Conditional formatting / Data validation / AutoFilter … Formatting rules or drop-down lists were dropped. Your numbers are safe; you'll need to set those up again.
Removed Part: /xl/sharedStrings.xml part with XML error. This is the serious one. Excel keeps every piece of text in your cells in one shared list. Lose it, and every text cell in the workbook comes back empty, while the numbers survive.
Removed Part: /xl/pivotCache/… or /xl/drawings/… A pivot table's data or a chart or picture was dropped.

Sheet numbers in the log follow the order the sheets were created, which isn't always the order of the tabs.

What usually causes it

The workbook was saved while Excel crashed, the computer lost power, or a USB drive was removed.
OneDrive, Dropbox, a download or an email attachment delivered an incomplete copy.
Software that exports to Excel (accounting, reporting, survey tools) wrote something Excel doesn't accept. If every export from that program does this, ask its maker for a fix.
A workbook shared and edited by several people in older Excel versions.

Free things to try first

Keep the original untouched and work on copies.
Click Yes to recover, then compare the result to what you remember, using the log above.
If you lost more than you'd like: File → Open → Browse, select the original, click the arrow next to Open → Open and Repair, then Extract Data. This sometimes keeps values that the first repair dropped.
Open the original in LibreOffice Calc or Google Sheets. They repair differently and sometimes keep the text that Excel threw away.
Look for an earlier version in Properties → Previous Versions or OneDrive version history.

Where Office File Repair helps

Excel tends to drop a whole part when anything in it is wrong. Office File Repair works the other way round: it keeps everything in each part up to the damage, so a shared text list that was cut short still gives you the text before the cut, and a damaged sheet keeps every complete row. It can also fix single damaged bytes using the checksum stored inside the file. You get a fresh .xlsx, and a preview of the recovered rows before paying. It runs in your browser; nothing is uploaded.

When nothing can help

If the text physically isn't in the file any more, because the file was cut short or overwritten with zeros, the matching cells stay empty whatever tool you use. Formulas removed by Excel in a file you've already saved over are also gone: always keep the original.

Common questions

Should I click Yes when Excel finds unreadable content?

Yes, but on a copy, and keep the original. Excel's recovery fixes small problems well, but it can remove whole parts, and once you save over the original those parts are gone.

Why are all my text cells empty after Excel repaired the file?

Excel stores the text from every cell in one shared list (sharedStrings.xml). If that part was damaged, Excel removes it and every text cell comes back empty. Try LibreOffice or a repair tool on the original file, which may recover the text up to the damage.

What does “Removed Records: Formula” mean?

Excel removed formulas it couldn't read. The cells keep their last calculated values, but they no longer update. You'll need to re-enter those formulas.

Is “Excel found unreadable content” the same as “We found a problem with some content”?

Yes. Older versions of Excel use the first wording and newer versions the second. The causes and fixes are the same.

Check your spreadsheet: the preview is free